322 



COMPARATIVE ANATOMY 



an uneven surface with wear, showing a characteristic pattern in 

 the different groups (cf. Figs. 234-236). 



The relations and number of the tubercles, as well as the form 

 of the teeth in general, are of great importance in elucidating the 

 ancestral history of the Mammalia, and attempts have been made 

 to trace the evolution of the various forms of molar met with in 

 the Class. According to one view, the tuberculated molar has arisen 

 by the gradual modification of a single conical tooth, which has 



FIG. 236. TEETH OF A CATAKRHINE MONKEY (Nasalis larwtiu*). 

 References as before. 



produced lateral outgrowths or buds. Thus taking a simple conical 

 form as the most primitive type of mammalian tooth, we find that 

 certain extinct Mammals (e.g. Triconodon) possessed teeth with a 

 main cone and two lateral cusps. It has been supposed that the 

 more complicated forms have been derived from this triconodont 

 tooth firstly by a rotation of the lateral cusps outwards in the 

 upper, and inwards in the lower one, thus forming a tritubercular 

 tooth, with three cusps arranged in a triangle ; and secondly by 

 the addition of accessory cusps (the first to appear being the 

 posterior heel or talon), as well as of connecting ridges. 1 



1 According to another hypothesis, the mammalian cheek-teeth were primarily 

 multitubercular, having originated by the fusion of a number of simple conical 

 teeth ; and certain facts in their development and the presence of multituber- 

 culate Mammals in the Triassic rocks indicate that in some cases, at any rate, 

 they may have become evolved in this manner. 



