Hind extremities proportionally longer with inner toe represented only by a small metatarsal bone.
Consequently the ankle-joint of birds is absolutely cruro-tarsal and tarso-metatarsal, i.e.
Outer toes small and rudimentary, or in some cases entirely suppressed; their metacarpal or metatarsal bones never complete.
The most interesting genera are, however, the Upper Oligocene and Lower Miocene Gelocus and Prodremotherium, which have perfectly selenodont teeth, and the third and fourth metacarpal and metatarsal bones respectively fused into an imperfect cannon-bone, with the reduction of the lateral metacarpals and metatarsals to mere remnants of their upper and lower extremities.
There is no trace of a first toe, and the fifth metatarsal is represented by a small nodule.
First hind toe rudimentary, clawless or absent; its metatarsal bone always present.
Hind foot long and narrow, mainly composed of the strongly developed fourth toe, terminating in a conical pointed nail, with a strong pad behind it; the first toe represented by a rudimentary metatarsal; the remaining toes completely developed, with claws, but exceedingly slender; the united second and third reaching a little way beyond the metatarso-phalangeal articulation of the fourth; the fifth somewhat shorter.
The three middle metatarsals become fused together into a cannon bone; the upper part of the third middle metatarsal projects behind and forms the so-called hypotarsus, which in various ways, characteristic of the different groups of birds (with one or more sulci, grooved or perforated), acts as guiding pulley to the tendons of the flexor muscles of the toes.
Reduction and final loss of outer pair of digits (second and fifth), with coalescence of the metacarpal and metatarsal bones of the two middle digits to form a cannon-bone.
The primitive Artiodactyla thus probably had the typical number (44) of incisor, canine and molar teeth, brachyodont molars, conical odontoid process, four distinct toes on each foot, with metacarpal, metatarsal and all the tarsal bones distinct, and no frontal appendages.
In porcupines and hares the tendons of the flexor digitorum longus and flexor hallucis longus are connected in the foot, while in the rats and squirrels they are separate, and the flexor digitorum longus is generally inserted into the metatarsal of the first toe.
In the feet the two middle (third and fourth) metacarpal and metatarsal bones, which are completely separate in the pigs, are united at their upper ends.
In the absence of any trace of the lower extremities of the metacarpal and metatarsal bones of the lateral toes the skeleton differs from the American deer, and resembles those hollow-horned ruminants in which these toes persist.
There is a tarsal, but no metatarsal gland and tuft.
In the Lower Miocene occurs Protomeryx or Gomphotherium, in which there is a considerable increase in the matter of bodily size, the two metacarpal and metatarsal bones (or those which unite in the latter forms to constitute the cannon-bones) being double the length of the corresponding elements in Protylopus.
- Lateral metacarpals as in Rangifer; antlers very variable in size, forming a marked angle with the plane of the face, without a brow-tine; when consisting of more than a simple prong, dichotomously forked, frequently with a subbasal snag, and always with the lower prong of the fork projected from the front edge of the beam, in @ome cases the lower, in others the upper, and in others both prongs again dividing; tail long; tarsal gland generally present; metatarsal gland very variable, both as regards presence and position; vomer dividing the inner aperture of the nostrils in the skull into two distinct chambers.
- Antlers large and complex, without a sub-basal snag, and the upper prong more developed than the lower one; metatarsal gland absent; tail short; face moderately long; face-gland and gland-pit well developed; upper canines usually present in male.
- Antlers small and simple, forming a single dichotomous fork; metatarsal gland absent; tail short; face moderately long; face-gland and gland-pit well developed; upper canines present in both sexes.
- Antlers in the form of simple unbranched spikes; metatarsal, and in one case also the tarsal gland absent; tail very short; face elongated; face-gland small and gland-pit deep and triangular; hair of face radiating from two whorls; upper canines sometimes present in old males.
- Skull and metacarpals generally as in Mazama; size very small; hair coarse and brittle; antlers in the form of short, simple spikes; cannon-bones very short; tail very short or wanting; no whorls in the hair of the face; face-gland moderately large, and gland-pit deep and oval; tarsal and metatarsal glands wanting; ectocuneiform bone of tarsus united with the naviculocuboid.
- Hair coarse and brittle; upper canines of male very long; no tarsal or metatarsal glands or tufts; lateral metacarpals represented by their lower extremities; lateral hoofs very large; tail very short; naked portion of muzzle extensive; male with a large abdominal gland.
The metapodals and phalanges resemble very closely those of the fore limb, but the principal metatarsal is more laterally compressed at its upper end than is the corresponding metacarpal.
Hind-feet with no trace of first toe externally, but the metatarsal bone is present.
They give the part of the tongue on which they occur the appearance and feel of a coarse rasp. The feet are furnished with round soft pads or cushions covered with thick, naked skin, one on the under surface of each of the principal toes, and one larger one of trilobed form, behind these, under the lower ends of the metacarpal and metatarsal bones, which are placed nearly vertically in ordinary progression.
They are small longsnouted gerbil-like animals, mainly nocturnal, feeding on insects, and characterized by the great length of the metatarsal bones, which have been modified in accordance with their leaping mode of progression.