THE LIBRARY Of THE ZOOLOGICAL SERIES APR 1 3 1944 OF UNIVERSITY Of ILLINOIS FIELD MUSEUM OF NATURAL HISTORY Volume 29 CHICAGO, MARCH 28, 1944 No. 10 MASTICATORY APPARATUS OF THE SLOTHS BY HARRY SICKER FOUNDATION FOB DENTAL RESEARCH, CHICAGO COLLEGE OF DENTAL SURGERY A descending processus of the zygomatic arch has been developed independently in widely separated orders of mammals. It is charac-teristic of the living Bradypodidae and of many extinct edentates (Glyptodontidae and Gravigrada). It is found in Diprotodon and indicated in the Macropodidae. In the Entelodontidae it reached its highest development. Although several explanations have been advanced for its functional significance in the entelodonts no attempt has been made to correlate its presence in the Bradypodidae to a specific functional adaptation of the masticatory apparatus. The present investigations disclose the curious fact that the mechanism of mastication in the two-toed sloth is, in a sense, of an opposite type from that in the three-toed sloth. The present study is based on the skeletal material of Field Museum of Natural History and the dissection of one specimen of Bradypus and one specimen of Choloepus from the United States National Museum. I wish to express my appreciation to these institutions for their generosity. The drawings are the work of Miss Peggy Collings. REVIEW OF LITERATURE Schulman (1906) gave a fairly accurate description of the mas-seter muscle in Choloepus. He states that "the masseter muscle of Ch. Hoffmani arises from both surfaces of the zygomatic arch, which is partly represented by a ligament, and mostly from the zygomatic bone; it does not split completely into an outer and inner layer according to its outer and inner origin. On the contrary, the mas-seter remains united immediately below the zygomatic arch." . . . "The main direction of the fibers is from anterior and superior to posterior and inferior." . . . "The masseter is a multipennate No. 557 161 ttOFULUB
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