Rodents may be characterized as terrestrial, or in some cases arboreal or aquatic, placental mammals of small or medium size, with a milk and a permanent series of teeth, plantigrade or partially plantigrade, and generally five-toed, clawed (rarely nailed or semi hoofed) feet, clavicles or collar-bones (occasionally imperfect or rudimentary), no canine teeth, and a single pair of lower incisors, opposed by only one similar and functional pair in the upper jaw.
In the typical Ungulata or Diplarthra, the feet are never plantigrade, and the functional toes do not exceed four - the inner digit being suppressed, Right Fore Foot of Indian at all events in all forms which Elephant.
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.
The largest of these ferocious animals, also forming a genus and family by itself, is the Cryptoprocta ferox; it is a plantigrade animal, 3 ft.
The build is stout and heavy, the limbs and tail are short, the ears moderate, the eyes minute and the feet five-toed and plantigrade.
Order Colymbiformes.-Plantigrade, nidifugous, aquatic. All toes webbed, fourth largest, hallux short; metatarsus laterally compressed; tibia with high, pyramidal crest.
The short feet of the penguins are an adaptation.