226 Zoological Society. 



The examination of the collection of skulls of the family in the 

 collection at the British Museum, has induced me to believe that the 

 recent species may be divided into three very distinct subdivisions, 

 and that there are at least seven distinct species. 



Synopsis of Genera. 



1. Cholcepus. — Hands two-clawed, feet three-clawed; front 

 grinder large, like a canine ; pterygoid bone rather swollen, sub- 

 vesicular. 



2. Bradypus. — Hands and feet three- clawed ; front grinder small ; 

 pterygoids swollen, hollow, vesicular. 



3. Arctopithecus. — Hands and feet three-clawed ; front grinder 

 small ; pterygoids compressed, crest-like, solid. 



I. Choloepus, Illiger (181 1) ; Bradypus, F. Cuvier, Dent. Mamm. 

 t. 77 ; Bradypus, sp. Linn. ; Tardigradus, sp. Brisson. 



Hands two-clawed, feet three-clawed. Grinders : front upper and 

 lower large, like canines ; the upper ones separated from the other 

 grinders by a broad space, with a deep concavity in front, at the back 

 edge of the teeth. Intermaxillary bones small, distinct, and produced 

 in front, with a long canal behind them ; pterygoid bones separate, 

 rather swollen, spread out on the sides, thick, with a moderate internal 

 vesicular cavity. 



Lower jaw much-produced in front between the teeth. 



The skull of this genus is well-figured by M. Cuvier, Oss. Foss. v. 

 t. 5, and M. De Blainville, Osteograph. Bradypus, t. 1 ; skeleton, t. 3. 

 f. 1, 2, old and young skull. 



1. Cholcepus didactylus. 



Bradypus didactylus, Linn. ; Cuvier^ Oss. Foss. v. 73. t. 6 ; t. 7. 

 f. 3, 5 ; skull, cop. Cuvier, Reg. An. Illust. t. 70. f. 2 ; Blainv. OstSog. 

 Bradypus, t. 1. 1. 3. f. 13 ; Gu6rin, Icon. R. A. t. 33. f. 2-2 «, skull. 



B. Unau and B. Curi, Link. 



We have three more or less perfect skulls from different-aged indi- 

 viduals of this species. 



The projection in the front of the lower jaw in the young specimen 

 is narrow and acute ; it then becomes thin, wider and rounded at the 

 end, and in the adult skull it is thickened, prolonged, and again be- 

 comes rather more acute. 



In the adult skull there are very large air-cavities between the pari- 

 etes of the bones, and a considerable cavity in the pterygoid bone. 



In the younger skull the pterygoid bone is small, and appears to be 

 nearly solid, but there is a very large circular perforation which com- 

 municates with a cavity under the pterygoid bones, which is nearly 

 entirely obliterated in the adult skull ; and the intermaxillary bones 

 of the two young skulls are much less projecting than those of the 

 adult one. 



The young skull exhibits a small, distinctly tapering, produced, 

 additional central nasal bone, which is not preserved (or not to be 

 found) in the adult one, or in any of the other skulls of the family 

 which have come under my observation. 



