_cs_, Chin-shields (anterior); _cs´_, chin-shields (posterior); _f_, frontal; _in_, internasal; _l_, loreal; _la_, labial (upper); _la´_, labial (lower); _m_, mental; _n_, nasal; _p_, parietal; _pf_, prefrontal; _pro_, preocular; _pto_, postocular; _r_, rostral; _sbo_, subocular; _so_, supraocular; _t_, temporals (first row); _t´_, temporals (second row); _v_, first ventral.] The periodical shedding of the outer layer of the epidermis in a single piece, including even the covering of the eye, is one of the most striking peculiarities of snakes, although paralleled in the Lizards of the family Anguidæ, to which our British Slow-worm belongs. The skin becomes detached at the lips, and is turned inside out from head to tail, without any sort of laceration when the snake is in good health. These exuviæ are transparent, but often carry a certain amount of pigment, especially those of the Vipers, in which the characteristic dark markings are perfectly visible; they usually exceed the length of the reptile, owing to stretching. In Sea-snakes the epidermis is cast piecemeal, and sloughing is a longer operation than in ordinary snakes. In Rattlesnakes each piece of the rattle, or "crotalon," in which the tail terminates, represents a retained portion of the sloughed epidermis. This remarkable appendage looks like a number of horny rings, but it consists in reality of hollow, bell-like pieces, similar to the terminal one, or "button," each with a circular constriction, in which the incurved free edge of the following piece fits, thus keeping the pieces together without impairing the mobility necessary to produce the rattling sound for which the apparatus is intended. At each exuviation one bell-shaped horny piece is added. The number of segments in the rattle is, therefore, not an index to age, as formerly believed; nor is it to the number of exuviations, for whilst segments are being added at the base of the apparatus the terminal ones break off and are lost. A _Crotalus_ sixteen months old may have six pieces to the rattle if there have been six exuviations and no loss. No rattle appears ever to comprise more than about twenty pieces, even in old specimens. The size of the terminal button shows whether it was formed at birth or at any later period, no growth taking place in the horny tissue.