The hindlimb musculature of six specimens of Amazona albifrons was dissected in order to (1) provide a detailed myological description, (2) provide a preliminary study and basis for a systematic analysis of the Psittaciformes, and (3) point out any myological peculiarities that might be associated with their unusually manipulative feet. Notable features include a peculiar positioning of M. extensor brevis digiti IV, a previously undescribed muscle to digit III, a peculiar arrangement of the short extensors of the hallux, and a branch of M. extensor digitorum longus to the hallux. These features are believed to be associated either with the zygodactyl digit configuration or the requirement of finer control over the individual digits. Received 15 March 1983, accepted 8 July 1983.

Department of Biology, College of the Holy Cross, Worcester, Massachusetts 01610 USA THE order Psittaciformes includes about 332 living species. If only external characters are considered, it is a relatively homogeneous group. Hence, there has never been any con- fusion as to whether or not a species belonged to the order. The Psittaciformes is so morpho- logically distinct from other avian orders, how- ever, that even comprehensive recent works (Sibley and Alquist 1972, Cracraft 1981) have failed to elucidate its systematic position. Re- lationships within the order are also uncertain. The fundamental argument has been over the reality of what appears to be a very uniform group. Using a variety of anatomical charac- ters, researchers have proposed as few as one and as many as 11 families (Garrod 1874, Sal- vadori 1891, Mivart 1895, Thomson 1899, Mudge 1902, Glenny 1957, Brereton 1963, Hol- yoak 1973, Homberger 1980). Despite the invaluable anatomical informa- tion provided by these workers, the relation- ships within the Psittaciformes continue to be a matter of considerable confusion. There is a need for comprehensive studies using appro- priate characters, along with a contemporary approach to analysis. Some of the more infor- mative current avian phylogenies have been based on appendicular myology (Borecky 1977; Raikow 1977, 1978; Bentz 1979; Raikow et al. 1980; Sweirczewski and Raikow 1981; Maurer and Raikow 1981). A similar analysis of the Psittaciformes thus seems appropriate. The present, primarily descriptive, account is the first in a planned series of studies of the hindlimb musculature of the Psittaciformes. A comprehensive description of a parrot hind- limb has not been previously written. This study thus will not only be a first step toward a systematic analysis of the group but will pro- vide a basis for future morphological compar- isons with other avian orders. MATERIALS AND METHODS I dissected one hindlimb of each of six preserved specimens of Amazona albifrons under a stereomicro- scope with a magnification of 6-25 x. I used iodine stain (Bock and Shear 1972) to demonstrate small muscles clearly and to elucidate fiber architecture. Drawings were made with a camera lucida. Anatom- ical nomenclature is in accordance with the "Nomina Anatomica Avium" (Baumel et al. 1979). DESCRIPTION OF HINDLIMB MUSCLES M. iliotibialis cranialis (Figs. 1, 2: IC).--This strap- shaped muscle arises by fleshy fibers from the apo- neurosis covering the dorsal surface of the most cau- dal vertebra, from the cranial 6 mm of the dorsal crest of the synsacrum, and from the cranial 3 mm of the aponeurosis that covers M. iliotrochantericus caudal- is. The muscle extends distally, defining the cranial border of the thigh. It narrows to about 4 mm to insert by fleshy fibers on the cranial cnemial crest of the tibiotarsus. In one specimen a few fibers insert on the aponeurosis of M. femorotibialis internus about 7 mm from the distal end of the femur. M. aiotibialis lateralis (Figs. 1, 2: IL).--Only a pre- acetabular portion is present. The sheet-like muscle originates by an aponeurosis that extends along the caudal one-half of the dorsal iliac crest to the fernoral trochanter. The aponeurosis completely covers M. il- iofemoralis externus and the caudal one-half of M. LC FCLP FCLA Fig. 1. Lateral view of the thigh of Amazona albifrons showing the superficial muscles. Note the absence of the postacetabular part of M. iliotibialis lateralis and the two heads of M. iliofibularis. Abbreviations for Figs. 1-4: AIF, ansa M. iliofibularis; CF, M. caudofemoralis; FCLA, M. flexor cruris lateralis pars accessoria; FCLP, M. flexor cruris lateralis pars pelvica; FCM, M. flexor cruris mediaIls; FTEP, M. femorotibialis externus pars proximaIls; FTI, M. femorotibialis internus; FTM, M. femorotibialis medius; IC, M. iliotibialis cranialis; IF, M. iliofibularis; IFE, M. iliofemoralis externus; IFI, M. iliofemoralis internus; IL, M. iliotibialis lateralis; ISF, M. ischiofemoralis; ITC, M. iliotrochantericus caudalis; ITCR, M. iliotrochantericus cranialis; ITM, M. iliotrochantericus medius; LC, M. levator caudae; OL, M. obturatorius lateralis; OM, M. obturatorius mediaIls; PIFL, M. pubo-ischio-femoralis pars lateralis; PIFM, M. pubo-ischio-femoralis pars mediaIls. iliotrochantericus caudalis. The fleshy belly of M. il- iotibialis lateralis is about 10 mm wide at its proximal end and lies superficial to Mm. iliotrochantericus caudalis and cranialis. As it passes down the lateral surface of the thigh, its cranial border lies just deep to M. iliotibialis cranialis and its caudal border just superficial to M. femorotibialis externus pars proxi- mails. Distally, the muscle fuses with the aponeurosis covering the lateral surface of M. femorotibialis ex- ternus. This fusion begins caudally and extends cra- nially in the distalmost part of the muscle. The apo- neurosis ultimately inserts on the patella. Just proximal to its insertion, the distalmost part of the belly of M. iliotibialis lateralis is about 2 mm wide and lies deep to the distal portion of M. iliotibialis cranialis. M. iliotrochantericus caudalis (Fig. 3: ITC).--This well- developed, fan-shaped muscle arises by fleshy fibers from the entire dorsal iliac fossa cranial to M. iliofe- moralis externus. Its caudal border lies just deep to this muscle. M. iliotrochantericus caudalis extends caudolaterally, its fibers converging on a broad ten- don to insert at the center of the femoral trochanter. The tendon narrows to about 2 mm at the point of its insertion. The muscle lies superficial to the prox- imal one-half of M. iliotrochantericus cranialis and to the entire iliotrochantericus medius. It lies deep to the aponeurosis that gives rise to M. iliotibialis lateralis. M. iliotrochantericus cranialis (Figs. 2, 3: ITCR).--The muscle arises by fleshy and tendinous fibers for about 9 mm from the ventral margin of the dorsal iliac fossa. As the fan-shaped belly extends caudoventral- ly, it narrows to about 4 mm to insert by fleshy fibers on the lateral surface of the femoral shaft about 8 mm from the proximal end of the greater femoral trochanter. The insertion is just deep to M. femoro- tibialis externus pars proximaIls. The distal 1 mm of the insertion fuses with the origin of a few fibers of M. femorotibialis medius. Proximally, M. iliotro- chantericus cranialis lies deep to M. iliotrochanteri- cus caudalis, and, distally, it lies deep to M. iliotibi- ITCR IFI IL l F'r FTI-- 'PIFM ;M Fig. 2. Medial view of the thigh of Arnazona albifrons showing superficial muscles. Abbreviations given in Fig. 1. alis lateralis. It lies immediately cranial to M. iliotrochantericus medius. M. iliotrochantericus medius (Fig. 4: ITM).--The mus- cle originates by fleshy fibers from the ventral rim of the dorsal iliac fossa for a distance of about 4 mm caudal to the origin of M. iliotrochantericus cranialis and superficial to that of M. iliofemoralis internus. The fan-shaped belly passes caudolaterally and ta- pers to about 2 mm. It inserts by a flat tendon on the lateral surface of the femur, just distal to the tro- chanter and proximal to the insertion of M. iliotro- chantericus cranialis. M. iliotrochantericus medius lies deep to M. iliotrochantericus caudalis and just caudal to M. iliotrochantericus cranialis. M. femorotibialis externus (Figs. 1, 4: FTEP).