_b._ The =dermal exoskeleton=. This has the form of bony =scutes= which underlie the epidermal scales along the dorsal surface of the trunk and anterior part of the tail. Except in very young individuals the epidermal scales are rubbed off from these scutes, which consequently come to project freely on the surface of the body. Each scute is a nearly square bony plate, deeply pitted or sculptured, and marked by a strong ridge on its dorsal surface, while its ventral surface is smooth. Contiguous scutes are united to one another by interlocking sutures. The scutes are arranged in two distinct areas, viz. (1) a small anterior =nuchal shield= which lies just behind the head and is formed of six large scutes more or less firmly united together, and (2) a larger posterior =dorsal shield= covering the whole of the back and anterior part of the tail, and formed of smaller scutes, which are arranged in regular transverse rows, and progressively diminish in size when followed back. The =teeth= are exoskeletal structures, partly of dermal, partly of epidermal origin. They lie along the margins of the jaws and are confined to the premaxillae, maxillae and dentaries. They are simple conical structures, without roots; each is in the adult placed in a separate socket, and is replaced by another which as it grows comes to occupy the pulp cavity of its predecessor. In the young animal the teeth are not placed in separate sockets but in a continuous groove. This feature is met with also in the Ichthyosauria. The groove gradually becomes converted into a series of sockets by the ingrowth of transverse bars of bone. The anterior teeth are sharply pointed and slightly recurved, the posterior ones are more blunt. The upper jaw bears about nineteen pairs of teeth, the lower jaw about fifteen pairs. The largest tooth in the upper jaw is the tenth, and in the lower jaw the fourth. The three living families of Crocodilia, the Crocodiles, Alligators and Garials, can be readily distinguished by the characters of the first and fourth lower teeth. In Alligators both first and fourth lower teeth bite into pits in the upper jaw; in Garials they both bite into notches or grooves in the upper jaw. In Crocodiles the first tooth bites into a pit, the fourth into a notch in the upper jaw. II. ENDOSKELETON. 1. THE AXIAL SKELETON. This includes the vertebral column, the skull, and the ribs and sternum. A. THE VERTEBRAL COLUMN. The vertebral column is very long, consisting of some sixty vertebrae. It can be divided into the usual five regions, the cervical, thoracic, lumbar, sacral, and caudal regions. [Illustration FIG. 41. FIRST FOUR CERVICAL VERTEBRAE OF A CROCODILE (_C. vulgaris_). (Partly after VON ZITTEL.)