232 Zoological Society. 
nent, while that of the younger individual is truncated and quite de- 
stitute of any convexity or keel, like the adult skull of 4. gularis. 
3. ArcropiTHECUS BLAINVILLII. 
Grey-brown, back and outside of the arms white varied, with an 
elongated narrow streak extending nearly the whole length of the 
back ; the forehead very convex and swollen over the back of the orbit. 
Teeth rather large ; front lower compressed. 
Lower jaw distinctly keeled up the symphysis, and slightly angu- 
larly produced on the front edge. 
B. tridactylus Braziliensis, Blainville, Osteog. t. 2, skeleton; 3, 
skull partly broken. 
Hab. Tropical America. 
We have three specimens of the animal agreeing with the skulls 
here described, but they offer no external character by which I can 
distinguish them from the preceding specimens (4. marmoratus) ; 
yet the skulls all agree in the greater convexity of the forehead and 
in the form of the ‘angle of the lower jaw. Two of the lower jaws 
have a distinct angular ridge up the front symphysis. 
It has been suggested that the differences in the form of the hinder 
part of the lower jaw, which, it should be observed, are not the only, 
but are the most easily described characters to separate these species, 
are not sufficient for specific distinction. I am willing to own that 
it is a fair question of discussion, and one that can only be settled by 
the comparison of more specimens than we at present possess. Should 
these variations prove only individual, and not specific, then it must 
lead us to be very cautious in the formation of species on the exami- 
nation of skeletons alone, as is of necessity the case in the animals now 
only found in a fossil state. 
b. Fur elongate, very flaccid, whitish; dorsal streak very short, 
indistinct, only seen where the hair is worn. 
4. ARCTOPITHECUS FLACCIDUS. 
Pale grey-brown; back, sides of the back and hinder part white 
varied, with a short blackish dorsal streak between the shoulders. 
Skull with a broad rather convex forehead. (3 spec.) 
Ai (seconde), Buffon, Hist. Nat. xii. 62. 
Jeunes Ais, Buffon, H. N. xii. t. 5 
Bradypus tridactylus, Temm. Ann. Gen. Sci. Phys. vi. 51, not 
Linn.; Pr. Max. Abbild. Nat. Braz.t.  . 2 & jun.; Betér. zur 
Nat. ii. 482. 
B. tridactylus, var. a. 2? ?, Desm., and var. d. ¢, Mamm. 
Var. 1. White gr ey-brown ; back of the hairs blackish, with a short 
black streak, and with a white spot on each side between the shoulders. 
(1 spec.) 
Hab. Venezuela; Mr. Dyson. 
Var. 2. Nearly uniform whitish grey-brown; base of the hairs 
blackish, without any dorsal streak. (1 spec.) 
Hab. "Para ; J.P. G. Smith, Esq. 