--This large, fusiform muscle occupies the caudolateral sur- face of the deep thigh. It and M. femorotibialis medi- us are intimately fused except at their origins, which are separated by the insertion of M. iliotrochanteri- cus cranialis. Both proximal and distal heads are pres- ent. The proximal head arises by fleshy fibers from the lateral and caudolateral surfaces of the femur, extending from the area just caudal to the insertion of M. iliotrochantericus cranialis to the distal one- quarter of the femur, where it fuses with the distal head. About half way down the thigh there develops a dense aponeurosis that covers both proximal and distal heads, as well as M. femorotibialis medius. Dis- tally, the aponeurosis thickens and contributes to the pateliar tendon. The insertion is on the pateliar crest of the tibiotarsus. The distal head is closely associated with the prox- imal head. It arises by fleshy fibers from the caudal surface of the femur, beginning in its distal one-half and extending for about 1 cm. Distal to this point it fuses with the proximal head and inserts as described above. M. femorotibialis medius (Figs. 2, 4.' FTM).--This well- developed muscle lies along the cranial and medial surfaces of the thigh. It arises by tendinous fibers from the medial surface of the trochanteric crest and by fleshy fibers from almost the entire length of the craniomedial surface of the femur. In its distal one- half M. femorotibialis medius is covered by an apo- neurosis that is continuous with that of M. femoro- tibialis externus. These two muscles are fused in the distal three-quarters of their length, and the aponeu- rosis contributes to the pateliar tendon. The belly of LC IFE ITC --ITCR ISF CF FCM FCLP PIFL Fig. 3. Lateral view of the thigh of Amazona albifrons showing the deep muscles. Abbreviations given in Fig. 1. The following muscles shown in Fig. 1 have been removed: M. femorotibialis externus pars proximaIls, M. iliotibialis cranialis, M. iliofibularis, and M. iliotibialis lateralis. M. femorotibialis medius, however, inserts by fleshy fibers directly on the proximal end of the patella. M. femorotibialis internus (Fig. 2: FTI).--This unipin- nate muscle arises by fleshy fibers from the caudal and caudomedial surfaces of the femoral shaft. The origin extends from immediately distal to the inser- tion of M. iliofemoralis internus to the level of the medial supercondylar crest. The fibers insert on a narrow aponeurosis that develops along the caudal border of the muscle. The muscle crosses the femo- rotibial joint and gives rise to a stout tendon that inserts on the medial surface of the cranial cnemial crest, just deep to the origin of M. gastrocnemius pars medialis. The cranial border of M. femorotibialis in- ternus lies deep to M. femorotibialis medius. The caudal border lies superficial to the insertion of M. pubo-ischio-femoralis pars mediaIls. M. iliofibularis (Figs. 1, 3, 6, 7, 8, 9, 10, 11: IF).--This large, fan-shaped muscle lies on the lateral surface of the thigh, just cranial to M. flexor cruris lateralis. Its cranial one-quarter arises from the aponeurosis that covers the muscles of the dorsal iliac fossa (Mm. il- iofemoralis externus and iliotrochantericus caudalis). In one of the six specimens dissected a small slip (about 1 mm wide) originates by tendinous fibers from the femoral shaft about 2 mm below the trochanter and by fleshy fibers from the lateral surface of M. femorotibialis externus, just distal to the origin of this muscle. Caudally, the origin is by fleshy and ten- dinous fibers from the dorsolateral iliac crest to about 3 mm from the terminal process of the ilium. Cranial and caudal parts of the muscle remain separate as they pass distally down the lateral surface of the thigh, superficial to Mm. ischiofemoralis, caudofemoralis, and pubo-ischio-femoralis. They fuse about four-fifths of the way down the thigh and converge on a strong, cylindrical tendon that passes through the ansa ilio- fibularis (biceps loop) and enters the shank muscu- lature between pars lateralis and intermedius of M. gastrocnemius. The insertion is on the caudal surface of the fibula. The ansa iliofibularis consists of three arms. The proximal femoral arm arises on the caudolateral sur- face of the femur about three-quarters of the way down its length. This arm is about 2 mm wide, and it expands distally as it approaches the M. iliofibu- laris tendon. It is about 8 mm long. The distal fernoral arm and the fibular arm are less well-developed. They arise from aponeuroses on the deep surfaces of Min. gastrocnemius pars lateralis and flexor perforans et perforatus digiti II, respectively. All three arms con- verge just caudal to the shank musculature and thick- en to form a sling that supports the tendon of M. iliofibularis. M. flexor cruris lateralis (Figs. 1, 3: FCLA and FCLP).--This large, parallel-fibered muscle lies on the lateral surface of the thigh just caudal to M. ilio- fibularis. It consists of two bellies, pars pelvica and pars accessoria. Pars pelvica originates by fleshy and tendinous fibers from the dorsolateral lilac crest, ex- tending from about 2 mm caudal to the ilioischiadic foramen to the terminal lilac process. Caudal to this point the origin continues from a tendon that gives rise to the ventral caudal muscles. The belly passes distally, lying superficial to the origin of M. ischio- femoralis and the lateral surfaces of Min. pubo-is- chio-femoralis and flexor cruris medialis and deep to the proximal part of M. caudofemoralis. About four- fifths of the way down the thigh the cranial one-half of pars pelvica inserts on a raphe that separates it from pars accessoria. At this point, the caudal one- half of pars pelvica inserts by tendinous fibers on the adjacent surface of M. flexor cruris mediaIls. Pars ac- cessoria crosses the distal portion of M. pubo-ischio- femoralis and inserts by short, tendinous fibers on the caudal surface of the femur just deep to the origin of the fernoral arm of the ansa M. iliofibularis. Pars accessoria is about 3 mm wide. As it arises from the raphe, pars accessoria is connected by a thin, liga- mentous band to the caudal edge of M. gastrocne- mius pars intermedia. M. caudofernoralis (Figs. 1, 3: CF).--Only pars caudofemoralis is present. It originates by short, ten- dinous fibers on the caudolateral surface of the femur about one-third of the way down its length. As it extends caudally, it lies deep to Min. iliofibularis and femorotibialis externus and superficial to M. pubo- ischio-femoralis and the caudal portion of M. ischio- femoralis. The strap-shaped belly passes between Min. flexor cruris lateralis and flexor cruris medialis and inserts by a slender tendon on the aponeurosis on the ventral surface of the rectrices. M. ischiofernoralis (Figs. 2, 3: ISF).--The muscle aris- es by fleshy fibers from the wing of the ischium cau- dal and ventral to the ilioischiadic foramen, and cov- ers the caudoventral one-half of that foramen. As the well-developed, fan-shaped belly passes cranially, an aponeurosis develops on the lateral surface. The fi- bers converge on the aponeurosis as it narrows to form a tendon about 3 mm wide. The insertion is on the lateral surface of the femur at the level of inser- tion of M. iliotrochantericus cranialis. M. ischiofe- moralis lies deep to Min. iliofibularis and caudofe- moralis and superficial to the origin of M. pubo-ischio- femoralis. ITM OM OL FTEP- Fig. 4. Lateral view of the upper thigh and cen- tral pelvis of Arnazona albifrons showing deep mus- cles. Abbreviations given in Fig. 1. The following muscles shown in Fig. 3 have been removed; M. caudofemoralis, M. iliofemoralis externus; M. iliotro- chantericus caudalis, M. iliotrochantericus cranialis, and M. ischiofemoralis. M. flexor cruris rnedialis (Figs. 1, 2, 3: FCM).