KINETOGENESIS. 359 



a direction the opposite of that which characterizes the 

 order Diplarthra (=Perissodactyla and Artiodactyla). 

 But the Litopterna present a most remarkable paral- 

 lelism to the Perissodactyla in the characters of both 

 the feet and the dentition. No genus is known as yet 

 which possesses more than three toes before and be- 

 hind, and these are of equal length in Macrauchenia 

 Owen. In this genus the teeth are not primitive but 

 are much modified. The most primitive dentition is 

 seen in the genus Proterotherium (Ameghino) where 

 the superior molars are tritubercular as in many Con- 

 dylarthra. In this genus (Fig. 112, A), there are three 

 toes, but the lateral ones are reduced, about as in the 

 Equine genus Anchitherium (p. 148). In the next 

 genus, Diadiaphorus Ameghino, the superior molars 

 are quadritubercular and crested, while the lateral toes 

 are reduced still more, being quite rudimental (Fig. 

 112, B, C), as in the equine genera Hippotherium and 

 Prothippus (p. 149; Fig. 70). The superior molars 

 have not progressed so far as in these genera, but are 

 not very different from those of Anchitherium. In the 

 third and last type (Thoatherium Ameghino), the lat- 

 eral digits have disappeared from both fore and hind 

 feet (Fig. 112, C,D), so that the condition is that of 

 the genus Equus (Fig. 81), but the splints in the Thoa- 

 therium crepidatum Ameghino are even more reduced 

 than in the known species of horse. The superior 

 molars have not assumed the pattern of the genus 

 Equus, but resemble rather those of Macrauchenia, 

 and could have been easily derived from those of Dia- 

 diaphorus. 



Here we have a serial reduction of the lateral digits 

 and their connections with the leg, and increase in the 

 proportions of the middle digit and corresponding in- 



