And two external, so that they were of the typical primitive bunodont structure.
They were five-toed, bunodont Condylarthra, with a decided approximation to the perissodactyle type in the structure of the feet.
There is usually a short gap between the canine and first premolar; the upper molars are short-crowned and transitional between the bunodont (tubercular) and selenodont (crescentic) types, with two outer concave tubercles and two inner conical ones; while the lower molars are crescentic, with three lobes in the last of the series..
The full typical series of 44 teeth was developed in each, but whereas in the Periptychidae the upper molars were bunodont and tritubercular, in the Pantolambdidae they have assumed a selenodont structure.
As regards the teeth, we have the passage of a simply tubercular, or bunodont ((30vv6s, a hillock) type of molar into one in which the four main tubercles, or columns, have assumed a crescentic form, whence this type is termed selenodont (v€X vn, the new moon).
It is noteworthy, however, that in some instances there appears to have been a retrograde modification from the selenodont towards the bunodont type, the hippopotamus being a case in point.
(See Oreodon.) The Eocene American genus Homacodon is regarded as representing a third family group, the Homacodontidae (= Pantolestidae), in which the molars were of a bunodont type, and approximate to those of the Condylarthra from which this family appears to have sprung, and to have given origin on the one hand to the Oreodontidae, and on the other to the Camelidae.
In the existing members of the group the cheek-teeth approximate to the bunodont type, although showing signs of being degenerate modifications of the selenodont modification.
This is the more remarkable seeing that Elotherium may be regarded as a kind of bunodont Anthracotherium.
Finally, we have in the Pliocene of India the genus Tetraconodon, remarkable for the enormous size attained by the bluntly conical premolars; as the molars are purely bunodont, this genus seems to be a late and specialized survivor of a primitive type..
Choeropotamus is a European Oligocene genus with bunodont molars which show a conspicuous basal cingulum in the lower dentition; the first premolar is absent.
Homacodon was an animal of the size of a rabbit, with five toes (of which only five were functional to each foot) and 44 teeth, of which the molars are tuberculated (bunodont), with six columns on those of the upper jaw; the premolars being of a cutting type.