--The muscle arises by fleshy and tendinous fibers from the caudal one-third of the lateral surface of the wing of the ischium and from the dorsal margin of the mem- brane covering the ischiopubic foramen. As the large, parallel-fibered belly extends distally, its cranial mar- gin lies superficial to M. pubo-ischio-femoralis pars mediaIls and deep to M. flexor cruris lateralis. About four-fifths of the way down the thigh, the muscle receives the tendinous insertion of M. flexor cruris lateralis pars pelvica. The muscle crosses to the me- dial surface of the shank and gives rise to a strong tendon, about 3 mm wide. The tendon enters the shank musculature between pars mediaIls and inter- media of M. gastrocnemius and inserts on the caudo- roedial surface of the tibiotarsus, just distal to the insertion of M. pubo-ischio-femoralis pars mediaIls. About 4 mm proximal to its tendon, M. flexor cruris mediaIls is connected to M. flexor cruris lateralis as the latter gives rise to its accessory head. M. pubo-ischio-fernoralis (Figs. 2, 3: PIFL, PIFM).-- The muscle arises by fleshy and tendinous fibers from the ventrolateral margin of the ischium, extending from the caudal border of the obturator foramen to about 6 mm from the terminal process of the ischium. The origin lies dorsal to the ischiopubic fenestra and superficial to the membrane that covers it. Although the fibers of pars lateralis and medialis are fused at their origin, the two bellies are distinguishable. Pars medialis extends caudally beyond pars lateralis for about 4 ram. The cranial two-thirds of pars mediaIls arises by an aponeurosis from the deep surface of pars lateralis. Both bellies are parallel-fibered, and they extend downward in a slightly cranial direction, passing deep to Min. iliofibularis and flexor cruris lateralis. Pars lateralis inserts by fleshy fibers on the caudal surface of the femur, extending from about the midpoint of the shaft to the level of the ansa M. GI AIF GL-- FPPD2 FPPD3 ITC Fig. 5. Lateral view of the shank of Amazona al- bifrons showing superficial muscles. Note the rela- tively large fibularis brevis and absence of the long tendon of insertion of M. fibularis longus. Abbrevi- ations for Figs. 5-11: AIF, ansa M. iliofibularis; EDL, M. extensor digitorum longus; FB, M. fibularis brevis; FDL, M. flexor digitorum longus; FHL, M. flexor hal- lucis longus; FL, M. fibularis longus; FPD2, M. flexor perforatus digiti II; FPD3, M. flexor perforatus digiti III; FPD4, M. flexor perforatus digiti IV; FPPD2, M. flexor perforans et perforatus digiti II; FPPD3, M. flexor perforans et perforatus digiti III; GI, M. gas- trocnemius pars intermedius; GL, M. gastrocnemius pars lateralis; GM, M. gastrocnemius pars medialis; TC, M. tibialis cranialis; TCF, M. tibialis cranialis ca- put femorale; TIC, tibial cartilage. iliofibularis. The caudal one-third of its fibers turn medially to insert on an aponeurosis on the caudal surface of the proximal end of M. gastrocnemius pars medialis. The cranial one-half of pars medialis inserts by tendinous fibers on the caudomedial surface of the femoral shaft for the 5 mm proximal to the me- dial condyle. The caudal one-half inserts on the apo- neurosis of M. gastrocnemius pars mediaIls, just me- dial to M. pubo-ischio-femoralis pars lateralis. M. obturatorius lateralis (Fig. 4: OL).--Only the ven- tral head is present. This short, parallel-fibered mus- cle arises from the pubis just cranial to the obturator foramen, and from the cranial and ventral rim of the obturator foramen. It extends laterally, just ventral to the tendon of M. obturatorius medialis, and inserts on the caudal surface of the femur about 2 mm distal to the trochanter. M. obturatorius medialis (Figs. 2, 3, 4: OM).--This well-developed, fan-shaped muscle lies on the me- dial surface of the synsacrura. It originates from the entire wing, as well as the caudal part of the body of the ischium, from the membrane across the ischio- pubic fenestra, and from the dorsal margin of the pubis from the obturator foramen to about 6 mm from the apex of the pubis. The fibers pass cranially, con- verging on a central tendon that forms about one- half way along the length of the muscle. As the mus- cle approaches the obturator foramen the tendon thickens and passes to the lateral surface of the mus- cle. Fibers continue to insert on the tendon as it ex- tends through the obturator foramen to insert on the caudolateral surface of the femoral trochanter just caudal to the insertion of M. iliofemoralis externus. M. iliofemoralis internus (Fig. 2: IFI).--This small, band-like muscle arises by fleshy fibers from the ven- tral margin of the ilium for a distance of about 3 mm from the acetabulum. It extends caudodistally to in- sert on the medial surface of the femur just distal to the attachment of the articular capsule of the hip joint. M. iliofemoralis internus passes medial to Mm. iliotrochantericus cranialis and femorotibialis medi- IlS. M. iliofemoralis externus (Fig. 3: IFE).--This small, fan-shaped muscle arises by fleshy fibers from the caudodorsal corner of the dorsal lilac fossa immedi- ately caudal to M. iliotrochantericus caudalis. It lies deep to the aponeurosis that gives rise to M. ilioti- bialis lateralis. The muscle extends across the proxi- mal end of the femur and gives rise to a short, flat tendon that inserts on the lateral surface of the tro- chanter between the insertions of Mm. iliotrochan- tericus caudalis and obturatorius medialis. M. gastrocnemius (Figs. 5, 10: GI, GL, GM).--Pars lateralis arises by a strong, flat tendon from the lat- eral epicondyle of the femur. The spindle-shaped belly extends about two-fifths of the way down the caudolateral surface of the shank. Here, the fibers insert on an aponeurosis that has developed on the lateral surface of the belly. The aponeurosis contrib- utes to a tendon of insertion shared with pars inter- media and medialis. This very strong tendon crosses the tibial cartilage and inserts on the proximal end of the hypotarsus, merging with an aponeurosis that covers the plantar surface of the tarsometatarsus. Pars lateralis is the most caudal of the shank muscles. It lies superficial to M. flexor perforans et perforatus digiti II. Pars intermedia lies just medial to M. flexor hal- lucis longus. It arises by tendinous fibers from the caudal surface of the medial femoral condyle. The strap-shaped, parallel-fibered belly extends distally as the most superficial muscle on the caudomedial surface of the shank. About one-half way down the shank the fibers insert on a broad aponeurosis that fuses with that of pars lateralis and medialis to form the common tendon of insertion. Pars medialis is the most superficial muscle on the medial shank. It arises by fleshy and tendinous fibers from the cranial cnemial crest and the medial femo- ral condyle, by fleshy fibers from the aponeurosis covering the proximal one-half of the medial surface of M. tibialis cranialis, and from a line along the me- dial surface of the proximal one-half of the femoral shaft. The belly is unipinnate, the fibers inserting on a dense aponeurosis that covers the medial surface of the muscle. The aponeurosis fuses with those of pars lateralis and pars intermedia about one-half way down the shank to form the common tendon of in- sertion. The belly of pars medialis extends to about 7 mm from the distal end of the tibiotarsus. M. tibialis cranialis (Figs. 5, 6, 10: TC, TCF).--This is the most cranial muscle of the shank. There are two heads, a superficial, or tibial, and a deep, or femoral. The tibial head is, by far, the larger. It arises by fleshy and tendinous fibers from the entire cranial cnemial crest, from the pateliar ligament, and from the cranial edge of the proximal 5 mm of M. fibularis longus. A few fleshy fibers arise from the underlying femoral head, as well as from M. extensor digitorum longus. The fusiform belly extends down the shank, superficial to these two muscles. The femoral head arises by a flat tendon from the cranial surface of the lateral femoral condyle just be- neath the patellar tendon. It extends for about 3 mm across the femorotibial joint and gives rise to a rather slender, parallel-fibered belly. When the pateIlar ten- don is removed, the tendon and proximal part of the belly are visible between the two heads of M. flexor perforans et perforatus digiti III. The femoral head extends distally, lying deep to M. flexor perforans et perforatus digiti III and superficial and lateral to M. extensor digitorum longus. About one-third of the way down the shank an aponeurosis develops on the craniolateral surface of the muscle. This fuses with an aponeurosis of the tibial head. Just distal to this point the tibial head divides to form two discrete heads, a deep and a superficial. The fernoral head is included in the deep head. Each of the two heads gives rise to a separate tendon. In five of the six spec- imens the tendons fuse just proximal to the retinacu- lum extensorium tibiotarsi on the cranial surface of the tibiotarsus. As it passes through the retinaculum, TCF AIF IF FHL FPPD2-- --FPPD5 FPD4-- TIC Fig. 6. Lateral view of the shank of Amazona al- bifrons showing deep muscles. Abbreviations given in Fig. 5. The following muscles shown in Fig. 5 have been removed: M. gastrocnemius pars lateralis and M. tibialis cranialis. the tendon lies superficial to that of M. extensor dig- itorum longus. The tendon inserts on a small tuber- osity on the craniomedial surface of the tarsometatar- sus about 6 mm from the proximal end of this bone. In one specimen the two tendons remain entirely separate to their point of insertion, which is as de- scribed above. M. extensor digitorum longus (Figs. 11, 13, 14, 16: EDL).