chap, vii.] MAMMALIA OF THE NEW WORLD. 
135 
also in the European Miocene and Upper Eocene formations, and 
constitutes a distinct family Hysenodontidae, allied, according to 
Dr. Leidy, to wolves, cats, hyrenas and weasels. The Ursidse 
are represented by only one species of an extinct genus, Leptar- 
chus , from the Pliocene of Nebraska. From the Pliocene of 
Colorado, Prof. Cope has recently described Tomardos , as a 
“short-faced type of dog;” as well as species of Cairn and 
Martes. 
Ungulata. — The animals belonging to this order being usually 
of large size and accustomed to feed and travel in herds, are 
liable to wholesale destruction by floods, bogs, precipices, drought 
or hunger. It is for these reasons, probably, that their remains 
are almost always more numerous than those of other orders of 
mammalia. In America they are especially abundant ; and the 
number of new and intermediate types about whose position 
there is much difference of opinion among Palaeontologists, ren- 
ders it very difficult to give a connected summary of them with 
any approach to systematic accuracy. 
Beginning with the Perissodactyla, or odd-toed ungulates, we 
find the Equine animals remarkably numerous and interesting. 
The true horses of the genus Equus , so abundant in the Post- 
Pliocene formations, are represented in the Pliocene by several 
ancestral forms. The most nearly allied to Equus is Pliohippus, 
consisting of animals about the size of an ass, with the lateral 
toes not externally developed, but with some differences of denti- 
tion. Next come Protohippus and Ilippcirion, in which the 
lateral toes are developed but are small and functionless. Then 
we have the allied genera, Anchippus , Merychippus , and Hyohip - 
pus, related to the European Hippotherium , which were all still 
smaller animals, Protohippus being only 2| feet high. In the 
older deposits we come to a series of forms, still unmistakably 
equine, but with three or more toes used for locomotion and with 
numerous differentiations in form, proportions, and dentition. 
These constitute the family Anehitheridse. In the Miocene we 
have the genera Ancliitherium (found also in the European 
Miocene), Miohippus and Mesohippus, all with three toes on each 
foot, and about the size of a sheep or large goat. In the Eocene of 
