TEETH OF UNGULATA. 



341 



deposits antecedent in time to those characterised by Palao- 

 or Anoplo-therium : they are afforded by Pliolophus, 1 and 

 Coryphodon. 2 The answerable parts of the grinding surface 

 will first be illustrated in the unsymmetrical series. In 

 Palceotherium, e. g. fig. 268, the tract of dentine, a, b, extending 

 along the outer side of the crown, has two indents, f, f, whereby 

 it is divided into two lobes, an anterior or ' ant-external lobe,' a, 

 and a posterior or e post-external lobe,' b. The tract of dentine 

 along the inner side of the crown is also divided by two deeper 

 and more oblique clefts or valleys into an ' ant-internal lobe,' c, m, 

 and a * post-internal lobe,' d : these lobes extend obliquely inward 

 and backward from the outer ones, of which they are direct con- 



268 



269 



Upper molar (m 2) : Palceotherium magnum. 



Upper molar (m 2) : Paloplotherium. 



tinuations. The anterior of the two inner clefts, e, i, extends 

 from the middle of the inner surface of the crown obliquely out- 

 ward and forward : the posterior one, g, h, enters at the posterior 

 side of the crown, and extends nearly parallel with e, i : both 

 valleys expand and deepen at their blind ends. At an early 

 period of the attrition of the crown they intercommunicate, and 

 extend to the anterior side of the crown, at /, as in the younger 

 molar of Paloplotherium, fig. 269. But the shallow communica- 

 ting passages between h and i, i and /, are soon obliterated, the 

 dentine of lobe d becoming continuous with b ; and that marked 

 e with a. In Paloplotherium a branch valley, also, extends 

 from e i, to the anterior side of the crown, k, cutting off the 

 part of the ant-internal lobe m from the rest of c ; but, by con- 

 tinued abrasion, this valley is also obliterated, and the tooth 

 assumes more of the palaeotherian pattern. In Equus, fig. 270, 

 the valleys are of less equal depth than in Palaotherium, and are 



1 cxv". p. 54. 



2 cxvi". p. 299. 



