XIII PHYLUM CHORDATA 347 



which are termed in their order from before backwards epiplastra, 

 hyoplastra, hypoplaMra and odphiplccstra. The median element, 

 probably corresponds to the episternnm of other Reptiles, the first 

 pair (epiplastra) to the 

 clavicles, the others pro- 

 bably being of the same 

 character as the abdominal 

 ribs of the Crocodilia. 



The carapace of the 

 Luth or Leather-backed 

 Turtle {Dermatochelys or 



Sphargis) is distinguished ^,„, osr.-Chelone midas. Transverso section of 



from that of the rest of skeleton, a cosUl platc ; CI eontrum ; M. mar- 



, - ... ginal plate ; P. lateral element of plastron ; R. nb ; 



the order in being com- v. expanded neural plate. (After Huxley.) 



posed of numerous poly- 

 gonal discs of bone firmly united together, and in not being con- 

 nected with the endoskeleton ; in the plastron the median bone 

 is absent. 



Carapace and plastron are firmly fixed together by bony union in 

 most instances, but sometimes the connection is ligamentous. 



The sternum in the Lacertilia is a plate of cartilage with a bifid 

 posterior continuation. In the Ophidia and Chelonia it is absent. 

 In the Crocodilia it is a broad plate with a posterior continuation 

 which bifurcates posteriorly. 



A series of ossifications — the abdominal ribs — lie in the wall of 

 the abdomen in the Crocodilia (Fig. 983, Sta), and similar ossifica- 

 tions occur also in the Monitors and in Sphenodon. As already 

 noticed, the posterior elements of the plastron of the Chelonia are 

 probably of a similar character. 



In the skull ossification is much more complete than in the 

 Amphibia, the primary chondrocranium persisting to a considerable 

 extent only in some Lizards and in Sphenodon ; and the number 

 of bones is much greater. The parasphenoid is reduced, and its 

 place is taken by the large basioccipital, basisphenoid, and pre- 

 sphenoid. 



A fairly typical Lacertilian skull has been described in the case 

 of Lacerta. Its principal characteristic features are the presence of an 

 interorbital septum, the presence'ef the epipterygoid andthe mobility 

 of the quadrate. The laSt of these features it shares with that of 

 the Ophidia. The epipterygoid is not universal in the Lacertilia, 

 being absent in the Geckos, the Amphisbsenians, and the Chamee- 

 leons. The skull of the Chamaeleons has a remarkable helmet- 

 like appearance owing to the development of processes of the 

 squamosal and occipital regions, which unite above the posterior 

 part of the cranial roof. The skull of the Amphisbsenians differs 

 from that of other Lacertilia and approaches that of Snakes in the 

 absence of an interorbital septum. 



