TEETH AND THEIR VARIATIONS. 213 



a tooth present the very characteristic pattern shown 

 in the figure, which at once enables us to say to which 

 group its owner belonged. This peculiar type of tooth 

 may be readily derived from the four-columned tooth 

 of the primitive Pigs (although it is not for one 

 moment to be supposed that Rhinoceroses, &c., are 

 descended from Pigs) by the union of the two outer 

 cones to form the outer ridge, and also by the junction 

 of each of the outer cones with the corresponding 

 inner cone to make the transverse crests. The crown 

 of the Rhinoceros's tooth, as seen in the figure, is com- 

 paratively low, and the valleys between the ridges and 

 crests are shallow, and not filled with cement. The 

 teeth of Tapirs are of the same general type, and so 

 also are those of all the extinct Tertiary Ungulates of 

 this group, from some of which, as Professors Huxley 

 and Marsh have so clearly shown, the modern horse 

 appears to have been evolved. 



the cheek-teeth of Horses (Fig. 72) have, however, 

 been so strangely modified from this generalised type 

 by the great increase in the height of their crowns, and 

 the complexity of their grinding-surfaces, produced by 

 foldings of the enamel of the longitudinal and trans- 

 verse ridges, that it is at first sight somewhat difficult 

 to see how the different portions of such a tooth corre- 

 spond with those of the tooth of a Rhinoceros. By the 

 aid, however, of Fig. 72, which represents a cheek-tooth 

 of an extinct horse-like animal, we may endeavour to 

 trace this correspondence, although the difficulty of 

 this is unfortunately somewhat increased by Figs. 72 



