The skeleton is mainly adapted for a terrestrial mode of life, and the hind limbs have the normal mammalian position. In almost every case the number of incisors is 3/3. Each jaw always has one specially modified _carnassial_ or sectorial tooth which bites like a scissors blade against a corresponding tooth in the other jaw. In front of it the teeth are always more or less pointed, while behind it they are more or less broadened and tuberculated. In the manus the first digit, and in the pes the first and fifth digits are never longer than the rest, and the digits of both limbs are almost invariably clawed. Some forms are plantigrade, some digitigrade, some subplantigrade. The group includes all the ordinary terrestrial Carnivora, and is divided into three sections:-- Æluroidea[135], including the cats, civets, hyaenas, and allied forms. Cynoidea, including the dog tribe. Arctoidea, including the bears, raccoons, weasels, and allied forms. _Suborder_ (3). PINNIPEDIA[136]. In this suborder the limbs are greatly modified and adapted for a more or less purely aquatic life, the proximal and middle segments of the limbs are shortened, while the distal segment, especially in the leg, is much elongated and expanded. There are always five well-developed digits to each limb, and in the pes the first and fifth digits are generally larger than the others. The digits generally bear straight nails instead of claws, but even nails are sometimes absent. There is no carnassial tooth, and the teeth in other ways differ considerably from those of Carnivora vera. The incisors are always fewer than 3/3; while the cheek teeth generally consist of four premolars and one molar, all of very uniform character, being compressed with conical crowns, and never more than two roots. The suborder includes three families--Otariidae (Eared Seals), Trichechidae (Walrus), and Phocidae (Seals). _Order 7._ INSECTIVORA[137]. This order contains a large number of small generally terrestrial mammals. The limbs are plantigrade or subplantigrade, and are generally pentedactylate. All the digits are armed with claws, and the pollex and hallux are not opposable. The teeth are diphyodont, heterodont, and rooted. The cheek teeth have tuberculated crowns, and there are never less than two pairs of incisors in the mandible; often the incisors, canines, and premolars are not clearly differentiated from one another, and special carnassial teeth are never found. The cranial cavity is small, and the facial part of the skull is generally much developed; often the zygomatic arch is incomplete. Clavicles are well developed (except in _Potamogale_), and the humerus generally has an ent-epicondylar foramen. The femur frequently has a ridge representing the third trochanter. There are two suborders: _Suborder_ (1). DERMOPTERA.