The geographical distribution of animals, with a study of the relations ...

chap, vii.] MAMMALIA OF THE NEW WORLD. 135also in the European Miocene and Upper Eocene formations, andconstitutes a distinct family Hyaenodontidse, allied,according toDr. Leidy, to wolves, cats, hyaenas and weasels. The Ursidaeare represented by only one species of an extinct genus, Leptarchus,from the Pliocene of Nebraska. From the Pliocene ofColorado, Prof. Cope has recently described Tomarctos, as a" short-faced type of dog ;" as well as species of Cains andMartes.Ungulata.—The animals belonging to this order being usuallyof large size and accustomed to feed and travel in herds, areliable to wholesale destruction by floods, bogs, precipices, droughtor hunger. It is for these reasons, probably, that their remainsare almost always more numerous than those of other orders ofmammalia. In America they are especially abundant ; and thenumber of new and intermediate types about whose positionthere is much difference of opinion among Palaeontologists, rendersit very difficult to give a connected summary of them withany approach to systematic accuracy.Beginning with the Perissodactyla,or odd-toed ungulates, wefind 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 severalancestral forms.The most nearly allied to Equus is Pliohippus,consisting of animals about the size of an ass, with the lateraltoes not externally developed, but with some differences of dentition.Next come Protohippus and Hipparion, in which thelateral toes are developed but are small and functionless. Thenwe have the allied genera, Anchippus, Merychippus, and Hyohippus,related to the European Hippotherium, which were all stillsmaller animals, Protohippus being only 2| feet high. In theolder deposits we come to a series of forms, still unmistakablyequine, but with three or more toes used for locomotion and withnumerous differentiations in form, proportions, and dentition.These constitute the family Anchitheridae. In the MioceDe wehave the genera Anchitherium (found also in the EuropeanMiocene), Miohippus and Mesohippus, all with three toes on eachfoot, and about the size of a sheep or large goat.In the Eocene of