398 CLASSIFICATION OF VERTEBRATES. 



others ; the digits bear nail-like hoofs. Restricted to the eastern hemisphere, 

 where they occur fossil in Europe and Asia since the pliocene, the living 

 species are all African. Three genera are distinguished by the number of 

 lower incisors : Hippopotamus with six, Tetraprotodon with four, and Chce- 

 ropus with two. Hippopotamus dates back to the pliocene, Chceropns is 

 living; Tetraprotodon is known only from the African pleistocene. The 

 common species, H. amphibius, has an enormous three-chambered stomach, 

 eleven feet in axial length. These animals are gregarious and herbivorous. 



The OREODONTID.E lived from the eocene to the pliocene of North Amer- 

 ica. They had the teeth i f , c \, p \, m f , and the feet four-toed ; and in Pro- 

 toreodon, from the eocene, the fore toes were five in number. Agriochcerus and 

 Oreodon miocene, Merychius (Ticholeptus) miocene and pliocene. The CA- 

 MELID^E (Tylopoda), which appear in the eocene, may have descended from 

 either the pantolestidas or the oreodontidae. They have the teeth / f to \, c ^-, 

 p 4 to f , m f , the molars selenodont, with a diastema between the premolars 

 and canines ; the feet four- or two-toed, the lateral toes completely lost in 

 the more recent species ; and in all except the older forms the metapodials 

 fused to a cannon bone. The stomach lacks the manyplies, while rumen 

 and honeycomb are sub-divided into numerous small cavities on the walls. 

 The placenta is diffuse. The living genera, Camelus, which inhabits Asia 

 and Africa, and Aitchenia of South America, appear in the pliocene. They 

 differ in the premolars, these being f and \ respectively. The camels are 

 two in number, the single-humped dromedary, C. dromedarius, and the two- 

 humped bactrian, C. bactrianus. Auchenia contains four species, the llama, 

 alpaca, vifuna, and guanaco. Among the extinct genera are Leptotragnlus, 

 eocene ; PoebrotJierium and Protolabis, miocene ; Procamelus, EscJiatius, 

 and Pliauchenia, pliocene. The ANOPLOTHERIID^E of the European eocene 

 and pliocene are noticeable from the fact that it was in this group that 

 Cuvier made many restorations. Anoplotherium, Dichobune, Ccenotlierium, 

 Xiphodon. 



The TRAGULID^E, or chevrotains, have the teeth i , c \,p f or |, ;// 1 ; 

 fibula complete, usually a cannon bone ; feet four-toed ; stomach three-cham- 

 bered ; placenta diffuse. These forms have been closely associated with the 

 musk deer. Tragulus of Asia contains the smallest existing ungulates. 

 Dorcatherium (Hyomoschns} from Africa. Leptomeryx, American miocene. 



The CERVICORNIA, in which the teeth are z'f to f , c \, p f to f m, f , the 

 upper canine being sometimes very large, sometimes small or absent, the 

 molars selenodont, are as a rule characterized by the development of horns 

 or antlers upon the frontal bones of the male, although they are occasionally 

 absent, or. as in the reindeer, they may appear in both sexes. These horns 

 consist of a bony outgrowth from the frontals ; and at first this is covered with 

 skin, which may persist through life, as in the giraffe, or, more usually, is 

 soon worn off, leaving the bone alone. Each year this horn is shed, and a 

 new antler takes its place, the later one displaying a greater number of 

 branches or ' tines, 1 so that these become an index of age. Metapodials 

 3 and 4 usually form a cannon bone ; the lateral metapodials are reduced, 

 and the toes do not reach the ground. The stomach and placenta are of the 

 ruminant type (p. 396). The species are very numerous, but none occur in 



