684 THE POPULAR SCIENCE MONTHLY. 



cies of this genus was. about as large as a cat. With Helohyus, this 

 o-enus forms a well-marked family, the Helohyidw. 



In the Uiplacodon horizon of the upper Eocene, the Selenodont den- 

 tition is no longer doubtful, as it is seen in most of the Artiodactyla 

 yet found in these beds. These animals are all small, and belong to 

 at least three distinct genera. One of these, Eomeryx, closely resem- 

 bles Ilomacodon in most of its skeleton, and has four toes, but its 

 teeth show well-marked crescents, and a partial transition to the teeth 

 of Hyopotamus, frum the Eocene of Europe. With this genus is an- 

 other, Parameryx, also closely allied to Ilomacodon, but apparently 

 a straggler from the true line, as it has but three toes behind. The 

 most pronounced Selenodont in the upper Eocene is the Oromeryx, 

 which genus appears to be allied to the existing Deer family, or Cervi- 

 dce, and if so is the oldest known representative of the group. These 

 facts are important, as it has been supposed, until very recently, that 

 our Eocene contained no even-hoofed mammals. 



In the lowest Miocene of the West, no true crescent-toothed Ar- 

 tiodactyla have as yet been identified, with the exception of a single 

 species of Hyopotamus ; but, in the overlying beds of the middle 

 Miocene, remains of the Oreodontidaz occur in such vast numbers as to 

 indicate that these animals must have lived in large herds around the 

 borders of the lake-basins in which their remains have been entombed. 

 These basins are now the denuded deserts so well termed Jfaicvaises 

 Terres by the early French trappers. The least specialized, and appar- 

 ently the oldest, genus of this group is Agriochoerus, which so nearly 

 resembles the older Hyopotamus, and the still more ancient Eomeryx, 

 that we can hardly doubt that they all belonged to the same ancestral 

 line. The typical Oreodonts are the genera Oreodon and Epormdon, 

 which have been aptly termed by Leidy ruminating hogs. They had 

 forty-four teeth, and four well-developed toes on each foot. The true 

 Oreodons, which were most numerous east of the Rocky Mountains, 

 were about as large as the existing peccary, while Eporeodon, which 

 was nearly twice this size, was very abundant in the Miocene of the 



Pacific slope. 



In the succeeding Pliocene formation, on each side of the Rocky 

 Mountains, the genus 3Ierychyus is one of the provailing forms, and 

 continues the line on from the Miocene, where the true Oreodons be- 

 came extinct. Beyond this, we have the genus Merychochoerus, which 

 is so nearly allied to the last that they would be united by many 

 naturalists. With the close of the Pliocene, this series of peculiar 

 ruminants abruptly terminates, no member surviving until the Post- 

 Tertiary, so far as known. 



A most interesting line, that leading to the camels and llamas, 

 separates from the primitive Selenodont branch in the Eocene, prob- 

 ably through the genus Parameryx. In the Miocene, we find m 

 Poebrotherium and some nearly allied forms unmistakable indications 