--The muscle lies on the cranial surface of the shank, deep to M. tibialis cranialis and medial to M. fibularis longus. It arises by fleshy fibers from the entire lateral cnemial crest and the cranial and lateral surfaces of the proximal one-third of the tibiotarsus. The bipinnate belly extends distally to the retinacu- lum extensorium tibiotarsi (about 7 mm from the dis- tal end of the tibiotarsus), its fibers inserting on a conspicuous tendon that develops on its craniolateral surface. The tendon passes through the retinaculum, deep to the tendon of M. tibialis cranialis, and then beneath the supratendinal bridge, which is unossi- fled. As the tendon crosses the intertarsal joint, it passes through a cartilaginous pad that covers the roedial condyle of the tibiotarsus. The tendon passes through the retinaculum extensorium tarsometatarsi on the dorsomedial corner of the proximal end of the tarsometatarsus, and immediately distal to this point it sends a branch to the hallux. The main tendon continues on to the dorsal surface of the foot and sends branches to each of the remaining three digits. The branch to the hallux turns onto the roedial surface of the tarsometatarsus and passes through a retinaculum on the first metatarsal and through another on the dorsal surface of the proximal end of the first phalanx of the hallux. The tendon then ex- tends along the dorsal surface of the first phalanx, crosses the interphalangeal joint, and expands slight- ly as it inserts on the base of the second phalanx. An elastic band originates from the dorsal surface of the first phalanx about two-thirds of the way down its length and extends to the deep surface of the tendon at its insertion. About two-thirds of the way down the dorsal sur- face of the tarsometatarsus the main tendon bifur- cates. The roedial division extends toward digit II. Before crossing the metatarsal-phalangeal joint, it gives rise to two broad, lateral extensions that con- tribute to the branches of the lateral division. The tendon extends on to the dorsal surface of digit II and passes through a retinaculum on the base of the first phalanx. The retinaculum is partially composed of fibers from the lateral division. The tendon crosses the dorsolateral surface of the first interphalangeal joint and has lateral and roedial attachments on the base of the second phalanx. It continues down the dorsal surface of the second phalanx, crosses the sec- ond interphalangeal joint, and inserts on the base of the third phalanx in the same manner as the tendon to digit I. An elastic band is present. A small, acces- sory tendon extends from the retinaculum on the base of digit II along the dorsal surface of the first phalanx and inserts on the base of the second pha- lanx with the main tendon to this digit. The lateral division bifurcates at the base of digit IV. One branch extends toward digit III. As it crosses the dorsolateral surface of the metatarsal-phalangeal joint, it receives the most proximal of the two broad extensions of the roedial division. Immediately distal to this point, the tendon passes through a sheath formed by an extension from digit IV and the more distal of the broad extensions from the roedial divi- sion. The sheath gives rise to three accessory tendons that will be discussed subsequently. The main tendon to digit III crosses the dorsolat- eral surfaces of the first and second interphalangeal joints. It has attachments at the bases of both pha- langes, like those of the tendon to digit II. As it ap- proaches the distal end of the third phalanx, the ten- don turns onto the dorsal surface and fuses with two of the three accessory tendons. The insertion, on the base of the terminal phalanx, is similar to those of digits I and II. No elastic band is present. The roedial accessory tendon to digit III arises from the sheath about one-quarter of the way down the dorsal surface of the first phalanx. It crosses to the roedial surface of the digit, and has attachments around the ventral surface of the first and second interphalangeal joints. It merges with the main tendon on the dorsal surface of the third digit, as described. The middle accessory tendon to digit III extends from the distal end of the sheath to the base of the second phalanx. Here, it expands and inserts. The lateral accessory tendon is the most slender of the three. It arises from the lateral margin of the sheath and extends along the lateral surface of the third dig- it. As it crosses the first and second interphalangeal joints, it has attachments to the main tendon. It merges with the main tendon on the dorsal surface of the distal end of the third phalanx. The most lateral division of the extensor digitorum longus tendon extends toward digit IV. As it passes along the base of the first phalanx it passes through a sheath, which is joined to that of digit III. This sheath gives rise to two accessory tendons that will be discussed subsequently. The main tendon passes along the dorsal surface of the digit, beneath a reti- naculum on the base of each of the proximal four phalanges. It expands to insert on the dorsal surface of the base of the fifth phalanx. No elastic band is present. The roedial accessory tendon to digit IV extends along the dorsal surface of digit IV, just roedial to the main tendon. It crosses the first and second inter- phalangeal joints and inserts on the base of the third phalanx just deep to the retinaculum for the main tendon. The lateral accessory tendon to digit IV lies lateral to the main tendon along the most proximal phalanx. From its attachment at the base of the sec- ond phalanx it extends to the lateral surface of the digit. After crossing the third interphalangeal joint, it passes dorsally to merge with the main tendon on the distal end of the fourth phalanx. M. fibularis brevis (Figs. 5, 6, 11: FB).--This well- developed muscle lies on the cranial surface of the shank, lateral to M. tibiaIls cranialis. There are two heads of origin. The proximal, superficial head be- gins its origin at the tip of the lateral cnemial crest along with that of M. flexor perforans et perforatus digiti III. About 8 mm distal to this point, a smaller, deep head arises by fleshy fibers from the lateral sur- face of the tibiotarsus and cranial surface of the fib- ula. It extends for about 7 mm and fuses with the superficial head. The origin of the superficial head continues distally from an aponeurosis that separates it from M. flexor perforans et perforatus digiti II! and from the craniolateral surface of the fibula to the level of the retinaculum M. fibularis. A dense aponeurosis develops on the lateral surface of the muscle in its distal one-half and gives rise to a strong tendon. As the tendon crosses the lateral surface of the intertar- sal joint, it is held in place by a ligament that spans the joint. It turns to the plantar surface and inserts on the lateral margin of the hypotarsus. M. fibularis longus (Figs. 5, 6: FL).