More than a hundred species of snakes, nearly all having a wide geographical range, are found in America, north of Mexico. They constitute the first order, Ophidia, of reptiles, and have long, cylindrical bodies, are footless, without a shoulder-girdle, and invested with a coat of scales, which is shed in the summer months. Snakes have no eyelids in the strict sense of the term. Their eyes are permanently covered by a delicate membrane that takes the place of the lid, and this explains the stony stare, so disagreeable to many, that all snakes have. The skeleton of snakes is so arranged as to allow the greatest freedom and flexibility. Numerous pieces of bone, hollow in front and convex behind, make up the long tapering backbone, which literally works on a ball-and-socket plan. Articular facets, that lock into each other, are found upon the processes of the vertebræ, and these strengthen and give to the backbone a greater degree of flexibility. A more remarkable arrangement, however, is found in the head, which enables the snake to prey upon animals that are larger than itself. The jaws seem a combination of elastic springs, having no gauge to their tension, the quadrate bones connecting the lower jaw with the skull being movable, thus allowing that enormous gape with which all are familiar who have seen a snake swallow its prey. Besides this, the bones of the jaw itself and palate are more or less movable, also tending to the larger distention of the throat. As snakes do not tear or mutilate their prey, their teeth are not set in sockets, but serve merely to poison and stupefy the prey, or to prevent its escape, acting as hooks by which the body is hauled over the victim. The bones of the lower jaw, as we have seen, are not fastened closely to each other; so in swallowing prey the teeth on one side are advanced, and then those on the other side, and so on until the victim is hauled, hand over hand, as it were, into the snake’s throat. Poisonous snakes, such as the rattlers, have two long, sharp fangs, each compressed and bent up, and forming a hollow tube, open at both ends. The upper portion of the hollow fang is fastened to a bone in the cheek, which moves with ease, so that, when not in use, the fangs can be packed away until needed.