Cope.] 4UU [April 15, 



The geographical distribution of these genera, so far as present knowl- 

 edge shows, is as follows : 

 North America alone — MesoMppus, Anchtppus. 



North America and Europe — Anchitherium, Hippotherium, ProtoMppus. 

 Europe only — AnoMlophus, Paloplotherium, PalmotTierium. 



EQUID^. 



The two genera of this family are distinguished as follows : 



Internal lobes of superior molars subequal Hippidium. 



Anterior internal lobes of superior molars much larger than the pos- 

 terior Equus. 



The genus Hippidium is extinct, and its species have been thus far found 

 only in North and South America, in beds of Pliocene age. Equus made 

 its appearance during the same period, and is represented by several exist- 

 ing species. 



Besides the reduction in the number of digits, which is carried farther 

 here than in any other family of Mammalia, there are several other char- 

 acteristics of specialization. Thus in the dentition, the spaces between the 

 tubercles are filled with cementum. These valleys are generally deep, owing 

 to the prismatic forms of the molars. The cups of the incisors are com- 

 pletely developed, and also filled with cementum. There are two bicipital 

 grooves of the humerus. The preceding characters are also found in the 

 MippotheriincB of the Palceotheriidce. 



The EquidcB adds another evidence of greater specialization than the 

 latter group in the structure of its feet, i. e., the distal metapodial keela are 

 completed forwards, as in most ruminants. 



The similarity of the modifications which have supervened on the Artio- 

 dactyle and Perissodactyle lines in attaining their most specialized extreme 

 has often been noticed. I repeat them here in tabular form in three 

 columns. These show (Table I) the modifications in which the EquidoB 

 and Bovidm are identical or nearly so, which place them at the heads of 

 their respective orders ; Table II, those in which the Equidce are the 

 more specialized of the two ; and Table III, those in which the BovidcB dis- 

 plays the highest differentiation. 



