342 



ANATOMY OF VERTEBRATES. 



so shallow midway that, at an early stage of attrition, the entry 

 of the posterior valley, g, is separated from its termination, h ; 

 and that of the internal valley, e, from its termination i ; the blind 

 ends of both valleys, moreover, are more extended and irregular, 

 than in Palceotherium, with the tendency to curve, so as to produce 

 the crescentic form of the islands, i, h, in fig. 270. The oblitera- 

 tion of the mid-part of the accessory valley, k, unites the dentinal 

 tract, m, to the rest of the lobe, c, as in Palceotherium, fig. 268: 

 but it long remains separate in Hipparion, as in Paloplotherium , 

 fig. 269. 



The Rhinoceros and Hyrax more closely adhere to the Palseo- 

 therium type : but the outer indents, f, f, are less marked. The 



270 



Upper molar (jn 2), Equus caballus. 



Upper molar (wi 2). Mogaccros. 



horse approaches nearest to the symmetrical type of the Rumi- 

 nants, in which the homologous parts of the crown can, mostly, 

 be well defined. 



In the unworn crown of the Ruminant molar, fig. 271, the 

 valley, g, h, extends across the crown more parallel with the 

 long axis of the jaw, than in fig. 268, curving with the concavity 

 outward : it communicates with the valley, i ; and, as in Paloplo- 

 therium, this is continued to the foreside of the crown, as at /, 

 fig. 269, severing the lobe c from a. In Ruminants, both the 

 anterior and posterior entries to this antero-posterior double- 

 curved cleft are so shallow that they are soon obliterated, and the 

 lobe h is continued by a tract of dentine, with d, along the hind 

 part of the crown : as the lobe a is continued into lobe c at the 

 fore part, as seen in the worn molar of the deer, fig. 271 : the 

 middle of valley, e, is separated from the end i, as in the horse : 

 but the course of this valley is more transverse, and more di- 

 rectly bisects the antero-posterior valley, h, i: thus the inner 

 lobes c and d are more parallel with and similar to the outer 



