HYVRACID.E 415 
only be possible here to give details of some of the more interest- 
ing ov best-known forms. 
The characters by which the skeleton of the feet of the Sub- 
ungulata are distinguished from those of the Ungulata Vera have 
been already mentioned on p. 275. In addition to these it may 
be observed that the feet frequently have five functional digits, 
and may be plantigrade; while the upper surface of the astragalus 
is generally flattened, instead of presenting the strongly-marked 
pulley-like ridges and groove so characteristic of the Ungulata 
Vera, 
Suborder HYRACOIDEA,. 
Family HYRACID.©. 
This division is constituted to receive a single family of mam- 
mals, the affinities of which have long constituted a puzzle to 
zoologists. They were first placed among the Rodents, to which 
animals their small size and general appearance and habits vive 
them much superficial resemblanee. Cuvier’s investigations into 
their anatomical structure, and especially their dental characters, 
led him to place them among the Ungulates, near the genus 
Rhinoceros, a position long accepted by many zoologists. Further 
knowledge of their organisation and mode of development. caused 
Milne-Edwards, Huxley, and others to disassociate them from this 
connection, and, failing to find any agreement with any other known 
forms, to place them in an order entirely apart. Paleontology has 
thrown no light upon the aftinities of this anomalous and isolated 
group, as no extinet animals possessing their distinetive characters 
have as yet been discovered. 
