PERISSODACTYLA. 715 



later and specialized forms are very diflferent from each otlier. Palceothe- 

 rium is chiefly found in the Upper Eocene, and Anchitherium commences 

 in America in the White River or Oligocene, an age between Eocene and 

 Miocene. In Europe it commences in the Middle Miocene, and has An- 

 chippus for a cotemporary. Hippotlierium existed only in the latter part of 

 the Miocene epoch, consistent!}' with the greatly specialized structure of its 

 limbs and teeth, and the nearly allied Protohippus lived with it, while in Europe 

 a species with the same type of molar teeth is found in the Pliocene epoch 

 (Forsyth-Major). These forms were cotemporary with the Equidoe, which 

 outlived them. They have many points of resemblance to that family, but 

 nevertheless remain at a considerable interval from them in the structure of 

 the feet. 



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

 edge shows, is as follows : 



North America alone — Auchippus. 



North America and Europe — AncJiifheritim , Hippotherium, Protohippus. 



Elurope only — Anchilophus, Paloplotlierium^ Pakeotherium. 



EQUID^. 

 The two genera of this family are distinguished as follows : 



Interual lobes of superior molars subequal Hippidiutn. 



Anterior iuternal lobe of suiierior molars mucli larger than the posterior Equus. 



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

 found 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 further 

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

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

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

 owing to the prismatic foi'ms of the molars. The cups of the incisors are 

 completely developed, and also filled with cementum. There are two bicip- 

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

 HlppjotheriiiKB of the Pakeotherikke. The Eqnidce adds another evidence of 