--This rather poor- ly developed muscle lies along the distal one-third of the lateral surface of the shank, caudal to M. fi- bularis brevis and cranial to M. flexor perforans et perforatus digiti III. It arises from the cranial edge of the distal 1 cm of the aponeurosis that separates M. fibularis brevis from M. flexor perforans et perforatus digiti III. The unipinnate belly extends distally, the fibers inserting on an aopneurosis that develops on its caudal surface. About 8 mm from the distal end of the tibiotarsus the aponeurosis gives rise to a thin, flat tendon. The tendon turns caudally to insert on the proximolateral corner of the tibial cartilage. The long tendon of insertion, described in some birds (George and Berger 1966), is absent. M. flexor perforans et perforatus digiti III (Figs. 5, 6, 7, 10, 15: FPPD3).--The muscle lies on the lateral surface of the shank, just caudal to M. fibularis brevis and cranial to M. flexor perforans et perforatus digiti II. There are two heads of origin. The cranial head arises by fleshy fibers from the lateral surface of the pateliar ligament and from the lateral cnemial crest. The caudal head arises from the head of the fibula and from the underlying M. flexor perforatus digiti III. The two heads fuse about 4 mm distal to their origin. As the bipinnate belly extends distally, fibers continue to arise from the shaft of the fibula and the aponeuroses that separate the muscle from M. flexor perforans et perforatus digiti II and from Mm. fibu- laris longus and brevis. About 1 cm from the distal end of the tibiotarsus the belly gives rise to a strong, cylindrical tendon. It passes through the tibial car- tilage as the most superficial and lateral of the flexor tendons. As it crosses the intertarsal joint, it lies me- dial to the tendon of M. flexor perforatus digiti IV in a sheath formed by M. flexor perforatus digiti III (Figs. 7 and 10). It remains the most superficial tendon as it passes along the plantar surface of the foot, lying just medial to the tendon of M. flexor perforatus dig- iti IV. The tendon crosses the third metatarsal-pha- langeal joint, and about one-third of the way down the proximal phalanx it perforates the tendon of M. flexor perforatus digiti III. The tendon is held in place by ligamentous bands on the bases of the first two phalanges. About one-half way down the second phalanx the tendon of M. flexor perforans et perfor- atus digiti III is perforated by the branch of M. flexor digitorum longus to this digit. The resulting two branches insert on lateral and medial surfaces, re- spectively, of the base of the third phalanx. The hypotarsus in this species consists of a super- ficial and deep compartment. The superficial com- partment is defined by two vertical ridges. The deep compartment is a bony tunnel. Only Mm. flexor dig- itorum longus and flexor hallucis longus pass through the tunnel. M. flexor perforans et perforatus digiti II (Figs. 5, 6, 7, 11, 15: FPPD2).--This muscle lies on the lateral sur- face of the shank just caudal to M. flexor perforans et perforatus digiti III and cranial to M. gastrocne- mius pars lateralis. It arises by tendinous fibers from the distal end of the lateral femoral condyle and the head of the fibula and by fleshy fibers from the lat- eral (distal) arm of the ansa M. iliofibularis. In its proximal one-third, it is fused with the belly of M. flexor perforans et perforatus digiti III. As the bipin- nate belly extends down the shank, it lies superficial to M. flexor perforatus digiti IV. About three-fifths of the way down the shank the muscle tapers to a flat tendon. The tendon turns to the caudal surface of the shank and passes through the tibial cartilage medial to and in the same plane as the tendon of M. flexor perforans et perforatus digiti III. It extends along the plantar surface of the foot and crosses un- der the tendon of M. flexor perforatus digiti II but never penetrates it. The tendon crosses the metatar- sal-phalangeal joint of digit II and extends along the first phalanx, superficial to the branch of the flexor digitorum longus tendon to this digit. It crosses the first interphalangeal joint and inserts on the medial surface of the base of the second phalanx. It is not penetrated by the branch tendon of M. flexor digi- torum longus to this digit. In one of the six specimens dissected, this muscle splits into two distinct heads about 8 mm distal to its origin. Each head gives rise to a separate tendon. They fuse about 8 mm from the distal end of the tibiotar- SUS. M. flexor perforatus digiti IV (Figs. 6, 7, 10, 15: FPD4).--The muscle lies on the caudolateral surface of the shank, deep to Mm. flexor perforans et per- foratus digiti II and III and superficial to M. flexor perforatus digiti III. There are two heads of origin. The proximal head arises from the caudal surface of the lateral femoral condyle and from the lateral sur- face of the aponeurosis that gives rise to M. flexor perforatus digiti III. The distal head arises from this aponeurosis and by long tendinous fibers from the distal arm of the ansa M. iliofibularis. The two heads fuse about one-third of the way down the tibiotarsus. Fibers continue to arise from the aponeurosis to about the level of origin of the distal head of M. flexor perforatus digiti III. Distal to this point the muscle tapers to a broad, flat tendon. It passes through the tibial cartilage deep to the tendons of Mm. flexor perforans et perforatus digiti II and flexor perforatus digiti III and medial to that of M. flexor perforans et perforatus digiti III. It passes through a superficial compartment in the hypotarsus with the tendons of Mm. flexor perforans et perforatus digiti II and III. As the tendon approaches the distal end of the tar- sometatarsus it enters a cartilaginous compartment on the trochlea of the fourth metatarsal. It crosses FHL -- FP FHL-- FPD2 FPD5 FPPD2-- FPP --FPD3 Fig. 7. Lateral view of the shank of Amazona al- bifrons showing deep muscles. Note the sheath formed by the tendon of M. flexor perforatus digiti III. Ab- breviations given in Fig. 5. The following muscles shown in Fig. 6 have been removed: M. fibularis brevis, M. fibularis longus, M. flexor perforatus digiti IV, M. flexor perforans et perforatus digiti II, M. flex- or perforans et perforatus digiti III, and M. tibialis cranialis. The tibial cartilage has been cut open. the metatarsal-phalangeal joint and turns caudally to lie along the plantar surface of the fourth digit su- perficial to the branch of M. flexor digitorum longus to this digit. The tendon is held in place by ligamen- tous sheaths at the bases of the second and third pha- langes. The tendon also sends a strong branch to the lateral surface of the base of the second phalanx. At the proximal end of the third phalanx the tendon is perforated by the branch of the flexor digitorum lon- gus tendon. The lateral branch inserts on the lateral surface of the base of the third phalanx. The medial branch inserts on the medial surface of the base of the fourth phalanx. M. flexor perforatus digiti III (Figs. 7, 8, 9, 11, 15: FPD3).--The muscle has two heads of origin. The proximal, and larger, head arises from a broad apo- neurosis beginning in the intercondylar area of the TIC l Fig. 8. Lateral view of the shank of Amazona al- bifrons showing deep muscles. Abbreviations given in Fig. 5. M. flexor perforatus digiti III, shown in Fig. 7, has been removed. femur and extending on to the head of the fibula and to a level just distal to the insertion of M. iliofibu- laris. This aponeurosis continues distally, lying su- perficial to M. flexor perforatus digiti II and provid- ing a surface of origin for M. flexor perforatus digiti IV. The belly of the proximal head of M. flexor per- foratus digiti III develops along the caudal margin of this aponeurosis in the distal three-quarters of the shank. The fibers insert on a tendon that develops along the caudal surface of the distal one-half of the belly. The distal head of M. flexor perforatus digiti III arises by long, tendinous fibers from the fibula ad- jacent to the distal interosseus foramen. The fusiform belly extends from the distal one-half of the tibiotar- sus, and fuses with the proximal belly about 1.4 cm from the distal end of this bone. As the single belly approaches the tibial cartilage, it gives rise to two flat tendons, corresponding to the originally separate heads, that are joined by a thin but substantial layer of connective tissue. Together, the two tendons form a sheath around those of Mm. flexor perforans et per- foratus digiti III and flexor perforatus digiti IV as they all pass through the tibial cartilage and the hy- FPD3 F )L TIC-- Fig. 9. Lateral view of the shank of Arnazona al- bifrons showing deep muscles. Abbreviations given in Fig. 5. M. flexor perforatus digiti II, shown in Fig. 8, has been removed. potarsus. The sheath lies lateral to the tendon of M. perforans et perforatus digiti II. Just distal to the hy- potarsus the sheath narrows to a single tendon that extends toward digit III. It is held in place by liga- mentous bands on the third metatarsal trochlea and on the base of the first phalanx. Immediately after crossing the third metatarsal-phalangeal joint, the tendon is perforated by that of M. flexor perforans et perforatus digiti III and by that of M. flexor digi- torum longus to this digit. The lateral and medial divisions thus formed insert on the lateral and me- dial surfaces, respectively, of the base of the second phalanx. M. flexor perforatus digiti II (Figs. 8, 15: FPD2).