Recent discoveries have modified the author’s earlier opinions as to the lines of descent of the titanotheres and still further changes are anticipated with increase of knowledge of the connections between Upper Eocene, or Uinta, titanotheres and those of the Lower Oligocene, or White River.
The main lines of division are indicated in the proportions of the limbs, whether cursorial, mediportal, or graviportal; the proportions of the skull, whether mesaticephalic, brachycephalic, or dolichocephalic; the development of fronto-nasal horns, whether accelerated or retarded; the molarization of the premolar teeth, whether accelerated or retarded; the presence or absence of incisor teeth; the abbreviate or elongate, the triangular or oval form of the fronto-nasal horns as developed in Oligocene times.
With these criteria the various phyla may readily be distinguished as follows:
Wind River titanotheres, face more elongate than cranium:
I. Lambdotheriinæ, light-limbed, cursorial.. Genus, Lambdotherium II. Eotitanopinæ, medium-limbed, mediportal Eotitanops I. Lambdotheriinæ, light-limbed, cursorial.. Genus, Lambdotherium II. Eotitanopinæ, medium-limbed, mediportal Eotitanops