Most of these extend through narrow apertures foramina pneumatica - into the hollow bones, sometimes, e.g.
A single pair, which may pass through foramina ("opesiules") in the cryptocyst to reach their insertion.
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 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.
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.
In some specimens of these genera the horny shields covering the bony scutes of the carapace have been preserved, and since the foramina, which often pierce the latter, stop short of the former, it is evident that these were for the passage of blood-vessels and not receptacles for bristles.
Foramina by which the optic, trigeminal and portio dura, and abducens nerves leave the skull.