680 THE POPULAR SCIENCE MONTHLY. 



ton strongly resembled the living tapir, whose ancestry, to this point, 

 seems to coincide with that of the rhinoceros we are considering. 

 Strangely enough, the rhinoceros line, before it becomes distinct, 

 separates into two branches. In the upper part of the Dinoceras 

 beds we have the genus Colonoceras, which is really a Hyrachyus 

 with a transverse pair of very rudimentary horn-cores on the nasal 

 bones. In the lower Miocene west of the Rocky Mountains this line 

 seems to pass on through the genus Dicer atherhim, and in the higher- 

 Miocene this genus is well represented. Some of the species nearly 

 equaled in size the existing rhinoceros, which Dieeratherium strongly 

 resembled. The main difference between them is a most interesting 

 one. The rudimentary horn-cores on the nasals, seen in Colonoceras, 

 are in Dieeratherium developed into strong bony supports for horns, 

 which were placed transversely, as in the ruminants, and not on the 

 median line, as in all existing forms of rhinoceros. In the Pliocene 

 of the Pacific coast, a large rhinoceros has been discovered, which 

 may be a descendant of Dieeratherium ; but, as the nasal bones have 

 not been found, we must wait for further evidence on this point. Re- 

 turning now to the other branch of the rhinoceros group, which left 

 their remains mainly east of the Rocky Mountains, we find that all 

 the known forms are hornless. The upper Eocene genus, Amynodon, 

 is the oldest known rhinoceros, and by far the most generalized of the 

 family. The premolars are all unlike the molars ; the four canines are 

 of large size, but the inner incisor in each jaw is lost in the fully adult 

 animal. The nasals were without horns. There were four toes in 

 front, and three behind. The genus Hyracodon, of the Miocene, which 

 is essentially a rhinoceros, has a full set of incisor and canine teelh ; 

 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, and 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, 

 doubtless, 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 mam- 

 mals; 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 



