CHAPTER XVI.
GENERAL ACCOUNT OF THE SKELETON IN REPTILES.
EXOSKELETON.
The exoskeleton both epidermal and dermal is exceedingly well developed in reptiles.
Epidermal Exoskeleton.
This generally has the form of overlapping horny scales which invest outgrowths of the dermis, and are found covering the whole body in most Rhynchocephalia, Ophidia, and Lacertilia, and many Crocodilia. In the Ophidia the ventral surface of the tail is commonly covered by a double row of broad scales, while the ventral surface of the precaudal part of the body is covered by a single row. In the burrowing snakes (Typhlopidae) and some sea snakes (Hydrophidae) these broad scales do not occur, the scales of the ventral surface being similar to those of the dorsal.
In the Chelonia with the exception of Dermochelys, Trionyx and their allies there is a well-developed system of horny shields having a regular arrangement which has been described in the account of the Turtle's skeleton[91].
The rattle of the rattlesnake is an epidermal structure formed of several loosely articulated horny rings, produced by the modification of the epidermal covering of the end of the tail, which instead of being cast off when the rest of the outer skin is shed is retained loosely interlocked with the adjoining ring or joint. New rings are thus periodically added to the base of the rattle, and in old animals the terminal ones wear away and are lost.
Horny claws occur on the ends of some or all of the digits in most living reptiles.