A. No diastemata.

a. Fourth inferior premolar unlike the first true molar. Last inferior molar with five lobes; superior premolars four;........

AA. A diastema behind the first premolar in both jaws. a. Last inferior premolar different from first true molar. Last inferior molar with heel; cross-crests of superior mo

Systemodon Cope.

..Hyracotherium Owen.

Pliolophus Owen.

II. External lobes of superior molars well separated and little flattened; inferior molars with perfect cross-crests (Protapirina).

Last inferior molar with heel; ? four premolars ;..............Isectolophus S. and O. Last inferior molar without heel; inferior premolars three ;..Protapirus Filh.

III. External lobes of superior molars flat, not well distinguished, forming a wall. Inferior molars with perfect cross-crests (Lophiodontina).

Last lower molar without heels, no horns;...................... Dilophodon Scott. Last lower molar?; "an attachment for a dermal horn on

each nasal bone;".

......... Colonoceras Marsh.

The above table shows that the modification which this family has undergone in its superior molars, has consisted in the confluence of the external tubercles into a more or less irregular external wall to the crown, and the confluence of the intermediate and internal tubercles into cross-crests. In the lower molars cross-crests have been formed. It is impossible to separate the Hyracotheriine sub-family as a family from the Lophiodontine, since the characters grade into each other completely. But it has been from the Hyracotheriine sub-family that the horse line was derived; the Protapirinæ gave origin to the Tapiridæ; while the rhinoceroses have descended from the Lophiodontinæ.

Among Hyracotheriinæ the genus Systemodon (Cope) holds the lowest place on account of the entire absence of diastemata from the dental series. It is as old as any of the genera, occur

PLATE XXX.

Hyracotherium venticolum, Cope, skeleton restored, one-third natural size; from the Wasatch beds of Wyoming, N. A. Original except ribs, which are after Osborn. Unshaded portions not preserved in specimen, which is in Coll. E. D. Cope.

ring in the Wasatch (or Suessonian) epoch. A common ancestor gave origin to Systemodon and Hyracotherium. It resembled the former in the absence of diastemata, and the latter in the

FIG. 5. Skull of Hyracotherium venticolum Cope, three-quarters natural size; from Wind River Eocene of Wyoming. Original. Fig. a from above; b from below.

In the Lophiodontine line we have Helaletes (Marsh) without diastema, like Systemodon. In Heptodon the diastema appears,

and in the succeeding genera the first premolars are lost. In Hyrachyus the heel of the last inferior molar vanishes, while the nasal bones bear traces of dermal horns in Colonoceras (Marsh). The transition from Hyrachyus to the Rhinocerontid series is not difficult. The nearest form is Canopus of the Canopodidæ, which only differs from Hyrachyus in the lack of the canine teeth above, and of the middle incisors. The passage to the Tapirida is equally easy, through the close resemblance of Protapirus (Filh.) to Tapiravus (Marsh) of the