714 THE EOGENE FAUNA. 



advance over the Chalicotheroid and other genera of the Lower Eocene. 

 In fact, it has not been found in the Lower Eocene, but commences in 

 the Upper Eocene in the genera Palceotherium and Paloplotherium. Thence 

 it extends to the very summit of the Miocene, and may even occur in the 

 PHocene {Hippotherium i^^'o^ohippus). Its members exhibit considerable 

 range of variation in the details of the teeth and feet, but no striking break 

 of family imjiortance occure. The most noteworthy interruption is that 

 which is found between the Palocotherince and Hippotheriince, where there is 

 a change in the form of the proximal extremity of the humerus from a 

 tapiroid to a horse-like form, and a modification of similar significance in 

 the molar teeth, by the addition of a deposit of cementum. 

 The characters of the genera are as follows: 



I. Palwotheriince. Bicipital groove of bum ei us simple; teeth Tvitliout cementum. 

 «. Oue or more internal tubercles of sHi)erior molars distinct. 



External Ys of superior molars not well distinguished externally Anchilophus. 



External Ys separated by a vertical rib; intermediate tubercles not connecting fore 



and aft Paloplotherium. 



External Ys separated; intermediate tubercles extended fore and aft Anchipims. 



««. Internal tubercles of superior true molars continuous with the transverse ridges. 



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



Inferior molars with cusps at the inner extremities of the Ys ; lateral toes small. 



Anchitherium. 



II. Hippotlieriinw. Bicipital groove 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 Mippotherium. 



aa. Internal tubercles of molars not distinct. 

 Inner lobes of inferior molars enlarged Protohippus. 



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

 the Chalicotheriidce, and they range from the Middle Eocene to the Pliocene. 

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

 anticipated, more nearly allied to the Chalicotheriidce than any other genus 

 of this family. Pachynolophus also is not far removed from it. Anchilophus 

 is Upper Eocene, and is allied to the genus just named, which connects 

 both genera with the Lophiodontidoc. These early genera constitute by their 

 similarity the bond of connection between the three families, which in their 



