In the Sperm whale, _Physeter_, the lower jaw bears a series of twenty to twenty-five stout conical recurved teeth, while in the upper jaw the teeth are vestigial and remain imbedded in the gum. An extinct form, _Physodon_, from the Pliocene of Europe and Patagonia is allied to the Sperm whale, but has teeth in both jaws. In the Grampus _Orca_, the teeth number about 12/12, and are very large and strong. In some forms the teeth are very much reduced in number; thus in _Mesoplodon_ the dentition consists simply of a pair of conical teeth borne in the mandible. In the Narwhal _Monodon_ the dentition is practically reduced to a single pair of teeth, which lie horizontally in the maxillae, and in the female normally remain permanently in the alveoli. In the male the right tooth remains rudimentary, while the left is developed into an enormous cylindrical tusk marked by a spiral groove. Occasionally both teeth develop into tusks, and there is reason for thinking that two-tusked individuals are generally or always female. In the extinct _Squalodon_ the dentition is decidedly heterodont, and the molars have two roots. The dental formula is _i_ 3/3 _c_ 1/1 _pm_ 4/4 _m_ 7/7, total 60. It is probable that the homodont condition of modern Odontoceti is not primitive, but due to retrogressive evolution. UNGULATA. Just as in the Cetacea a piscivorous dentition is most typically developed, so the Ungulata are, as a group, the most characteristic representatives of a _herbivorous_ dentition in its various forms. UNGULATA VERA. _ARTIODACTYLA_. As regards the living forms, the Artiodactyla can be readily divided into two groups, namely those with bunodont and those with selenodont teeth. It has, however, been shown that selenodont teeth always pass through an embryonic bunodont stage[157]. The bunodont type is best seen in Pigs and Hippopotami and such extinct forms as _Hyotherium_. In _Hippopotamus_ the dental formula is _i_ (2-3)/(1-3) _c_ 1/1 _pm_ 4/4 _m_ 3/3. The incisors and canines of _Hippopotamus_ are very large and grow continuously. The genus _Sus_, which affords a good instance of an _omnivorous_ type of dentition, has the regular unmodified Mammalian dental formula _i_ 3/3 _c_ 1/1 _pm_ 4/4 _m_ 3/3, total 44. The canines, specially in the male, are large and have persistent pulps, and the upper canines do not have the usual downward direction but pass outwards and upwards. In the Wart Hog, _Phacochaerus_, they are enormously large, but a still more extraordinary development of teeth is found in _Babirussa_. In the male _Babirussa_ the canines, which are without enamel, are long, curved and grow continuously. Those of the upper jaw never enter the mouth, but pierce the skin of the face and curve backwards over the forehead. The dental formula of _Babirussa_ is _i_ 2/3 _c_ 1/1 _pm_ 2/2 _m_ 3/3, total 34.