220 EVOLUTION OF MAMMALIAN MOLAR TEETH 



lief erring to Osborn's figures of Dryolestes (Fig. 206), Gidley * says: 

 ' But there are two important cusps not noted by Osborn, one an 

 external cusp placed anterior to the main external cusp, the other 

 a small but well-defined intermediate cusp appearing on the posterior 

 transverse ridge. Thus there are five distinct cusps instead of three, 

 as stated by Osborn, and these do not form a trigon in the sense that 

 this term has been used, for the main external cusp is in the middle 

 of the base of the triangle instead of forming one of its angles. 



o O 



" Considering the outer portion of the Dryolestes molar as 

 homologous to the three cones and two fangs of Triconodon, the 

 derivation of this type of tooth is much simplified, it being not so far 

 removed from the primitive reptilian condition, and though diverging 

 on different lines, is no more specialized, as a whole, than the Triconodon 

 type of tooth, the differentiation being carried on more rapidly in the 

 latter in the special development of the anterior and posterior lateral 

 cones and their accessory cusps, while in Dryolestes the specialization 

 has apparently been centralized in the development of the high, 

 narrow, heel-like cusp and its supporting fang on the inner side of 

 the molar. 



' This view is strongly supported by the evidence obtained from 

 still another characteristic Atlantosaurus-beds type of molar represented 

 by Dicrocynodon. In this form, PL V,t fig. 4, the same primitive 

 arrangement of three cusps and two fangs is preserved in the outer 

 portion of the tooth, while on the internal side a large secondary 

 cusp has been developed differing widely in character from that of 

 Dryolcstes. This cusp is a laterally compressed cone supported by two 

 rudimentary fangs and is joined to the outer portion of the tooth by 

 a high, wedge-shaped ridge. The base of the inner cone is greatly 

 expanded antero-posteriorly, curving gently outward toward the 

 external portion of the tooth. Thus the crown, as a whole, is greatly 

 constricted medially with the inner and outer portions superficially 

 resembling each other. 



" From these observations two important conclusions may be drawn : 

 First, that, leaving out of consideration the multituberculates, there 

 are among the mammals of the Atlantosaurus beds at least three 

 distinct forms of upper molars representing three primitive types of 

 about equal specialization apparently leading off' in entirely independent 

 lines. Probably only one of these, Dryolcstes, represents an ancestral 

 type from which the Upper Cretaceous and later forms possessing 

 trigonodont molars may have been derived. Second, that the evidence 



*" Evidence bearing on Tooth-Cusp Development," Proc. Washington Acad. Sci., 

 Vol. VIII., 190(i, p. 96. 



t[Fig. 207.] 



