486 UNGULATES. 



There are, however, a number of allied extinct animals which connect the true 

 rhinoceroses with more generalised extinct types of Odd-toed Ungulates. Such for 

 instance is the Amynodon, from the Miocene Tertiary of North America, which 

 was a rhinoceros-like animal with no hom, and the full typical number of forty-four 

 teeth. That is to say, there were three incisors, a tusk, and seven cheek-teetli on 

 each side of both jaws ; the front teeth being like those of ordinary mammals, and 

 not having the peculiarly modified form presented by those of the true rhinoceroses. 

 Moreover, the whole of the three upper molar teeth were alike ; and none of them 

 had the processes projecting into the middle valley which are found in those of all 

 true rhinoceroses. Probably the Amynodon also occurred in the lower Miocene 

 and upper Eocene rocks of France. There were other allied types, but the above 

 example is sufficient to show that the earlier rhinoceroses were far less different 

 from tapirs and some extinct generalised forms to be noticed later on than are 

 their modern representatives. 



We must not, however, take leave of the rhinoceros family without referring 

 to a most remarkable creature known as the elasmothere, which flourished during 

 the Pleistocene period in Siberia. This creature was probably as large as Burchell's 

 rhinoceros, and like that species had no teeth in the front of the jaws. The skull 

 had a bony partition in the cavity of the nose, and carried on the forehead an 

 enormous protuberance which, during life, doubtless supported a hom of very large 

 size. The most remarkable feature about the elasmothere is, however, to be found 

 in the structure of its cheek-teeth, which while formed on the type of those of the 

 rhinoceroses, are greatly elongated, and have their enamel so much folded as to 

 present some resemblance to those of the horse. Indeed, the elasmothere may be 

 regarded as a highly-specialised grass-eating creature, presenting a relationship to 

 an ordinary rhinoceros somewhat similar to that which the horse exhibits to 

 certain extinct Ungulates noticed in the sequel. 



The Horse Tribe. 



Family EQUID^E. 



Under the general title of horses, zoologists include not only the animals to 

 which that name is restricted in ordinary language, but likewise the asses, zebras, 

 and quaggas, together with certain nearly-allied extinct animals. All these are 

 characterised by having very high-crowned cheek-teeth, in which the enamel is 

 thrown into a series of complicated foldings, and the deep valleys between the 

 component columns completely filled up with cement. In the upper cheek-teeth, 

 as shown in B and C of the accompanying figure, the outer columns (pa, me) of 

 each tooth are flattened, and the premolars somewhat exceed the molars in size ; 

 while in the lower jaw the ridges are crescent-like, although much complicated by 

 the foldings of the enamel. So different, indeed, are the molars of the horses from 

 those of other Odd-toed Ungulates, that it is at first sight somewhat difficult to 

 realise their fundamental unity of structure. A comparison of the three figures in 

 the accompanying illustration will, however, clearly indicate how the structure of 

 the tall-crowned molar of the horse is essentially the same as that of the low- 



