[90] Free use has been made of L.C. Miall's _Studies in Comparative Anatomy_, I., _The Skull in Crocodilia_, London, 1878. See also W.K. Parker, _Tr. Z.S._, vol. XI. 1885, p. 263. 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. Owen's Chameleon bears three epidermal horns, one arising from the nasal and two from the frontal region. In the Chelonia, some of the Theromorpha such as _Udenodon_ and _Dicynodon_, probably also in the Pterosauria and _Polyonax_ among the Dinosaurs, the jaws are more or less cased in horny beaks. The horny beaks of Chelonia are variable; sometimes they have cutting edges, sometimes they are denticulated, sometimes they are adapted for crushing. DERMAL EXOSKELETON. Nearly all Crocodilia, many Dinosauria, some Rhynchocephalia and Pythonomorpha, and some Lacertilia such as _Tiliqua_, _Scincus_ and _Anguis_ have a dermal exoskeleton of bony scutes, developed below and corresponding in shape to the epidermal scales. Sometimes as in _Caiman sclerops_, _Jacare_ and _Teleosaurus_, the scutes completely invest the body, being so arranged as to form a dorsal and a ventral shield, and a continuous series of rings round the tail. In _Crocodilus_ they are confined to the dorsal surface, and in _Alligator_ to the dorsal and ventral surfaces. The scutes of some extinct forms articulate with one another by a peg and socket arrangement as in some Ganoid fish.