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 A. gularis. 



'3. Arctopithecus 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 (A. 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 Jlaccid, 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), Buff on, Hist. Nat. xiii. 62. 



Jeunes Ais, Buffon, H. N. xiii. t. 5. 



Bradypus tridactylus, Temm. Ann. Gen. Sci. Phys. vi. 51, not 

 Linn. ; Pr. Max. Abbild. Nat. Braz. t. . <j> & jun. ; Beitr. zur 

 Nat. ii. 482. 



B. tridactylus, var. a. £ ?, Desm., and var. d. <$, Mamm. 



Var. 1 . White grey-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. 



