A FOSSIL CHEL03HAN REPTILE, FROM THE MIDDLE PURBECKS. 6? 



the islands of Mauritius and Eodriguez, but are now extinct. 

 The Chelonian family (Tortoise and Turtle), in common with the 

 Reptilian Order, are cold-blooded and air breathers. Like the 

 Avian Order, the skull is articulated with the vertebral column, 

 by a single condyle ; a peculiar bone, called the quadrate lone, 

 unites the lower jaw with the skull ; it is fixed, and forms the 

 greater portion of the tympanic cavity. The jaws are unarmed 

 with teeth, their office being fulfilled by a horny cutting sheath, 

 similar to the bill of a bird. 



The shell of a Tortoise is formed by an osseous structure, 

 covered over with an epidermal coating ; the bones participating 

 in the formation of the shell are the vertebrae, the sternum, and 

 the ribs, the latter being so dilated that their edges form sutures 

 with the next in succession ; the humeral and pelvic bones are 

 included within the shell. The vertebral column is free only, 

 and moveable at its caudal and cerebral portion?. The dorsal 

 and lumbar vertebrae being useless as a support to the body, 

 or as an attachment to the muscles, are only of a secondary 

 importance, owing to the development of the carapace, and are 

 much reduced, the limbs too are considerably modified. The 

 scapular arch is three-branched, so as to admit the attachment of 

 powerful muscles, which afford additional strength to the fore- 

 limbs, -nrhich its habits of life required. The plastron or breast- 

 shield is composed of four pairs of bones, and a single one, the 

 precise nature of which is uncertain; in their relation to the 

 architype they probably represent the sternum in part, the rest 

 being integumentary ossifications. The carapace or upper shell 

 consists of a series of vertebral plates, with a series of costal 

 plates on either side, also a marginal series, which has LO 

 correspondence with the bones of the endoskeleton. 



The Chelonian Order is divided into four groups : 



1. Land Tortoises. Testudinulae. 



2. Freshwater Tortoises. Eu.d'dpc. 



3. Freshwater Turtles. Tryoni 'd e. 



4. Sea Turtles. Cheloi:ida>. 



