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.
This is the more remarkable seeing that Elotherium may be regarded as a kind of bunodont Anthracotherium.
They were five-toed, bunodont Condylarthra, with a decided approximation to the perissodactyle type in the structure of the feet.
And two external, so that they were of the typical primitive bunodont structure.
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.
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.
Choeropotamus is a European Oligocene genus with bunodont molars which show a conspicuous basal cingulum in the lower dentition; the first premolar is absent.
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..