66 
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 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. 
The hinder angle of the lower jaw of the two skulls, the one of a 
young and the other of an adult animal, in the Museum collection, is 
nearly similar in form. The condyloid process of the young is short 
and truncated behind, that in the older jaw being produced and bent 
back at the tip. 
In the British Museum collection there are five skins of adults, two 
very young, one dry, the other in spirits, and three skulls more or 
less perfect. 
The very young specimen in spirits in the British Museum is figured 
in Griffith’s Animal Kingdom, and Seba figures the fetus from spirits. 
II. Brapyrus. ?Acheus pars, F. Cuvier, Dent. Mamm. t. 78 ; 
Guérin. Bradypus pars, Linn. Bradypus, Illiger. Tardigradus, sp. 
Brisson, Arctopithecus, Gesner. 
Hands and feet three-clawed. Skull flattened above on the fore- 
head. Grinders: front upper small, cylindrical; front lower small, 
transverse, compressed. Intermaxillary bones none, or very rudi- 
mentary. The upper process of the zygomatic arch with a broad 
process in front, forming a back edge to the orbit. Pterygoids sepa- 
rate, much-swollen and raised, very thin, enclosing a large vesicular 
cavity. 
ee jaw produced in front between the teeth, flattened. 
Cuvier, Oss. Foss. v. 88, described the skull of this subgenus. 
Blainville (Osteograph. Bradypus, t. 3) figured an imperfect skull 
of a young animal under the name of B. torquatus, but it does not 
