1881.] '^yy [Cope. 



The characters of the genera are as follows : 



I. Palmotheriinm. Bicipital groove of humerus simple ; teeth without 

 cementum. 



a. One or more internal tubercles of superior molars distinct. 



External Vs of superior molars not well distinguished externally 



AncMlopJius. 

 External Vs separated by a vertical rib ; intermediate tubercles not con- 

 necting fore and aft , Paloplotlierium. 



External Vs separated ; intermediate tubercles extended fore and aft 



Anchippus. 

 aa. Internal tubercles of superior true molars continuous with the 

 transverse ridges. 



Inferior molars with two Vs only; lateral toes large Palceotherium. 



Inferior molars with distinct internal tubercles ; lateral toes small ; a short 



fifth metacarpal MesoMppus. 



Inferior molars with cusps at the inner extremities of the Vs ; lateral toes 

 small ; no fifth metacarpal AncMilierium, 



II. Hippotheriinm. Bicipital grooove of humerus double ; molars with 

 cement in the valleys. (Intermediate tubercles connected fore and 

 aft. ) 



a. One or more internal tubercles of superior molars distinct. 

 Inner lobes of inferior molars enlarged Eippotherium. 



aa. Internal tubercles of molars not distinct. 

 Inner lobes of inferior molars enlarged ProtoMppus. 



The genera of this family are generally of less antiquity than those of 

 the GhalicotJieriidce, and they range from the Middle Eocene to the Plio- 

 cene. Paloplotherium is found in the Middle Eocene, and is, as might have 

 been anticipated, more nearly allied to the Ghalicotlieriidce. than any other 

 genus of this family. PropalmotTierium is not far removed from it. An- 

 cMlophus is upper Eocene, and is allied to the genus just named, and also 

 to Pachynoloplius among the LopModontidce. These early genera consti- 

 tute by their similarity, the bond of connection between the three families 

 which in their later and specialized forms are very diflFerent from each 

 other. PalcBotherium is chiefly found in the Upper Eocene, and MesoMp- 

 pus is only known from the White river or Oligocene, an age between 

 Eocene and Miocene. AncMtherium commences in the Middle Miocene 

 and has Anchippus for a cotemporary. Hippotherium existed only in the 

 latter part of the Miocene Epoch, consistently with the greatly specialized 

 structure of its limbs and teeth, and the nearly allied ProtoMppus 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 cotem- 

 porary with the Bquidw, which outlived them. They have many points of 

 resemblance to that family, but nevertheless remain at a considerable inter- 

 val from them in the structure of the feet. 



