xrii 



PITYLUM CTTORDATA 



••'.-17 



Fin. OST.— Chelone ixiidas. Transverse section of 

 skeleton. C. costal plate; C. ' ecntriini ; 3/. mar- 

 ginal plate ; V. latei-al clement of jjlastron ; /{. rib ; 

 V. expanded nevu'al plate. (After Iluxley.) 



which aro tevnicil in their order from before backwanhs r.piphistrn, 



hyopJadra, hypopladra and xiphiphi^rn. The median element, 



probably corresponds to the episternnm of otlier Reptiles, the first 



pair (epiplastra) to the 



clavicles, tiie others pro- C ^..^^tf^s^r^ 



bably being of the same 



character as the abdominal 



ribs of the Crocodilia. 



The carapace of the 

 Luth or Leatlier-backed 

 Turtle (Dcnnatochc/i/s or 

 Sphargifi) is distinguished 

 from that of the rest of 

 the order in being com- 

 posed of numerous pol}^- 



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 sternuvh 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 wdth a posterior continuation 

 which bifurcates posteriorly. 



A series of ossifications — the ahdominal rihs — lie in the wall of 

 the abdomen in the Crocodilia (Fig. 083, 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 iil'ull 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 chai-acteristic features are the presence of an 

 interorbital septum, the presenceof theepipterygoidandthe 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 Amphisba^nians, and the Chama?- 

 leons. The skull of the Chama3leons 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 Amphisba^nians differs 

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

 absence of an 'interorbital septum. 



