1074 THE UNGULATES, OR HOOFED MAMMALS 



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 horn 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-specialized grass-eating creature, presenting a relationship to 

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

 extinct Ungulates. 



THE HORSE TRIBE 

 Family 



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 

 characterized 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 com- 

 ponent 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 fold- 

 ings 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 realize their 

 fundamental unity of structure. A comparison of the three figures in the accom- 

 panying 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-crowned 

 molar of the extinct anchithere, while that of the latter does not differ very widely 

 from the molars of the rhinoceros represented on p. 1051. Remembering that the 

 figured molar of the anchithere belongs to the opposite side of the jaws to those of 

 the horses, it will be apparent that it would only require a heightening of its 

 columns and ridges; accompanied by the formation of a series of foldings in their 

 investing enamel, and the filling up of the deepened intervening valleys with 

 cement, to produce a very similar type of tooth. It is almost superfluous to add 

 that the tall-crowned molars of the horse, with their completely-filled valleys and 

 their alternating ridges of harder and softer constituents, are far more efficient in- 

 struments of mastication than the low-crowned teeth of the anchithere, with their 

 perfectly-open valleys. Indeed, while the horse's are adapted for a grinding action, 

 and have nearly flat surfaces, the anchi there's molars are suited to a champing 

 motion, and have ridged surfaces. 



Another peculiarity in the dentition of the horses is that the incisor 



or front teeth in both jaws have an infolding of the enamel at the 



summit of their crowns, as shown in the figures A, B, C, on p. 1078. This peculiar 



