.--On 20 March 1969, a Darwin's Rhea (Pterocnemla pennata) egg was opened at the iNew York Zoological Park and was found to contain conjoined twin embryos. The egg was laid on 30 January 1969 and six days later, was placed in a forced-air incubator. By 17 March, movement was heard within the shell. Movement continued until noon on 19 March, when no signs of life could be detected. Other Darwin's Rhea eggs incubated under the same conditions (97 ø F and 85 per cent relative humidity) had an average incubation period of 37 to 39 days. After 43 days of incubation, this egg was opened and the conjoined twins discovered. Fc. 1. Embryo of conjoined twin Darwin's Rhea. 219 220 THE WILSON BULLETIN June 1970 Vol. 82, No. 2 The embryos are joined and single ventrally from the lower mandible, to the sternum, and yolk sac. Dorsally the embryos are double with two sets of vertebrae, two sets of limbs, and basically, two separate bodies. Two separate upper mandibles fused at their base into one head with only one pair of eyes. At the back of the skull, two sets of vertebrae meet. The cranium was incomplete and 2.5 cm 2 of the brain was exposed. (see Fig. 1.) The twin embryo weighed 364 g while the yolk and yolk sac weighed approximately 100 g for a total of 464 g. Thirty-five normal newly hatched chicks averaged 426 g but varied from a low of 327 g to a high of 491 g. The eggshell itself weighed 84 g and varied from 0.040 to 0.042 inch in thickness. All of the waste material and the membranes from around the embryos totaled 65 g. The egg had lost 55 g or 8.2 per cent of its weight during development. An 8.2 per cent weight loss is 0.8 per cent above the average weight loss found in 30 Darwin's Rhea eggs, at the New York Zoological Park, but is well below the maximum of 8.8 per cent that occurred in one other Darwin's Rhea egg which hatched successfully. The specimen is preserved in a buffered formalin solution. I thank W. G. Conway for comments upon the manuscript.--DoNALD ]]RUNING, New York Zoological Society, Bronx Park, Bronx, New York 10460, 8 May 1969.