The characteristics of these groups will be found under their respective headings, with the exception of the Barypoda and Condylarthra, for which see Arsinoitherium and Phenacodus.
ARSINOITHERIUM (so called from the Egyptian queen Arsinoe), a gigantic horned mammal from the Middle Eocene beds of the Fayum, Egypt, representing a sub-order of Ungulata, called Barypoda.
Although the brain is relatively larger, the bones of the limbs, especially the short, five-toed feet, approximate to those of the Amblypoda and Proboscidea; but in the articulation of the astragalus with both the navicular and cuboid Arsinoitherium is nearer the former than the latter group. It is probable, however, that these resemblances are mainly due to parallelism in development, and are in all three cases adaptations necessary to support the enormous weight of the body.
No importance can be attached to the presence of horns as an indication of affinity between Arsinoitherium and the Amblypoda; and there are important differences in the structure of the skulls of the two, notably in the external auditory meatus, the occiput, the premaxill.ae, the palatal foramina and the lower jaw.
From the Proboscidea Arsinoitherium differs broadly in skull structure, in the form of the cheek-teeth, and in the persistence of the complete dental series of forty-four without gaps or enlargement of particular teeth.
Whether there is any relationship with the Hyracoidea cannot be determined until we are acquainted with the forerunners of Arsinoitherium, which is evidently a highly specialized type.
Arsinoitherium is the precursor of the horned Ungulata; while Moeritherium and Palaeomastodon undoubtedly include the oldest known elephants.