The beak is simply a horny tract of skin which has become hardened for its special uses. It is not even distinctive of the bird; for turtles, particularly the snapping turtles, have beaks which are not only precisely like those of birds, but are equally effectual when turned to aggressive ends. It is a commonplace of knowledge that the bill or beak presents an almost endless variety of form, which is associated with an equally diversified use. The remarkable shovel-shaped bill of the duck is suitable for dabbling in soft mud, just as is the hooked beak of the hawk or owl for tearing living prey. The most prevalent form of bill is that possessed by most passerine birds, a conical longer or shorter bill. The relatively enormous beak of the toucan is serrated along the free edge, which enables its possessor to obtain a firmer grasp of the fruits upon which it feeds. The ridges upon the inner surface of the beak in the ducks serve an analogous purpose; the same structure is seen in the bill of the Flamingo, though the outline of the bill is unlike that of the duck, and gave rise to the idea, or at any rate had something to do with the former impression, that the flamingo was a long-legged duck. But, as a matter of fact, there is a stork in which there is precisely the same ridging of the beak, and it is more usual now to place the flamingo among the storks, or near to them. The Spoonbill, as its name denotes, has a beak which is at the extreme of the series of beaks which are useful for sifting the mud at the bottom of pools and rivers; the extremity is widened and flattened out. Most singular is the recurved bill of the Avocet, and equally so the under-jawed Rhynchops, the terms used implying the peculiarities in each case. There is no living bird which lacks a beak; but in some of the extinct and toothed birds, which are again referred to later, the beak was absent. Its place was taken in them by the teeth. Feet. [Illustration: FIG. 5.--FOOT OF PELICAN.] [Illustration: FIG. 6.--FOOT OF PERCHING BIRD.] [Illustration: FIG. 7.--FOOT OF KINGFISHER.]