ORIGIN AND DEVIlLOPMENT OF THE EXISTING HORSES. 



71 



Fig. 15. 



Rightsuperior molar of a species of Hip- 



becomes confluent with the posterior ridge /^^. The lobe I which was conic in 



ancitherium is elongated in a transverse direction 

 to the crown so as to close in the posterior valley 

 and Join the posterior external cusp pe with the 

 posterior crest pec. Additional vertical pillars are 

 -acc developed on the cross-ridges. In the inferior 

 molars (Fig. 16) the lobe ai' is now completely 

 separated and the ridge k rises to a level with the 

 other cusps. The heel h is also elevated and 

 connected by a strong ridge. The filling up of 

 the valleys by a deposit of cementum and the 

 /o/A^rtwwz with cementum removed. For conscqucnt attrition in mastication produce a 



worn condition of same see Fig. 17, nat. ^^^^^t^ changc in appcarancc from that seen in 



Ancitherium but by close observation the strictest homology is seen to exist. 



Figs. 17 and i8 represent an under and upper view of the cranium of Hip- 

 potherium speciosum, Leidy, from Nebraska. 



But one other genus remains between Hippoihe. 

 rium and Equus. This is the genus Frotohippus, 

 Leidy. The only character in which it differs from 

 Hippotheriunt is found in the superior molars. Here 

 the anterior internal lobe ai is confluent with the an- 

 terior cross crest acc as in the present horse. This 

 lobe in Hippotherium remains distinct. The feet of 

 the two genera do not differ. Figs. 19 and 20 show (after Gaudry) 

 two views of a skull of this animal. Fig. 21 represents a right posterior foot of 

 the same individual. 



K 



ae 



Fig. 16. 



Left inf. molar c f Hippo:heriutn 

 gracile, three-fourths natural size, 



Fig. 17. 



Fig. 17. — Cranium of Hippotherium sptcietum, under view, (after Cope). 



We come next to the Equidcs containing one extinct genus, Hippidium, Owen, 

 and one living, Equus or horse. The outer toes are still further abbreviated and 

 the metapodals terminate in imperfect distal extremities constituting the so- 

 called ''splint bones" (Fig. 22). The fibula is not distinguishable at its distal 



