24:8 THE STRUCTURAL EVIDENCE OF EVOLUTIOIT. 



resent this type ; the last molar of Lopliiodon appears to he Amm- 

 hodont. 



Trichecliodonts. — Tubercles confluent into two or more trans- 

 verse crests. 



The Dinotlierium represents this form, so does Elephas, Tri- 

 checus (the manati), and a number of the larger Marsupialia. 



2. Amcebodonts. 



PalcBotheriodonts. — External tubercles longitudinal, subcres- 

 centic in section ; the inner united with them by transverse oblique 

 crests. 



Palmotherium and AncMtherium present this type, which only 

 differs from the Tapirodont in the alternation of the opposing 

 tubercles. 



Symborodonts. — External tubercles longitudinally compressed 

 and subcrescentic in section ; the inner indejoendeut and unal- 

 tered, i. e., conic. 



To this group are to be referred the types of Palwosyops, Me- 

 nodus, and Symhorodon. They, of all Lopliodonts, approach near- 

 est to the Bunodonts. 



Batlimodonts. — The posterior pair of tubercles approximated 

 and connected, together compressed and subcrescentoid in section ; 

 the anterior outer connected Avith the anterior inner by an oblique 

 crest forming a V with the preceding. 



Batlimodon and Metalopliodon. — The homologies of the crests 

 are difficult to make out ; the subcrescentic crest of the second 

 molar may include only the posterior outer tubercle, and thus be 

 entirely homologous with the posterior crescent of Palseosyops. In 

 support of this view we have the structure of the premolars, where 

 it becomes the only external crescent, while the anterior transverse 

 crest turns round on its inner side, supporting the inner anterior 

 tubercle of the tooth. [Note to this edition : The latter view is 

 the correct one, as the posterior inner tubercle is wanting.] 



Loxolopliodonts — Anterior inner tubercle connected with the 

 two external by oblique crests ; the posterior inner tubercle rudi- 

 mental or wanting. 



UintatJierium and probably Tillotherium represent this group, 

 both being like those of the last, extinct genera from the Eocene 

 of Wyoming. 



