84 ON THE HOMOLOGIES AND ORIGIN OF THE 



molars of the latter is continued to the posterior end of the series in Uintatherium 

 (Fig. 29); that is, the last molar of the latter consists of a V and an oblique cross- 

 crest. 



IV. Relations of the types of Dentition to types of Foot Structure. 



I hope that I have succeeded in showing that the Bunodont and Lophodont 

 types of dentition form two homologous series, similar to those already indicated 

 among Batrachia, Cephalopoda, etc.* That this relation indicates descent of the 

 corresponding terms of the one series from those of the other has also been ren- 

 dered highly probable. This conclusion has also been previously stated as a theo- 

 rem,t as follows: "V* The heterologous terms or genera in the later series are 

 modified descendants of those of the earlier series ;" in other words, that certain 

 groups higher than genera are produced from others of a similar high value by 

 "descent with modification." 



As already pointed out, the Bunodont primary genera belong to the older geo- 

 logic epoch of the Eocene, while most of the derivative ones belong to later periods. 

 Some were contemporary with the primary forms, but doubtless have descended 

 from pre-existent members of the same type as yet unknown to us. The genus 

 Achce^iodon, Cope, is especially generalized in three respects: (1) the simplicity of 

 the construction of its molars; (2) the same simplicity of the premolars, which 

 are without inner or posterior lobes ; (3) the absence of all diastemata and conse- 

 quent continuity of the dentalseries. Hence it may be regarded as more primi- 

 tive than Palwochoerus, Choeromorus, Dicotyles or Elotherium, in all which there are 

 marked diastemata. The two series may then be arranged as follows : with the 

 understanding that in some cases names of genera used, represent rather family 

 groups, in which the special generic lines have not yet been made out. 



* See Origin of Genera, p. 53.. • f Loc. cit. p. 79. 



