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. 
CuoLta@pus.—Hands two-clawed, feet three-clawed; front 
W idek large, like a canine; pterygoid bone rather swollen, sub- 
vesicular. 
2. Brapypus.—Hands and feet three-clawed ; front grinder small ; 
pterygoids swollen, hollow, vesicular. 
3. ARcCTOPITHECUS.—Hands and feet three-clawed ; front grinder 
small; pterygoids compressed, crest-like, solid. 
I. CuoLapvus, Llliger (1811) ; Bradypus, #. Cuvier, Dent. Mamm. 
t. 77; Bradypus, sp. Linn. ; Tardigradus, sp. Brisson. 
Hands two-clawed, feet three-clawed. Grinders: front upper and 
lower large, like canes; 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 fr ont, with a long canal behind them; pterygoid bones separate, 
rather swollen, spr a out on the sides, thick, ae 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, Ostéograph. Bradypus, t. 1 ; skeleton, t. 3. 
f. 1, 2, old and young skull. 
1. CHOL@PUS DIDACTYLUS. 
Bradypus didactylus, Linn.; Cuvier, Oss. Foss. v.73. t.6; t. 7. 
f. 3,5; skull, cop. Cuvier, Rég. An. Illust. t. 70. f. 2; Blainv. Ostéog. 
Bradypus, t.1.t. 3.f.13; Guérin, Icon. R. A. t. 33. f. 2-2 a, 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 inthe 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 tapermg, 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. 
