The feet of birds are as varied in different species as are their wings. In birds of prey the claws are powerful and hooked. In some the foot is flat and the claw straight and adapted for walking (Figs. 56, 57, 58). The great toe is generally the strongest, but this is not an absolute law: a projection which is found on the leg of some birds, and is designated a spur, is a formidable weapon in some species (Figs. 59, 60). Some birds walk by bringing their feet forward alternately; others by a simultaneous motion, or a succession of leaps. Some run with great velocity, while others walk with great difficulty, and that only on a flat surface. Many have their toes joined by thin membranes, which act as paddles by which they propel themselves through the water. The beak, or bill, of birds is composed of two bony pieces, called mandibles, surrounded by a horny substance, differing both in form and thickness according to the habits of the species. In the genus FALCO the bill is shorter than the head; the upper mandible is furnished at the base with a bare coloured skin, of a peculiar dense texture, called the cere; its outline slightly convex as far as the edge of the cere (Fig. 61), then curved so as to form about the third of a circle, and evidently destined, in connection with its formidable claws, to tear its prey. [Illustration: Fig. 61.] [Illustration: Fig. 62.] [Illustration: Fig. 63.] In the TOUCANS RAMPHASTIDÆ the bill is half a foot long, hollow within, thin, and nearly transparent; and the mandibles are so disposed as to combine, with their great bulk, strength and lightness, and assisting by their digestive power to assimilate both animal and vegetable food (Fig. 62). In the PELICANIDÆ, as in the Common Cormorant, _Phalacrocorax carbo_, the bill is long, straight, and compound; the upper mandible curved towards the point, the lower compressed; the base inserted in a small membrane which extends under the throat. In the back part of the head is an additional bone (Fig. 63, _a_), attached in such a manner to the occiput as to admit of great expansion, which permits of its swallowing plaice and other flat fish of considerable size. The CRANE, _Grus cinerea_ (Fig. 64), has the bill rather longer than the head, strong, straight, compressed, and pointed at the extremity; the sides of the mandible deeply channelled with nostrils, and closed backwards by a thin membrane. [Illustration: Fig. 64.]