Pachydermia (Gr., thick, and, skin), the name given by Cuvier to a group of herbivorous mammals, generally large and unwieldy, with a thick skin, naked or sparingly covered with hair. Among its living members are the elephant, hippopotamus, rhinoceros, manatee, tapir, hog, and peccary, and among the extinct genera the mastodon, dinotherium, palseotherium, lophiodon, macrauchenia, and toxodon. They are allied to ruminants by the fossil anoplotherium, and to rodents by the hyrax, while the hiatus between the rhinoceros, tapir, and elephant was filled by the extinct lophiodon, macrauchenia, palosotherium, and their allied genera. In the present epoch the genera and species are few, but during the tertiary period they existed under a far greater variety of form and in more northern habitats. In the system of Cuvier the pachyderms included all non-ruminating hoofed quadrupeds, divided into proboscidia (elephants), solidun-gula (horse, &c), and ordinary pachyderms subdivided according to the odd or even number of the hoofs.
Wagner makes sections: I., anisodactyla, with hoofs in a single series around the bottom of the foot, and with skin usually naked, including the four families of elephant, tapir, hippopotamus, and rhinoceros; II., zygodactyla with two hoofed toes for walking and two others placed higher up, including the hog family; and III., lamnun-gia, with flattened nails instead of hoofs, including the hyrax family. According to (J wen's cerebral system (see Mammalia), the pachyderms would comprise all the hoofed quadrupeds except the ruminants and solidun-gulates, with the addition of the sirenoid muti-lata. Van der Hoeven combines the systems of Cuvier and Owen, as follows: order pachy-dermata, with phalanx I., proboscidia, with the elephant family; II., perwuodactyla, with an odd number of toes, with the families nasicor-nia (rhinoceros), lamnungia (hyrax), tapirina, and solidungula; and III., artiodactyla, with even toes, including the hog and hippopotamus families. Huxley divides the old pachyderms into proboscidca, hyracoidea, and the peris-sodactyl and artiodactyl ungulata, the latter including the ruminantia. Most modern sys-tematists, since Owen, do not use the term, dividing its members among various orders, according to the hoofs and toes.
The skeleton is generally massive, indicating great strength but inactive habits; the thoracic cavity is enormous, in proportion to the great bulk and weight of the viscera; the limbs are robust, though adapted for running in the smaller members like the hog; there are no clavicles; the peculiarities of the skeleton and teeth are given in the articles devoted to the different animals. The stomach is generally simple, and the intestines very long and voluminous, in accordance with the bulky and vegetable character of their food; the brain is well developed, and the complexity of the convolutions ranks them with the subclass gyrencepliala of Owen, but, with the exception of the hog, below the ruminants and carnivora; the nasal apparatus is richly endowed with nerves, forming a delicate organ of touch, and in some of prehension. They occur in the warmer climates of all parts of the world except Australia, in the present epoch.