The astragalus has a pulley-like surface above for articulation into the tibia, but its lower surface is flattened and unites to a much greater extent with the navicular than with the cuboid, which bone is of comparatively less importance than in the Artiodactyles.
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.
The navicular and the external cuneiform bones are broad and flat.
I A), and never fuse into a complete cannon-bone;, and the navicular and cuboid bones of the tarsus are separate.
The trapezoid and magnum of the carpus, and the cuboid and navicular of the tarsus are distinct.
Union of the navicular and cuboid, and sometimes the ectocuneiform bone, of the tarsus.
Lower articular surface of the astragalus divided into two nearly equal facets, one for the navicular and a second for the cuboid bone.