34 



fully adult animal. The nasals were without horns. There 

 were four toes in front, and three behind. The genus Hyrac- 

 odon, of the Miocene, which is essentially a Rhinoceros, has 

 a full set of incisor and canine teeth; and the molars are so 

 nearly like those of its predecessor Hyrachyus, that no one will 

 question the transformation of the older into the newer type. 

 Hyracodon, however, appears to be off the true line, for it has 

 but three toes in front. In the higher Miocene beds, arid 

 possibly with Hyracodon, occurs a larger Rhinoceros, which 

 has been referred to the genus Aceratherium. This form has 

 lost the canine and one incisor above, and two incisors below. 

 In the Pliocene are several species closely related, and of large 

 size. Above the Pliocene in America, no vestiges of the 

 Rhinoceros have been found, and our American forms doubt- 

 less became extinct at the close of this period. 



The Tapir is clearly an old American type, and we have 

 seen that, in the Eocene, the genera Helaletes and Hyrachyus 

 were so strongly tapiroid in their principal characters, that the 

 main line of descent probably passed through them. It is 

 remarkable that the Miocene of the West, so greatly developed as 

 it is on both sides of the Rocky Mountains, should have-yielded 

 but a few fragments of tapiroid mammals, and the same is true 

 of the Pliocene of that region. In the Miocene of the Atlantic 

 Coast, too, only a few imperfect specimens have been found. 

 These forms all apparently belong to the genus Tapiravus, 

 although most of them have been referred to Lophiodon, a lower 

 Eocene type. In the Post-Tertiary, a true Tapirus was abun- 

 dant, and its remains have been found in various parts of North 

 America. The line of descent, although indistinct through 

 the middle and upper Tertiary, was doubtless continuous in 

 America, and several species exist at present, from Mexico 

 southward. It is worthy of notice that the species North of 

 the Isthmus of Panama appear all to be generically distinct 

 from those of South America. 



In addition to these three Perissodactyle types which, as 

 the fittest, have alone survived, and whose lineage I have 



