Most of these extend through narrow apertures foramina pneumatica - into the hollow bones, sometimes, e.g.
The incisive foramina are large and usually confluent; the bony palate is very narrow from before backwards; there is no alisphenoid canal; the fibula is welded to the tibia, and articulates with the calcaneum; and the testes are permanently external.
They are small rat-like rodents, with one pair of upper premolars, which are mere pins, as is the last molar, and the two pairs of limbs of normal length, with the metatarsals separate; the infra-orbital opening in the skull being triangular and widest below, while the incisive foramina in the palate are elongated.
The incisive foramina of the palate are moderate and distinct; the fibula does not articulate with the calcaneum; and the testes are abdominal, and descend periodically only into the inguinal canal.
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.