cies of this genus was. about as large as a cat. With Helohyus, this genus forms a well-marked family, the Helohyidæ.
In the Diplacodon horizon of the upper Eocene, the Selenodont dentition 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 resembles Homacodon 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, from the Eocene of Europe. With this genus is another, Parameryx, also closely allied to Homacodon, 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 Cervidæ, 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 Artiodactyla 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 Oreodontidæ 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 Mauvaises Terres by the early French trappers. The least specialized, and apparently the oldest, genus of this group is Agriochœrus, 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 Eporeodon, 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 Merychyus is one of the provailing forms, and continues the line on from the Miocene, where the true Oreodons became extinct. Beyond this, we have the genus Merychochœrus, 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, probably through the genus Parameryx. In the Miocene, we find in Pœbrotherium and some nearly allied forms unmistakable indications
