We commence the Artiodactyla, or even-toed Ungulates, with the hog tribe. These are represented by species of peccaries, (_Dicotyles_) from the Pliocene of Nebraska and Oregon; and by an allied form _Thinohyus_, very like _Dicotyles_, but having an additional premolar tooth and a much smaller brain-cavity. From the Miocene are three allied genera, _Nanohyus_, _Leptochoerus_, and _Perchoerus_. Professor Cope, however, thinks _Leptochoerus_ may be Lemuroid, and allied to _Menotherium_. The Anthracotheridæ, a family which connects the Hippopotamidæ and Ruminants, and which occurs in the Miocene of Europe and India, are represented in America by the genus _Hyopotamus_ from the Miocene of Dakota, and _Elotherium_ from the Miocene of Oregon and the Eocene of Wyoming; the latter genus being sometimes classed with the preceding family, and lately placed by Professor Marsh, in the new order, Tillodontia. Professor Cope has since described three other genera from the Eocene of New {138}Mexico: _Meniscotherium_, having resemblances to _Palæosyops_, _Hyopotamus_, and the Limnotheridæ; _Phenacodus_, the size of a hog, of doubtful position, but perhaps near _Elotherium_; and _Achænodon_, as large as a cow, but more hog-like than the preceding. Another new genus from the Miocene of Colorado--_Pelonax_--is said by Professor Cope to come between _Elotherium_ and _Hippopotamus_. The Camelidæ are very abundant, and form one of the most striking features of the ancient fauna of America. _Procamelus_, _Homocamelus_, and _Megalomeryx_, are extinct genera found in the Pliocene formation; the first very closely allied to the Old World camel, the last smaller and more sheep-like. In the Miocene two other genera occur, _Poebrotherium_ and _Protomeryx_, the former allied to both the camel and the llama. Deer are represented by a single species of _Cervus_ in the Pliocene, while two extinct genera, _Leptomeryx_ and _Merycodus_, are found in the Miocene deposits, the latter indicating a transition between camels and deer. Two other genera, _Hypisodus_ and _Hypertragulus_, of very small size, are said by Professor Cope to be allied to the Tragulidæ and to _Leptomeryx_. The Bovidæ, or hollow-horned ruminants, are only represented in the Newer Pliocene by a single species of an extinct genus, _Casoryx_, said to be intermediate between antelopes and deer. We now come to an exclusively American family, the _Oreodontidæ_, which consisted of small animals termed by Dr. Leidy, "ruminating hogs," and which had some general structural resemblances to deer and camels. They abounded in North America during the Pliocene, and especially during the Miocene epoch, no less than six genera and twenty species having been discovered. _Merychus_ contains the Pliocene forms; while _Oreodon_, _Eporeodon_, _Merychochoerus_, _Leptauchenia_, and _Agriochoerus_ are Miocene. The last genus extends back into the Eocene period, and shows affinity to the European Anoplotheridæ of the same epoch.