_Mammals._—It is but in animals who suckle their young (_Mammalia_) that the extremities attain their greatest state of developement, and it is in the higher orders alone that they become useful as organs of prehension, their office being limited in most of the lower tribes to support and locomotion. In the lowest order of the series, or whale tribe (_Cetacea_), the members, in the number of pieces of which they are composed, bear a considerable resemblance to the fins of fishes, though they are somewhat differently constructed, according as the animal may be destined exclusively for an aquatic existence, like the common sperm whale, or be more or less herbivorous, and consequently terrestrial in its habits, like the dugong. In the former case, the extremities are converted into broad paddles adapted for swimming, and the feet are webbed; in the latter, the members are better developed, the toes being free, and furnished with nails at their termination. Of those orders of _Mammalia_ which are provided with four members—the ordinary quadrupeds—some are furnished with a foot which appears to be entirely without toes, their place being supplied by a mass of horny tissue—the hoof. This is the case with the ruminants—animals that chew the cud—as the cow, and with the horse; but in the latter, as the name of the order implies (_Solidungula_), the hoof is solid, while in the ruminants it is cleft; and hence by some naturalists these latter animals have been termed _Bisulca_. Some slight differences are perceptible in the construction of the feet of ruminants: thus the chamois, the antelope, and other of the deer tribe, are furnished with an additional toe, which, projecting backwards, affords a firmer support to the body. In the thick-skinned animals (_Pachydermata_) the toes can hardly be said to be free, being still partially enclosed in the tough skin. It is from characters afforded by the foot that several tribes of animals belonging to this order are mainly characterised: thus the elephant has five toes to each foot, the rhinoceros three, and the hog and hippopotamus four. Among those of the mammalia which are characterised by the presence of four members with distinct and separate toes and nails, but few are furnished with a true hand—that is, a member with a thumb capable of being brought into apposition with the other fingers. Carnivorous animals have either a foot with sharp recurved and retractile claws, as the cat and tiger, or with straight blunt claws, not retractile, as the bear; or the feet are small, almost completely enveloped in the skin, and furnished with a membranous expansion connecting the toes, as in the aquatic seal and walrus, or river-horse: but in all these animals, although the fore-legs may be more or less employed as instruments of prehension, they are not used for the purpose of conveying food to the mouth. In some of the gnawing animals, as the squirrel, the anterior extremity commences to