TEETH OF UNGULATA. 343 



lobes a, b. Whether the accessory lobule m, be a homologue 

 of the end, so marked, of lobe c in Palceothcrium, Paloplotherium, 

 and Equus, or a special development at the entry of valley e may 

 be doubtful. 



In the Hippopotamus, fig. 272, the valley commencing at the 

 inner side of the crown at e, extends straight across the tooth to 

 n, bisecting the crown trans- 

 versely: it is also bisected, 272 



antero-posteriorly, by a shal- /a\^/^^^\ s^ \f 



lower valley, answering to 

 h, i 3 fig. 271. At the stage 

 of attrition shown in fig. 

 272, the remnant of the 

 latter valley is seen at h and 

 i : the deeper transverse 

 valley, e, n, remains : the 

 shorter indents,/,/, g, k, give 

 the trefoil character to the 

 two chief divisions of the crown characteristic of Hippopotamus. 



Another exposition of the homologous parts of the complex 

 crowns of the Ungulate molars assumes the crucial division into 

 four quarters or lobes to be the primitive modification. The fore- 

 and-aft cleft has already begun to be filled by the mid-lobules in 

 Pliolophus : the arrest of the outer end of the transverse cleft 

 produces the continuity of a with b : that of its mid-part, of d 

 with e : the obliteration of both ends of the antero-posterior cleft 

 insulates that cleft, as in the Ruminant. The obliteration of the 

 middle of the transverse cleft produces the continuation of a, b, 

 with d, c ; while the oblique continuation of e with i, and the 

 retention of the continuity of g with h, leads to the type of 

 Palceotherium and Rhinoceros. 



A sub-type of grinding surface is produced by the existence of 

 a transverse without an antero-posterior valley, dividing the 

 crown into a pair of transverse ridges ; as in the Tapir ; which, 

 however, is mainly the greater development, and more transverse 

 disposition, of the tracts b, d, and a, c, in Palaotherium, fig. 268. 

 The ( bilophodont ' sub-type becomes more marked in Dinotherium, 

 fig. 288, and in the anterior small molar of Mastodon : the suc- 

 cessive multiplication of the transverse ridges completes the 

 transition into the molar character of Elephas. 



B. Artiodactyla. The extinct Cheer op otamus, Anthracotherium, 

 Hyopotamus and Hippohgus, had the typical dental formula, and 

 this is preserved in the existing representative of the same section 



