358 REPTILIA. 



immovably anchylosed together, and the two rami are also 

 united in front by a true bony union. There are also no 

 teeth, and the edges of the jaws are simply sheathed in horn, 

 constituting a sharp beak. As regards the second of these 

 points, the bony case in which the body of a Chelonian is 

 enclosed consists essentially of two pieces, a superior or dorsal 

 piece, generally convex, called the " carapace," and an inferior 

 or ventral piece, generally flat or concave, called the "plastron." 

 The carapace and plastron are firmly united along their edges, 

 but are so excavated in front and behind as to leave apertures 

 for the head, tail, and fore and hind limbs. The limbs and 

 tail can almost always be withdrawn at will under the shelter 

 of the thoracico-abdominal case formed in this way by the 

 carapace and plastron, and the head is also generally re- 

 tractile. 



The carapace or dorsal shield is composed of the flattened 

 spinous processes of the dorsal vertebrae, the expanded ribs, 

 and usually a series of marginal bones the whole covered by 

 horny epidermic plates or by a leathery skin. The plastron 

 or ventral shield is composed of nine bony pieces, which are 

 probably integumentary ossifications, but which are sometimes 

 regarded as composing a modified and greatly - expanded 

 breast-bone. The scapular and pelvic arches, supporting re- 

 spectively the fore and hind limbs, are placed within the cara- 

 pace. As in the Crocodilia, clavicles are wanting. 



From the aquatic habits of many of the members of this order 

 they are by no means uncommon in the fossil condition. The 

 Turtles frequent the sea, and thus come naturally to be fossils 

 in marine deposits ; and the preservation of all the Chelonians 

 alike is rendered easy by the indestructible nature of the case 

 in which their bodies are enclosed. 



The Chelonians may be divided into sections according as 

 the limbs are natatory, are adapted for an amphibious life, or 

 are fitted for terrestrial progression. In the first of these sec- 

 tions are the true Turtles (Chdoniidce), which frequent the 

 sea, and are distinguished by their depressed and flattened 

 carapace, and by their oar-like limbs. In the second section 

 are the River and Marsh Tortoises, comprising the Soft Tor- 

 toises (Trionyddce) and the Terrapins (Emydidee). In the 

 third section are the true Land-tortoises (Testudintdce), distin- 

 guished by their strongly convex carapace, and limbs adapted 

 for walking upon the land. All these three sections are repre- 

 sented in past time, the Turtles, Trionycidce^ and Emydida 

 appearing for the first time, so far as is certainly known, in the 

 Jurassic series, whilst the Testudinida do not appear till the 



