SUHLE 307 



slight convexity projects, in some species of Bhinoceros, giving 

 a gently undulated surface to that side of the tooth. The valley 

 (e) is more expanded at its termination (i), in the Bhinoceros ; 

 and, in some species, it bifurcates and deepens, so that one 

 branch may form an insulated circle of enamel when the crown 

 is worn. The posterior valley (g) is usually deeper and more 

 extended. The ordinary lobes (a, b, c, 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 esta- 

 blished as transverse ridges in the Lophiodont or Tapiroid 

 group. A basal ridge (/■) 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 Palaeontological catalogues. 



The extinct Cheer •opotamus, Anthracotherium, Hyopotamus, 

 and Hippohyus, had the typical dental formula, and this is 

 preserved in the existing representative of the same section of 

 non-ruminant Artiodactyles, the hog. The first true molar 

 when the permanent dentition is completed, exhibits the 

 effects of its early development in a more marked degree than 

 in most other Mammalia, and in the Wild Boar has its 

 tubercles worn down and a smooth field of dentine exposed 

 by the time the last molar has come into place ; it originally 

 bears four primary cones, with smaller sub-divisions formed by 

 the wrinkled enamel, and an interior and posterior ridge. The 

 four cones produced by the crucial impression, of which the 

 transverse part is the deepest, are repeated on the second true 

 molar with more complex shallow divisions, and a larger tuber- 

 culate posterior ridge. The greater extent of the last molar is 

 chiefly produced by the development of the back ridge into 

 a cluster of tubercles ; the four primary cones being distin- 

 guishable on the anterior main body of the tooth. The crowns 

 of the lower molars are very similar to those above, but are 



