EQU1D.E. 397 



of enamel when the crown is worn. The posterior valley (g) is 

 usually deeper and more extended. The ordinary lobes (a, b, 

 e, d) are very similar, and produce, by the confluence of a 

 with c, and of b with d, the two oblique tracts of dentine 

 which are more decidedly established as transverse ridges in 

 the Lophiodont or Tapiroid group. A basal ridge (V) girts 

 more or less completely the inner and the fore and hind 

 parts of the base of the crown. Not fewer than twenty 

 species of extinct rhinoceroses are entered in Paheontological 

 catalogues. 



Equidcc. — Eemains of quadrupeds with the limbs and den- 

 tition of the horse-kind have first been seen in strata of mid- 

 tertiary or miocene age. Such deposits at Eppelsheim in 

 Germany, in the department of Yaucluse, France, and in the 

 Sewalik hills of India, have yielded upper molar teeth differ- 

 ing from those of modern Fquidce chiefly in the distinctness 

 or greater extent of separation of the interlobal 

 or inner column (fig. 149, m) ; and have revealed 

 the highly interesting structure shewn by the 

 retention of the small digits and hoofs (fig. 150, 

 ii. and iv.), appertaining to the rudiments of 

 their metapodials, called splint-bones by vete- 

 rinarians, which alone are retained in modern 

 horses, zebras and asses* The small hoofs in L PP er molar. 



Hipparion. 



question, ib. ii. and iv., dangled by the side of 

 the large and functional hoof iii., like the pair of spurious 

 hoofs behind those forming the cloven foot in the ox (fig. 161). 

 They would cause the foot of the Hipparion to sink less deep 

 into swampy soil, and be more easily withdrawn, than the 

 more simplified horse's foot. From another point of view, as 

 the small digits ii. and iv. answer to the outer and inner toe of 

 the foot of the Palaeothere, the miocene Hipparion, on the 



* The tridactyle horse was called Hipparion (Gr. for "little horse ") by M. 

 Christol (1832), and HippotJierivm, by Dr. Kaup (1835). 