--This rather poorly developed muscle lies deep to M. flexor perforatus digiti III along the caudal surface of the fibula and superficial to M. flexor hallucis longus. There is no fernoral origin, as described in other birds. The single head originates by fleshy fibers from the entire caudal surface of the fibula and distal to this from a raphe that separates the muscle from the cra- nial margin of M. flexor hallucis longus. About 9 mm from the distal end of the tibiotarsus M. flexor per- foratus digiti II gives rise to a tendon that enters one GI --GM PPD$ FPD4 Fig. 10. Medial view of the shank of Arnazona al- bifrons showing superficial muscles. Abbreviations given in Fig. 5. of the superficial compartments in the tibial carti- lage. The tendon lies medial to and on the same plane as the M. flexor perforatus digiti III sheath. After crossing the intertarsal joint, the tendon passes through the hypotarsus as the most medial of the flexor tendons. About one-half way down the tarso- metatarsus, the tendon crosses superficial to that of M. flexor perforans et perforatus digiti II, coming to lie lateral to it. It crosses the second metatarsal~pha- langeal joint and inserts on the lateral surface of the base of the first phalanx of this digit. M. flexor hallucis longus (Figs. 6, 7, 8, 9, 10, 11, 12, 15: FHL).--This very well-developed muscle lies on the caudal surface of the shank deep to M. gastrocne- mius. The single head has two origins, by tendinous fibers from just proximal to the intercondylar area of the femur, and by short, tendinous fibers from the caudal part of the knee joint capsule and medial semilunar cartilage. As the fusiform belly extends distally, a dense aponeurosis develops on its entire caudal surface. The belly ends about 1.2 cm from the distal end of the tibiotarsus. A strong, flat tendon arising from the aponeurosis passes through a deep L FDL FPD5 FDL -- --FHL D2 Fig. 11. Medial view of the shank of Amazona al- bifrons showing deep muscles. Abbreviations given in Fig. 5. The following muscles shown in Fig. 10 have been removed: M. gastrocnemius pars inter- media, M. gastrocnemius pars mediaIls, and M. tibi- ails cranialis. compartment in the tibial cartilage, lateral to the ten- don of M. flexor digitorum longus. In two of the six specimens there is a minute distal head that originates from the caudal joint capsule and extends along the cranial margin of the proximal head to fuse with it about 1 cm distal to the proximal end of the tibiotarsus. M. flexor digitorum longus (Figs. 8, 9, 11, 12, 15: FDL).--This unipinnate muscle is the deepest muscle on the caudal surface of the shank. There are two heads of origin. The lateral head arises by fleshy fi- bers from the caudomedial surface of the fibula for the entire length of this bone. The medial head arises by fleshy and tendinous fibers from the caudomedial surface of the tibiotarsus, from the proximal end of this bone to the level of the distal end of the body of the fibula. Distal to this point the two heads fuse, and their fibers insert on a raphe corresponding to Fig. 12. Dorsal view of the flexor hallucis longus- flexor digitorum longus tendons of Amazona albifrons. The relationship is that of Type X of Gadow's clas- sification, rather than Type I, as has previously been described in the Psittaciformes. Abbreviations are given in Fig. 5. their original boundaries. About 1 cm from the distal end of the tibiotarsus the belly ends, and the raphe continues as the tendon of insertion. Tendon of Mm. flexor hallucis longus and flexor digi- toturn longus (Fig. 12: FHL, FDL).--The tendons re- main separate to about one-half way down the sur- face of the tarsometatarsus, that of M. flexor digitorum longus lying deep to that of M. flexor hallucis longus. At this point, the flexor hallucis longus tendon splits into four branches, one extending to each of the dig- its. The flexor digitorum longus tendon splits into three branches, to digits II, III, and IV. Each fuses with its corresponding branch of M. flexor hallucis longus and extends toward its digit. As the tendons extend along the caudal surfaces of their digits, they pass through ligamentous rings at each interphalan- geal joint, each inserting on the base of the most distal phalanx. These are the deepest of the flexor tendons to each digit. The branch to the hallux has a vinculum extending from its deep surface about one-half way down the length of the first phalanx to the connective tissue on the distal end of this digit. The branches to digits II, III, and IV have vincula attaching them to the Fig. 13. Dorsal view of the foot of Amazona albifrons showing the divisions of the tendon of M. extensor digitorum longus. Note the branch to the hallux, a character otherwise known only in the Coliiformes. penultimate phalanges of their respective digits in a manner similar to that seen in digit I. M. flexor hallucis brevis (Fig. 15: FHBD, FHBP).-- This is a very well-developed muscle on the caudo- medial surface of the tarsometatarsus. There are two heads. The larger, proximal head has an extensive origin by fleshy fibers from the distal end of the hy- potarsus, caudomedial surface of the proximal one- half of the tarsometatarsus, and deep surfsace of the plantar aponeurosis adjacent to the hypotarsus. The EPD3A EPD3 EHLD'. ED Fig. 14. Dorsal view of the foot of Amazona albifrons, with the extensor digitorum longus tendon removed. M. extensor hallucis longus has a distal tendon joining that of M. extensor digitorum longus. It is uncertain whether both pars proximaIls and distalis of M. extensor hallucis longus are present or if both bellies correspond to pars distalis. M. extensor propflus digiti III accessorius has not been previously described in birds. Note that the caudal direction of digit IV results in M. extensor brevis digiti IV acting more as an adductor than an extensor of this digit. Abbreviations for Figs. 13-15: ABD4, M. abductor digiti IV; EBD4, M. extensor brevis digiti IV; EDL, M. extensor digitorum longus; EHLD, M. extensor hallucis longus pars distalis; EHLP, M. extensor hallucis longus pars proximaIls; EPD3, M. extensor propflus digiti III; EPD3A, M. extensor proprius digiti III accessorius; FHBD, M. flexor hallucis brevis pars distalis; FHBP, M. flexor hallucis brevis pars proximaIls; LM, M. lumbricalis. Other abbreviations given in Fig. 5. FPPD$-- :DL "PPD2 FPD FPD4 LM Fig. 15. Plantar view of the foot of Amazona albifrons showing intrinsic muscles and insertions of flexor tendons. Abbreviations given in Figs. 5 and 14. Retinacula of the flexor tendons have been removed. Note the presence of only a single slip of M. lumbricalis and the strong development of M. flexor hallucis brevis. With digit IV directed caudally, M. abductor digiti IV acts more as an extensor of this digit than as an abductor. fan-shaped belly extends distally, superficial to the tendons of Mm. flexor digitorum longus and flexor hallucis longus. As it reaches metatarsal I, the muscle gives rise to a broad, flat tendon that turns around the trochlea, crosses the metatarsal-phalangeal joint and inserts on the lateral surface of the base of the first phalanx. The insertion does not expand over the flexor hallucis longus tendon, as described in many birds. A much smaller, distal head arises by tendinous fibers from the caudomedial surface of the tarso- metatarsus about 3 mm from the proximal end of this bone. The flat belly extends parallel and superficial to that of the proximal head. The distal head inserts by fleshy fibers along the caudomedial surface of the proximal head, then gives rise to a short, flat tendon that fuses with the main tendon before crossing the metatarsal-phalangeal joint. M. extensor hallucis longus (Figs. 14, 16: EHL).