CHOLOPUS. 185 
2. CHOLOPUS. 
Cholepus, Illiger, Prodr. Syst. Mamm. &c. p. 108 (1811). 
Cholopus, Sclater, P. Z. S. 1872, p. 861. 
The most obvious external difference between this genus and the last lies in the fore 
foot, only two of the digits being functionally developed in Cholopus. In the skull the 
tympanic remains in a very imperfect state throughout life; while, on the other hand, 
the intermaxillaries are better developed than in Bradypus, and soon become ankylosed 
with the maxillaries, forming an angular projection in front of the palate, which cor- 
responds to the curious “ spout-like process” into which the front part of the mandible 
is produced. ‘There are either six or seven cervical vertebra, and only four molars on 
each side in the lower jaw; but the anterior pair of teeth, both above and below, are 
much larger than the rest, being developed into large triangular canine-like weapons, of 
which the creature can make formidable use. 
For long the only known member of the genus was its type, the Unau or Two-toed 
Sloth of Brazil, Bradypus didactylus of Linneus. In 1858 Professor Peters described 
a second species, from Costa Rica, under the name of Cholapus hoffmanni, characterized 
by its long hair and short white claws; and a few years later he was enabled to show, 
from the examination of no less than six skeletons, that the normal number of cervical 
vertebre was s7z in C. hoffmanni, instead of seven as in C. didactylus. 
This curious discovery has since been amply confirmed by the examination of other 
specimens; but, on the other hand, the external characters pointed out by Dr. Peters 
do not prove quite constant when a large number of examples are compared. It is 
true that the hair of C. hoffmanni is usually longer, of a paler brown, and more often 
tipped with white than that of C. didactylus; but many intermediate specimens occur 
which it is very difficult to assign to either species. Dr. v. Frantzius observes that the 
animal is dark, short-haired, and woolly in youth, but becomes longer-haired and paler 
in colour as it grows up. The comparative length of the claws appears also to depend 
on age, as suggested by Gray. On the characters of the skull, which Professor Peters 
considered to be more convex and shorter in the rostrum than that of C. didactylus, 1 
can place no weight; for Dr. Krauss has shown how variable are the cranial features 
of the latter species*, and Dr. v. Frantzius found just as little constancy in Hoffmann’s 
Sloth f. 
Mr. O. Thomas’s recent discovery of a skin and skeleton of C. hoffmanni in a collec- 
tion formed in Ecuador, shows that the range of the form is not so well defined as 
has hitherto been supposed; and it even suggests a doubt whether the reduction of the © 
vertebrae, on which the separation of the Central-American race appears solely to rest, 
may not occur more or less frequently as an individual variation in other parts of the 
* Arch. f. Naturg. xxxv. 1, pp. 124-129. t+ Tom. cit. p. 313. t P. Z. 8. 1880, p. 402. 
BIOL. CENT.-AMER., Mamm. Vol. 1, December 1880. 2B 
