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.
The Wart Hog has a very anomalous dentition, for as age advances all the teeth except the canines and last molars show signs of disappearing; both pairs of persisting teeth are however very large.
Various extinct Ungulata such as Anoplotherium have teeth which are intermediate in character between the bunodont and selenodont types. Anoplotherium has the regular mammalian series of forty-four teeth. The crowns of all the teeth are equal in height, and there is no diastema—an arrangement found in no living mammal but man.
We come now to the selenodont Artiodactyla.
The Tylopoda—camels (Camelidae) and Llamas (Aucheniidae) when young have the full number of incisors, but in the adult the two upper middle ones are lost. The molars are typically selenodont and hypsodont[158]. In the Camel the dental formula is i 1/3 c 1/1 pm 3/2 m 3/3, total 34. The upper incisors, canines and first premolars of the Camel are very small teeth, and the first premolar is separated by a long diastema from the others.
The Tragulina or Chevrotains have no upper incisors, while the canines are largely developed, especially in the male.
The Ruminantia or Pecora are very uniform as regards their dentition. The upper incisors are always absent, for though their germs are developed they are reabsorbed without ever becoming visible, and as a rule the upper canines are absent too, while the lower canines are incisiform. The grinding teeth are typically selenodont, and in the lower jaw form a continuous series separated by a wide diastema from the canines. The dental formula is usually
i 0/3 c 0-1/1 pm 3/3 m 3/3.
The canines are largely developed in the male Muskdeer (Moschus) and in Hydropotes.
Perissodactyla. The premolars and molars have a very similar structure and form a continuous series of large square teeth with complex crowns. The crowns are always constructed on some modification of the bilophodont[159] plan, as is easily seen in the case of the forms with brachydont teeth, but in animals like the Horse, in which the teeth are very hypsodont, this arrangement is hard to trace. All four premolars in the upper jaw are preceded by milk teeth, while in Artiodactyla the first has no milk predecessors.