This order falls naturally into two sub-orders:
| "I. | A third trochanter on the femur, and a fossa for the round ligament: no alisphenoid canal; superior incisors present Pantodonta. |
| "II. | No third trochanter, nor fossa for the round ligament; an alisphenoid canal; no superior incisors. Dinocerata." |
This sub-order, Dinocerata, includes at present three distinct genera, Uintatherium, Leidy, Dinoceras, Marsh, and Loxolophodon, Cope (also probably Megacerops, Leidy). These three genera, in addition to the characters above given, are marked by the possession of two or more osseous projections from the upper surface of the head; and of these the posterior pair, developed from the parietals, are the largest.
Synopsis of genera of
DINOCERATA.
| A. | Cervical vertebra? long; median horn-like processes anterior to the orbit; nasal tuberosities do not overhang the nasal tips. | |
(a) Last molar, with or without a tubercle, occupying the entrance of the valley between the lobes, and but one on posterior basal ridge. Temporal fossæ not continued beyond the base of the parietal processes. Occipital condyles projecting. | Uintatherium. | |
(b) Last molar never has a tubercle at entrance of valley, and has two on posterior basal ridge. Temporal fossæ continued very far back; condyles sessile. | Dinoceras. | |
| B. | Cervical vertebræ short; median horn-like processes directly over the orbit; nasal tuberosities overhang the entire tips. | Loxolophodon. |
Synopsis of species of
UINTATHERIUM.
| A. | With a tubercle occupying the entrance of the valley between the lobes of the last upper molar. | U. robustum. |
| B. | No tubercle at the entrance of the valley. | |
(a) Nasals divided by a deep groove; slender zygomatic arch; dorsal vertebræ compressed. | U. Leidianum. | |
(b) No nasal groove; stout zygomatic arch; dorsal vertebræ sub-cylindrical | U. princeps. |