--This very small muscle consists of two parts. For reasons given in the Discussion, the homologies with pars proximaIls and pars distalis are questionable. The larger, more proximal part arises from the medial surface of the tarsometatarsus just caudal (plantar) to the M. tibialis cranialis tuberosity and deep to the undivided tendon of M. extensor digitorum longus. The parallel-fibered belly follows the branch of this tendon to the base of the dorsal surface of the hallux. Here, it gives rise to two thread-like tendons. One inserts on the base of the proximal phalanx. The oth- er extends distally alongside the extensor digitorum longus tendon and fuses with it on the distal end of the proximal phalanx. There is an accessory extensor of the hallux con- sisting of only a few fibers. It arises from the medial surface of the tarsometatarsus about 3 mm distal to ABSENT Fig. 16. Diagrammatic cranial view of the tarsometatarsus showing a possible scheme for the evolution of the unique branch of the extensor digitorum longus tendon to the hallux in the Psittaciformes and the Coliiformes. A. Hypothetical ancestral condition with branches only to digits II, III, and IV and both pars proximalis and pars distalis of M. extensor hallucis longus present. B. Psittaciform condition with branches to digits I, II, III, and IV. Pars proximaIls of M. extensor hallucis longus is absent, and its tendon has presumably shifted to that of M. extensor digitorum longus. C. Coliiform condition with branches to digits I, II, III, and IV. Pars proximaIls of M. extensor hallucis longus is also absent here, and a similar shift in its tendon to that of M. extensor digitorum longus is proposed. Note the strong development of M. extensor hallucis longus pars distalis as compared with that of the parrots dissected. Abbreviations given in Fig. 14. the M. tibialis cranialis tuberosity. It extends along the cranial surface of the first metatarsal, passing through the retinacula with the tendons of its prox- imal counterpart and M. extensor digitorum longus. It inserts on the base of the proximal phalanx. M. extensor proprius digiti III (Fig. 14: EPD3).--This is the most conspicuous of the cranial intrinsic foot muscles, lying just deep to the extensor digitorum longus tendons. It arises by fleshy fibers from the dorsomedial surface of the distal one-half of the tar- sometatarsus and extends distally to cross the third metatarsal-phalangeal joint. It inserts as a broad apo- neurosis across the entire dorsal surface of the base of the proximal phalanx. There is a small muscle lying superficial to M. ex- tensor proprius digiti III. It arises from the cranial surface of the tarsometatarsus for about 2 mm prox- imal to the origin of M. extensor proprius digiti III. The slender belly extends distally, over the medial edge of this muscle for about 7 mm. It then gives rise to a delicate tendon that crosses the medial surface of the metatarsal-phalangeal joint, extends along the dorsal surface of the proximal phalanx, and inserts on the base of the second phalanx. It is entirely in- dependent from M. extensor proprius digiti III. This muscle has not been previously described in birds, and I have called it M. extensor proprius digiti III accessorius (EPD3A). M. extensor brevis digiti IV (Fig. 14: EBD4).--This very small muscle originates from the craniolateral surface of the tarsometatarsus about 6 mm from its distal end. The belly is only about 1.5 mm long. It gives rise to a substantial tendon that passes around the lateral surface of the fourth metatarsal and inserts on the dorsolateral surface of the base of the proxi- mal phalanx. It appears to function more as an ad- ductor than an extensor. M. abductor digiti IV (Fig. 15: ABD4).--This well- developed muscle arises from the deep lateral surface of the hypotarsus and from the caudolateral surface of the tarsometatarsus along a line from the lateral condyle to about 2 mm from the distal end of the fourth metatarsal. About two-thirds of the way down its length a tendon develops on its caudal margin. The tendon crosses the fourth metatarsal-phalangeal joint and inserts on the caudomedial surface of the base of the proximal phalanx. In an anisodactyl po- sition this muscle would function as an abductor, but with the fourth digit directed caudally it acts as an extensor of the digit. M. lumbricalis (Fig. 15: LM).--Only one rather poor- ly developed slip is present. It arises from the branches of the flexor digitorum longus tendon to digits II and III. It extends along the medial surface of the branch to digit III and inserts on the trochlea for this digit. The following muscles are absent in A. albifrons: M. ambiens, M. plantaris, M. popliteus, M. abductor digiti II, M. adductor digiti II, M. extensor brevis dig- iti III. DISCUSSION The thigh and shank.--The muscles of both the thigh and the shank of A. aIbifrons are relatively unspecialized. The few distinctive features that do occur are often variable from one specimen to another and have obscure functional signif- icance. In the thigh the absence of the postacetabu- lar portion of M. iliotibialis lateralis and the separation of the two heads of M. iliofibularis may be useful in determining parrot relation- ships after comparative studies are carried out. The two tendons of insertion of M. tibialis cranialis found in one of the six specimens is of uncertain significance. It is very probably an anomolous variation. Because this condition has been described only in Bubo and Otis (George and Berger 1966), however, it could have sys- tematic implications if consistently found in other parrots. The relative development of Mm. fibularis brevis and fibularis longus is quite variable among birds (George and Berger 1966). It is worth noting, however, that in A. aIbifrons M. fibularis brevis is by far the larger, and M. fi- bularis longus lacks the long tendon of inser- tion (Fig. 6). Only the two flexors of digit II exhibit no- table characters. The tendon of M. flexor per- forans et perforatus digiti II is not perforated by that of M. flexor digitorum longus, a feature shared only with woodpeckers and a few oth- ers (George and Berger 1966). M. flexor perfor- atus digiti II lacks the femoral origin described for most birds. The relationships of the flexor digitorum longus-flexor hallucis longus tendons are Type X of Gadow's (1893-1896) classification. This arrangement was illustrated, but not described, by Gadow in HeIiornis. Gadow classified parrots as Type I, in which the tendon of M. flexor hal- lucis longus inserts on the hallux but is con- nected by a vinculum to the tendon of M. flex- or digitorum longus. In Type X the two tendons are not connected by a vinculum. Instead, the flexor hallucis longus tendon divides into four branches, three of which fuse with those of the flexor digitorum longus tendon (Fig. 12). In the case of both types I and X, the hallux is acted upon only by M. flexor hallucis longus, where- as the other three digits are flexed by both mus- cles. The foot.