_Allied Extinct Genera._—In the Tertiary deposits of both the Old and New World occur remains of Pig-like animals which, so far as we can judge, appear to connect the Peccaries so closely with the true Pigs as to render the _Dicotylidæ_ really inseparable from the _Suidæ_. Of these the American genus _Chænohyus_ has the lower canine with a triangular cross section and received into a notch in the upper jaw, as in the Peccaries, but the fourth upper premolar is simpler than the molars, as in the under-mentioned genus _Hyotherium_. The typical forms have only three premolars, but in others, which it has been proposed to separate generically as _Bothriolabis_, there are four of these teeth. _Hyotherium_, of the Pliocene and Miocene of the Old World, is a generalised form allied both to _Sus_ and _Dicotyles_ as well as to certain extinct genera. The upper molars (Fig. 110) are characterised by their square crowns, the last having no distinct third lobe, and coming into use before the first is much worn, while the last premolar is simpler than the true molars. The canines, which have an oval section and are scarcely larger than the incisors, are not received into a notch in the upper jaw. In the Pliocene of India there occurs an apparently allied genus known as _Hippohyus_, in which the crowns of the molars are much taller, and have lateral infoldings of the enamel, producing a very complex pattern on the worn crowns. The European Miocene genus _Listriodon_, with the dental formula _i_ ³⁄₃, _c_ ¹⁄₁, _p_ ³⁄₃, _m_ ³⁄₃, differs from all the preceding in having the anterior and posterior pairs of tubercles of the molars united into ridges running across their crowns, so that these teeth resemble the lower molars of the Tapir. The genus is also found in the Lower Pliocene of India. EXTINCT TRANSITIONAL ARTIODACTYLES. In this place it will be convenient to notice briefly a few of the extinct types of Tertiary Artiodactyles which connect the existing bunodont Suina with the more specialised selenodont groups mentioned below so closely as to show that in a strictly palæontological classification such groups cannot be maintained. It should be mentioned that while some of these extinct forms were in all probability actual ancestral links between the bunodonts and selenodonts, others, like the Anoplotheres, died out entirely without giving rise to any more specialised descendants. [Illustration: FIG. 111.—The imperfect third left upper molar of _Hyopotamus giganteus_, Miocene, India. (From the _Palæontologia Indica_.)]