.--Fusion of the tar- sals and metatarsals in most birds has produced one of the more characteristic arian skeletal elements, the tarsometatarsus. It is of interest that at least one other verte- brate group, the Dipodidae (Mammalia, Old World Jerboas) has developed a strikingly similar skeletal element. In contrast to modern birds the dipodids have fused only the three central metatarsals, not including the tarsals (see Figure 1A, 1D), to form a single metatarsal bone. Some genera (e.g. Dipus) have completely lost metatarsals I and V, whereas small splints with accompanying digits are retained in others (e.g. Allactaga; see Schaub 1934, Abhandl. Schweiz. Palaeont. Gesell. 57: 5 for il- lustration of additional genera). The most significant differences of the dipodid metatarsus from the avian tarsometatarsus occur on the proximal end and include the lack of a distinct hypotarsus and the lack of differentiation of internal and ex- ternal cotyla with an intervening intercotylar prominence. The dipodids instead have three rather flat, nearly horizontally oriented facets with no intervening protuberances Figure 1. Fused metatarsus of Dipus sowerbyi (AMNH No. 55979, male), Dipodidae, Mammalia: A, posterior; B, proximal; C, distal; D, anterior (X 11/.2). accommodating the unfused tarsals (Figure lB, 1D). Additionally on the distal end (Figure 1C), the trochleae are centrally ridged on the posterior surface and smooth on the anterior surface instead of being centrally grooved as in birds. The central trochlea may be the smallest of the three (as in Dipus) or subequal to trochleae II and IV (e.g. Alactagults) unlike the case in most birds where trochlea III is commonly largest or second largest of the three. Dipodids further lack the anterior metatarsal groove with accompanying proximal foramina as well as the distal foramen located between trochleae III and IV in most birds. Thanks are due Malcolm C. McKenna and Richard H. Tedford for pointing out the occurrence of metatarsal fusion in the Dipodidae and to Ray Gooris for drawing Figure 1. Work was supported by an American Association of University Women Fellowship for 1971-72 and a grant to the University of Colorado Museum by the Smithsonian Institution.--P^T VXCKERS Rxc, Department of Geology, Columbia University, New York, New York. Present address: Department of Vertebrate Paleontology, American Museum of Natural History, Central Park West at 79th Street, New York, New York 10024. Accepted 30 Jun. 72.