These, it is suggested, may have been related to the ancestral Hyracoidea.
The remaining and less typical subordinal groups - sometimes ranked as orders by themselves - include among living animals the Proboscidea, cr elephants, and the Hyracoidea, or hyraxes, and among extinct groups the Amblypoda, Ancylopoda, Barypoda, Condylarthra, Litopterna and Toxodontia.
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.
For many years extinct representatives of the Hyracoidea were unknown, partly owing to the fact that certain fossils were not recognized as really belonging to that group. The longest known of these was originally named Leptodon graecus, but, on account of the preoccupation of the generic title, the designation has been changed to Pliohyrax graecus.
It is now possible to define the suborder Hyracoidea as including ungulates with a centrale in the carpus, plantigrade feet, in which the first and fifth toes are reduced in greater or less degree, and clavicles and a foramen in the lower end of the humerus are absent.