Of the metatarsals the fifth occurs as an embryonic vestige near the joint; the first is reduced to its distal portion, and is, with the hallux, shoved on to the inner and posterior side of the foot, at least in the majority of birds.
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.
The short feet of the penguins are an adaptation.
The middle metacarpals and metatarsals generally confluent, the outer ones (second and fifth) slender but complete, i.e.
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.
The pelvis and hind-limbs much resemble those of a running bird, such as those of an emu or the extinct moa; but the basal bones (metatarsals) of the three-toed foot remain separate throughout life, thus differing from those of the running birds, which are firmly fused together even in the young adult.
In the Jaculidae the metatarsals are greatly elongated, and in some of the species, as jerboas, they are welded together.
They are small rat-like rodents, with one pair of upper premolars, which are mere pins, as is the last molar, and the two pairs of limbs of normal length, with the metatarsals separate; the infra-orbital opening in the skull being triangular and widest below, while the incisive foramina in the palate are elongated.
The Dipodinae, on the other hand, are leaping rodents, with the metatarsals elongated, a small upper premolar present or absent, and the crowns of the molars tall.
The least specialized genus is Zapus, containing the jumping-mice of North America, with one outlying Siberian species, in which the five metatarsals are free, as are also the cervical vertebrae, the small upper premolar being retained.
In the forelimbs the bones corresponding to the third and fourth metacarpals of the pig's foot are fused into a cannon-bone; and a similar condition obtains in the case of the corresponding metatarsals in the hind-limbs.
(X i .) Lateral digits of both fore and hind feet almost always present, and frequently the lower ends of the metacarpals and the metatarsals as well.
Lower ends of the lateral metacarpals and metatarsals never present.
Here the metacarpals and metatarsals have partially united to form cannonbones, the skull has assumed the elongated form characteristic of modern camels, with the loss of the first and second pairs of upper incisors, and the development of gaps in front of and behind each of the next three teeth, that is to say, the third incisor, the canine and the first cheek-tooth.
In one species of Procamelus the metacarpals and metatarsals coalesced into canon-bones late in life; but when we come to the Pleistocene Camelops such union took place at an early stage of existence, and was thoroughly complete.
The hind-limb is typically avine, with intertarsal joint, distally reduced fibula, and the three elongated metatarsals which show already considerable anchylosis; reduction of the toes to four, with 2, 3, 4 and 5 phalanges; the hallux is separate, and as usual in recent birds posterior in position.
In all the more typical members of the family the three middle metatarsals of the long hind-legs are fused into a cannon-bone; and in the true jerboas of the genus Jaculus the two lateral toes, with their supporting metatarsals, are lost, although they are present in the alactagas (Alactaga), in which, however, as in certain allied genera, only the three middle toes are functional.
Further, at an early stage of development the fibula is a complete and separate bone, while the three metatarsals, which subsequently fuse together to form the cannon-bone, are likewise separate.
2 The three metatarsals in the penguins are not, as in other birds, united for the whole of their length, but only at the extremities, thus preserving a portion of their originally distinct existence, a fact probably attributable to arrest of development, since the researches of C. Gegenbaur show that the embryos of all birds, so far as is known, possess these bones in an independent condition.