594 REPTILIA [CH. 



with them both. The transverse process is represented by the 

 expansion of the neural plate which meets the costal plate. The 

 almost horizontally directed outer ends of the ribs are received into 

 a series of dermal bones called marginals, which form the edge 

 of the carapace. 



The plastron is formed of one unpaired and several paired bones 

 (Fig. 293) The median bone, called the entoplastron, is believed 

 to correspond to the interclavicle of other Reptiles. The first pair 

 are called epiplastra and probably represent the clavicles of 

 other forms. The posterior pairs are called hyoplastra, hypo- 

 plastra and xiphiplastra respectively; they are firmly joined to 

 the marginals. 



In front and behind the plastron and carapace are separated by 

 soft flexible skin ; their edges project so as to form roof and floor 

 to cavities into which the head and neck and arms in front and the 

 legs and tail behind can be withdrawn. A study of the develop- 

 ment of modern Chelonia and of the anatomy of fossil species makes 

 it probable that the ancestors of the present forms were provided 

 with ,a carapace composed entirely of osteoderms underlying the 

 horny scales, just as is the case with Crocodilia. This dermal cara- 

 pace however was gradually replaced by the development of bony 

 expansions of the ribs and neural arches ; though remnants of it 

 persist in the nuchal, pygal and marginal plates. 



There is no trace of sternal ribs or sternum ; but the pectoral 

 and pelvic girdles occupy the peculiar position of being within 

 instead of outside the ribs, a consequence of the almost horizontal 

 direction of these. The girdles are in fact converted into pillars or 

 struts which keep the plastron and carapace apart. In front the 

 scapula forms a vertical pillar which has a ventral process the 

 acromion projecting inwards beyond the articulation with the 

 coracoid. This process is unique among recent Reptilia but existed" 

 in the extinct Plesiosauria. The coracoid slopes backwards and 

 inwards. The ilium and pubis serve to support the carapace 

 posteriorly. The pelvic girdle is similar to that of a Lizard but 

 the pectoral girdle has no epicoracoid. The limbs are essentially 

 similar to those of the Lizard but the toes are shorter and blunter. 

 The neck is extraordinarily flexible ; the vertebrae composing it fit 

 one another by cup and ball joints, one is amphicoelous, the next 

 is biconvex and so on. The dorsal vertebrae have flat faces. 



The skull is devoid of teeth and the prem axilla and maxilla 

 are short. Both they and the dentary have sharp cutting edges 

 ensheathed in horn so as to form a beak. In all species the orbit is 



