All Bats, except a few tropical genera, feed on insects, and possess teeth like those of the preceding order of Mammals (p. 30). The leading feature is the characteristic modification of the fore limbs into a flying apparatus. The bones of the fore arm (Fig. 18, _u, r_), the metacarpels (_mc_), and the phalanges (except in the case of the thumb, which possesses a sharp claw (_p_)) are of great length; and between the long fingers, between the fore and hind limbs, and, last of all, between the two hind limbs there is an elastic membrane, serving both for flight and touch. Sight ill developed, since the bat is a nocturnal animal; a delicate sense of touch has its seat not only in the flying-membrane, but also in the skin of the ears, which are often very large, and in the membranous flaps which, in a few genera (the “leaf-nosed” bats), occur on the nose and lips. As is well known, bats sleep in the day; and they also hibernate in chimneys, hollow trees, ruins, and other similar places of concealment. They principally devour night-flying moths, and spiders; and, since they use a great quantity of nourishment, are of great service, since the caterpillars of many of the species they destroy are very injurious to agriculture or forestry. About nine species live in Britain, but there is no use in enumerating them here. ORDER: =Rodentia= (GNAWING MAMMALS). [Illustration: FIG. 19.—Skull of Squirrel. ] Two long incisors (Fig. 19), the crowns of which are continually being worn down, while a corresponding growth takes place at the root-end. These incisors are used for gnawing, in which process the lower jaw is rapidly moved backwards and forwards. Gnawing wears down these teeth less in front than behind, owing to the presence of a thick layer of enamel in the former position. Their crowns, therefore, maintain a chisel-edge. That the incisors never stop growing is clearly seen when the usual wearing down does not take place, as, _e.g._, when the lower jaw is placed obliquely under the upper jaw, or when a tooth is absent in one jaw, under which circumstances the corresponding incisor in the other jaw is not worn down. In such a case the incisors continue to grow, ultimately curving upwards or downwards, and becoming tusk-like structures (Fig. 20). The Rodents have no canines. In those Rodents which feed both on animal and vegetable food (the “Omnivora,” _e.g._ squirrel, common mouse, brown rat, etc.), the crowns of the back teeth are completely covered with enamel; in the purely vegetable feeders (“Herbivora,” _e.g._ hare, rabbit), they are compound teeth (p. 22). In most Rodents the hind feet are longer than the fore, giving a springing gait. Eyes large. Many forms have “cheek-pouches,” in which the food they obtain can be stored up for some time. When the pouches are full, a muscle contracts by which their ends are drawn backwards; they are emptied by the animal pressing them with its fore feet. The majority of