--The most distinctive features of the hindlimb of A. albifrons are found in the pes, and it is very likely that they are related to the zygodactyl digit configuration or to its unusual manipulative ability. Most species of Amazona use their feet to at least some degree in grasp- ing and handling food. Flexion of the hallux is brought about by two muscles. M. flexor hallucis longus, in the shank, acts when flexion of the individual phalanges is required, but M. flexor hallucis brevis, an intrinsic foot muscle, causes flexion of the digit as a unit. Similarly, extension of the individual phalanges of the hallux is effected from the shank by M. extensor digitorum longus, whereas extension of the entire digit is brought about by an intrinsic foot muscle, M. extensor hallucis longus. There are two possible explanations for the condition of the extensor hallucis longus (Fig. 16). It is likely that pars proximalis has been lost and that the large, distally located belly with the accessory insertion on the distal pha- lanx of the hallux is pars distalis. The very small, distal muscle would then be either a structure previously undescribed among birds or, more probably, the homologue of M. extensor hal- lucis longus pars accessorius, previously de- scribed only in the Coliiformes (Berman and Raikow 1982). The necessity for cranial rotation of the hallux in the mousebirds could account for the difference in position of this new mus- cle in the two orders. In parrots, both the ac- cessory insertion of pars distalis and the pres- ence of this "new" muscle could simply provide for more delicate control of the hallux. This explanation could also account for the branch of the extensor digitorum longus tendon to the hallux. This condition has been found in only one other avian order, the Coliiformes (Berman and Raikow 1982). In Coliiformes, because of the position of the muscle belly that is present, and its insertion, it is reasonably certain that M. extensor hallucis longus pars proximalis has been lost. Moreover, as in the parrots dissected, the branch of the extensor digitorum longus tendon to the hallux has a very proximal origin (Fig. 13), suggesting that it might have been captured from M. extensor hallucis longus pars proximalis when this muscle was lost. The se- lection pressure in Coliiformes was probably toward more efficient operation of the digits in the zygodactyl configuration (Berman and Rai- kow 1982: 55). Parrots are permanently zygo- dactyl, and Colius occasionally uses this digit configuration. The extension of both caudally directed digits by the action of one muscle would be functionally advantageous over a sys- tem operating them separately, as occurs with the conventional anisodactyl configuration. This branch of the extensor digitorum longus tendon has not been found in the zygodactyl Piciformes (Swierczewski 1977) and Cuculi- formes (George and Berger 1966), but neither of these groups uses the foot extensively in grasping or manipulating objects. A second interpretation of M. extensor hal- lucis longus in A. albifrons is that the large belly is pars proximalis and its insertion on the distal hallux is thus as in other birds. The accessory structure would then be a much reduced pars distalis. Although this is the more conservative explanation, it raises problems. If the large bel- ly of this muscle is pars proximalis, it has an unusually distal location on the tarsometatar- sus, corresponding to the position of M. exten- sor hallucis longus pars distalis in those birds that have both parts. Also, with this interpre- tation, the branch of the extensor digitorum longus tendon to the hallux would have to be a new structure. Although this is no more un- likely than the appearance of a new muscle, there is no functional explanation for its very proximal origin from the main tendon. I pres- ently favor the first explanation. It is to be hoped that future comparative dissections will allow additional insight. There are two intrinsic foot muscles that sup- plement the action of M. extensor digitorum longus on digit III. M. extensor proprius digiti III is in a conventional position but very well- developed. M. extensor proprius digiti III accessorius (Fig. 14) has not been previously described in birds, and it seems likely that it produces a finer control over the digit. George and Berger (1966) describe a second extensor of this digit, M. extensor brevis digiti III. It orig- inates from the extreme distal end of the tar- sometatarsus and thus seems an unlikely hom- ologue for the muscle described in this parrot. Both intrinsic foot muscles affecting digit IV have undergone changes in their actions as a result of the caudal direction of this digit. M. abductor digiti IV (Fig. 15) functions more as an extensor than as an abductor here, whereas M. extensor brevis digiti IV acts as an adductor (Fig. 14). ACKNOWLEDGMENTS I am grateful to Marion J. Mengel of the University of Kansas Museum of Natural History for providing the preserved specimens for dissection and to Robert J. Raikow for his invaluable constructive criticism of the manuscript. The manuscript was greatly im- proved by the suggestions of Richard Zusi, Gregory Bentz, and one anonymous reviewer. Thanks are also due to Doris Carrigan for typing the manuscript. This study was partially supported by a grant from the Charles and Rosanna Batchelor (Ford) Foundationß awarded to me by the Holy Cross College Committee on Professional Standards. LITERATURE CITED BAUMEL, J. J., A. S. KINGß A.M. LUCAS, J. E. BREAZILE, & H. E. EVANS (Eds.). 1979. Nomina Anatomica Avium. Londonß Academic Press. BENTZ, G. D. 1979. The appendicular myology and phylogenetic relationships of the Ploceidae and Estrildidae (Aves: Passeriformes). Bull. Carnegie Mus. Nat. Hist. No. 15: 1-25. BERMAN, S. L., & R. J. RAIKOW. 1982. The hindlimb musculature of the mousebirds (Coliiformes). Auk 99: 41-57. BOCK, W. J.,  R. SHEAR. 1972. A staining method for gross dissection of vertebrate muscle. Anat. Anz. 130: 222-227. BORECKY, S.R. 1977. The appendicular myology and phylogenetic relationships of the avian "corvid assemblage." Unpublished Ph.D. dissertation. Pittsburghß Pennsylvaniaß Univ. Pittsburgh. BRERETON, J. L. 1963. Evolution within the Psitta- ciformes. Proc. 13th Intern. Ornithol. Congr.: 499-517. CRACRAFT, J. 1981. Toward a phylogenetic classifi- cation of the Recent birds of the world (class Aves). Auk 98: 681-714. GADOW, H. 1893-1896. Muscular system. Pp. 602- 620 in A dictionary of birdsß part 3 (A. Newtonß Ed.). London, Adam and Charles Black. GARROD, A.H. 1874. On some points in the anato- my of parrots which bear on the classification. Proc. Zool. Soc. London 1874: 586-598. GEORGE, J. C., & A. J. BERGER. 1966. Avian myology. New Yorkß Academic Press. GLENNY, F. H. 1957. A revised classification of the Psittaciformes based on carotid artery arrange- ment patterns. Ann. Zool. Agra. 2: 47-56. HOLYOAKß D.T. 1973. Comments on taxonomy and relationships in the parrot subfamilies Nestori- nae, Loriinae, and Platycercinae. Emu 73: 157- 176. HOMIERGER, D. G. 1980. Funktionelle-morpholo- gische Untersuchungen zur Radiation der Er- nihrungs-und Trinkmethoden der Papageien (Psittaci). Bonner Zool. Monogr. No. 13. MAURERß D. R., & R. J. RAIKOW. 1981. Appendicular myology, phylogeny, and classification of the avian order Coraciiformes (including Trogoni- formes). Ann. Carnegie Mus. Nat. Hist. 50: 417- 434. MIVART, G. 1895. On the hyoid bones of certain par- rots. Proc. Zool. Soc. London 1895: 162-174. MUDGE, G. P. 1902. On the myology of the tongue of parrots, with a classification of the orderß based upon the structure of the tongue. Trans. Zool. Soc. London 16: 211-278. RAIKOW, R.J. 1977. The origin and evolution of the Hawaiian honeycreepers (Drepanididae). Living Bird 15: 95-117. 1978. Appendicular myology and relation- ships of the New World nine-primaried oscines (Aves: Passeriformes). Bull. Carnegie Mus. Nat. Hist. 7: 1-43. ß P. J. POLUMBO, & S. R. BORECKY. 1980. Ap- pendicular myology and relationships of the shrikes (Aves: Passeriformes: Laniidae). Ann. Carnegie Mus. Nat. Hist. 49: 131-152. SALVADORI, T. 1891. Catalogue of the Psittaci, or parrots, in the collection of the British Museum. London, Longmarts and Co. SIBLEY, C. G., & J. ALQUIST. 1972. A comparative study of the egg white proteins of non-passerine birds. Bull. Peabody Mus. Nat. Hist. 39: 159-168. SWIERCZEWSKI, E. V. 1977. The hindlimb myology and phylogenetic relationships in the avian or- der Piciformes. Unpublished Ph.D. dissertation. Pittsburgh, Pennsylvania, Univ. Pittsburgh. ß & R. J. RAIKOW. 1981. Hind limb morphol- ogy, phylogeny, and classification of the Pici- formes. Auk 98: 466-480. THOMSON, D.W. 1899. On characteristic points in the cranial osteology of the parrots. Proc. Zool. Soc. London 1899: 9-46.