The theory that the rattle is the remains of cast-off cuticle, as some herpetologists have supposed, may be dismissed at once; for what would cause such vestiges to harden into a complicated and symmetrical form? To Dumeril we owe some of our best conceptions of the growth of the rattle, which, whether it has or has not been evolved from the mere horny spine that terminates the tails of so many snakes, has certainly _now_ an express provision for its production. Like hair, claws, or nails, the rattle is horny matter excreted and hardened. In his _Elementary Lessons in Physiology_, Prof. Huxley shows us how in the growth of a nail new epidermic cells are added to the base, constraining it to move forward. ‘The nail, thus constantly receiving additions from below and from behind, slides forward over its bed and projects beyond the end of the finger.’ If the reader will look at his finger nail, and suppose the end bone of the Crotalus spine to be the ‘bed’ of the nail, he will to a certain extent be able to comprehend how the rattle grows out; but that the links become detached in succession is a phenomenon so astonishing and at the same time so difficult to comprehend, that few naturalists have ventured to state positively how this occurs. Conjecturally only and diffidently do I, therefore, presume to offer a supposition; and if my readers will once more pardon reference to human nails, and lend the aid of their imagination, they may be able to evolve a true theory out of my crude idea. The young readers of _Aunt Judy’s Magazine_ were also, a few years ago,[84] invited to lend the aid of their pink little finger nails to the illustrative development of a supposed rattle; and we will again imagine the whole tip of a finger to be covered with a round nail-cap, proceeding from the first joint, and to have grown so from birth. In growing out, this curious, cup-like nail, being never cut, would become hollow like a thimble. Pointed or tapering it would of course be, because, as the baby finger grew, the base or new portion of nail grew larger with it. We will also suppose that the joint whence the nail sprang was in constant activity, and so articulated that it _could_ move with a quick and regular action or vibration; the hollow nail-cap, having attained a certain size, would become withered, and (as the constant bending of a piece of card or metal in time divides it) would be worn, and at length detached at its base. Meanwhile the growth of nail has not been arrested, but a new cap is forming within. The old, dry, and withered cap has now nothing to retain it, and would drop off, on account of its simple, conical form, like a loose-fitting thimble. But Dumeril explains to us that the terminal bones of the rattlesnake’s spine present a peculiar form, several of them coalescing.