A very marked feature of the terrestrial Carnivora is to be found in the structure of the teeth. The incisors are nearly always six, and are somewhat feebly developed in many cases. The canines are almost invariably very large strong teeth, and are always present. In some of the extinct Cats they reached enormous dimensions. The number of cheek teeth is not always identical; but the last premolar in the upper jaw and the first true molar in the lower jaw, known as the "carnassial" or "sectorial" teeth, mark a difference in structure between the anterior and the posterior crushing teeth; those in front of the carnassial tooth have cutting edges, and are often merely small, conical teeth; those behind have broader crowns and are tuberculate; those of simpler forms often trituberculate; those of others {388} with numerous tubercles. The carnassial tooth is often, but by no means always, very much larger and especially longer than the rest of the molar and premolar series. It is less pronounced in some of the omnivorous Arctoidea. The skull of the Carnivora is longer in the more primitive types, such as the Canidae, and shorter in the more specialised Felidae. The orbit is hardly ever completely shut off by bone, though the postorbital process of the frontal sometimes approaches the corresponding upward process of the zygomatic arch. The palate, which is completely ossified, sometimes reaches back for some distance behind the teeth; it always extends as far as the last molar. The tympanic bulla is often very inflated, and if flatter, as in the Bears, is at any rate large and conspicuous. The lower jaw has a high coronoid process, and the condyle is transversely elongated, this part of the bone being rolled into an almost cylindrical form; it fits very closely into the glenoid cavity, and the articulation is thereby very strict--an obvious advantage in a creature with so great a need for power of jaw. [Illustration] FIG. 191.--A, Atlas of Dog. Ventral view, x 1/2. B, Axis of Dog. Side view, x 2/3. _o_, Odontoid process; _pz_, posterior zygapophysis; _s_, spinous process; _sn_, foramen for first spinal nerve; _t_, transverse process; _v_, vertebrarterial canal. (From Flower's _Osteology_.)