chap, vii.] MAMMALIA OF THE NEW WORLD. 135 



also in the European Miocene and Upper Eocene formations, and 

 constitutes a distinct family Hyaenodontidse, allied, according to 

 Dr. Leidy, to wolves, cats, hyaenas and weasels. The Ursidae 

 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 Tomarctos, as a 

 " short-faced type of dog ;" as well as species of Cains 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 Hipparion, 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 Anchitheridae. In the MioceDe we 

 have the genera Anchitherium (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 



