Birds possess an ear-muscle which at least acts as a tensor tympani; it arises near the occipital condyle, passes through a hole into the tympanic cavity, and its tendon is, in various ways, attached to the inside of the membrane and the neighbouring extracolumellar processes.
The apex of the cochlea is turned towards, and almost reaches the anterior wall of the occipital condyle; at most it makes but half a twist or turn; it possesses both Reissner's membrane and the organ of Corti.
The skull generally lacks a sagittal crest; and the condyle of the lower jaw is transversely elongated.
Odontoid process of second vertebra semi-cylindrical; skull with a sagittal crest; and the condyle of the lower jaw rounded.
In the Oreodontinae or typical section of the family, which includes several genera nearly allied to Oreodon, the skull is shorter and higher than in the camels, with a swollen brain-case, a preorbital glandpit, the condyle of the lower jaw transversely elongated, the tympanic bulla hollow, and the orbit surrounded by bone.