136 DISTRIBUTION OF EXTINCT ANIMALS. [part ii. 



Utah and Wyoming, we get a step further back, several species 

 having been discovered about the size of a fox with four toes in 

 front and three behind. These form the genus Orohippus, and 

 are the oldest ancestral horse known. Prof. Marsh points out the 

 remarkably perfect series of forms in America, which, beginning 

 with this minute ancient type, is gradually modified by gaining 

 increased size, increased speed by concentration of the limb-bones, 

 elongation of the head and neck, the canine teeth decreased in 

 size, the molars becoming longer and being coated with cement — 

 till we at last come to animals hardly distinguishable, specifically, 

 from the living horse. 



Allied to these, are a series of forms showing a transition to the 

 tapirs, and to the Palccothcrium of the European Eocene. In the 

 Pliocene we have Parahippus ; in the Miocene Lophiodon, found 

 in the same formation and in the Eocene of Europe, and allied 

 to the tapir ; and in the Eocene, Palccosyops, as large as a rhino- 

 ceros, which had large canines and was allied to the tapir and 

 Palceotherium ; Limnohyus, forming the type of a family Limno- 

 hyidse, which included the last genus and some others mentioned 

 further on ; and Hyrachyus, allied to Lophiodon, and to Hyracodon 

 an extinct form of rhinoceros. Besides these we have Zophiothe- 

 rium (also from the Eocene of Europe) ; Diplacodon allied to 

 Lim7iohyus,h\it with affinities to modern Perissodactyla and nearly 

 as large as a rhinoceros ; and Colonoceras, also belonging to the 

 Limnokyidse, an animal which was the size of a sheep, and had 

 divergent protuberances or horns on its nose. A remarkable 

 genus, Bathmodon, lately described by Professor Cope, and of 

 which five species have been found in the Eocene of New Mexico 

 and Wyoming, is believed to form the type of a new family, 

 having some affinity to Palcvosyops and to the extinct Bronto- 

 theridse. It had large canine tusks but no horns. 



The Rhinocerotidre are represented in America by the genus 

 Rhinoceros in the Pliocene and Miocene, and by Accrathcrium 

 and Hyracodon in the Miocene. Both the latter were hornless, 

 and Hyracodon was allied to the Eocene Hyrachyus, one of the 

 LophiodontidcB. In the Eocene and Miocene deposits of Utah, 

 and Oregon, several remarkable extinct rhinoceroses have been 



