ORDINAL TVI'KS OF MOLARS: I'KKISSODAf 'TV I.A 



175 



modelling of the cusps. In the Titanotheres, fur example (Figs. 1G7- 

 171), the protocone and hvpocone remain Imnoid, the paracone and 

 inetacone have become selenoid. and the small ridges tunned by th<- 



D 



E 



FIG. 161. Evolution of the upper molars in the Equida;. E. after Kowalevsky. The 

 series figured is not a phylogenetic one, since animals belonging to several different lines of 

 descent are represented ; but it is a morphological series and (with Figs. 1(32-1(54) shows thu 

 principal successive stages of molar evolution in the F.quidse. All figures natural size. 



A. Hiiracothenum, Lower Eocene. 



R. Pachynolophus, Middle Eocene. Note ps, ms, and crescentie />c, me, 



C. Aitchilophus, Lower Oligocene. Note mts and hypostyle Its. 



D. Hfi.>hi{itit'x, Middle Oligocene. 



E. Anchitherium, Lower Miocene. 



pro to- and metacomiles have become vestigial, the crown thus consisting 

 of two outer crescents and two inner cones ; but even in this specialized 

 crown traces of the primitive triangular arrangement of three primary 

 cusps remain. In the little La mix kit her inm (Fig. 167) we have a 



postfosaette = c- 



crochet 



antecrochet 



Fiu. 102. Upper milk molar of M ,-i/r/,i / i l ni.< sp. from the Upper Miocene, showing the 

 completed ground plan of the molar pattern of the modern Horse. (Cf. Figs. liiO, lid.) x | 



perissodactyl (probably an ancestral Titauo there), in which the trituber- 

 cular derivation of the molars is indisputable. In these teeth among 

 the horses, the styles and intermediate conules d>1, ml) play an important 

 role (p. 85). The key to the evolution of the teeth of the horses as 

 compared with that of the Titanotheres is given in Figs. 135, 159, 

 160. The type attained is lopho-selenodont. 



