Archaeoceti. Zeuglodon has the following dentition, i 3/3 c 1/1 pm and m 5/5, total 36. The incisors and canines are simple and conical; the cheek teeth are compressed and have serrated cutting edges like those in some seals.
In the Mystacoceti, or whalebone whales, calcified tooth germs probably belonging to the milk dentition are present in the embryo, but they are never functional, and are altogether absent in the adult. The anterior of these germs are simple, the posterior ones are originally complex, but subsequently split up into simple teeth like those of the anterior part of the jaw. Hence according to Kükenthal, who described these structures, the Cetacean dentition was originally heterodont.
In the living Odontoceti the dentition is homodont and monophyodont. In some cases traces occur of a replacing dentition which never comes to maturity, and renders it probable that the functional teeth of the Odontoceti are really homologous with the milk teeth of other mammals. Some of the dolphins afford the apparently simplest type of mammalian dentition known. The teeth are all simple, conical, slightly recurved structures, with simple tapering roots and without enamel. The dentition is typically piscivorous, being adapted for seizing active slippery animals such as fish. The prey is then swallowed entire without mastication. Sometimes the teeth are excessively numerous, reaching two hundred or more (fifty to sixty on each side of each jaw) in Pontoporia. This multiplication of teeth is regarded by Kükenthal as due to the division into three parts of numbers of trilobed teeth similar to those of some seals.
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 Killer 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.