118 MAMMALIAN DENTITION 



viously noted, indicate progression. The molars of both jaws 

 decrease in size from the first to the third and their crowns 

 are rhomb oidal with a somewhat rounded contour, characters 

 similar to those of the human molars and attained as the re- 

 sult of parallel evolution. In the upper series there is a well 

 marked trigon and a pronounced hypocone. An oblique ridge 

 connects the protocone with the metacone as in Anthropoids 

 and in Man. In the lower molars the paraconid is lost and the 



Fig. 39. — Dentition of White-bellied Spider-monkey (Ateleus belzebuth, E. Geoffroy; 

 9.882-2). The left upper incisors are wanting. Note the rounded molar crowns and 

 the presence of an oblique ridge on the upper molars. 



metaconid extends its base as far forward as that of the proto- 

 coled with which cusp it is connected in the first and second 

 molars by a low transverse ridge. The talonid is large and 

 basin-shaped and possesses a small hypoconulid. 



In contradistinction to the omnivorous characters of the 

 dentition of Ateleus are the purely herbivorous features of the 

 teeth in Alouatta, the Howler (Fig. 40), an animal the diet 

 of which is stated to consist entirely of leaves. The dental 

 formula is the same as in Ateleus. 



