XIII 



PHYLUM CHORDATA 



323 



are likewise immovable ; a short distance from its origin each 

 passes into a large bony costal plate (67), and the series of costal 

 plates uniting by their edges form a large part of the carapace on 

 either side of the row of neural plates. The carapace is made 

 up of the neural and costal plates supplemented by a row of 

 marginal plates (Figs. 930 and 931, in) running along the edge, 

 and nuchal (Nil) and pyyal (Py) plates situated respectively in 

 front of and behind the row of neural plates. 



The bony elements of the plastron of the Chelonia are an 

 anterior and median plate and six pairs of plates the six pairs 

 probably being of similar nature to the abdominal ribs of the 

 Crocodilia. 



The carapace of the Luth or Leather-backed Turtle (Derma- 

 tochelys) is distinguished from that of the rest of the order in being 

 composed of numerous polygonal discs of bone firmly united 



together, and in not being 

 connected with the en- 

 doskeleton ; and in the 

 plastron the median bone 

 is absent. 



The sternum in the 

 Lacertilia is a plate of 

 cartilage with a bifid pos- 

 terior continuation. In 

 the Ophidia and Chelonia 

 it is absent. In the 

 Crocodilia it is a broad 



plate with a posterior continuation or hypostemum, extending 

 backwards as far as the pelvis. 



A series of ossifications the abdominal ribs, with a mesial 

 abdominal sternum- -lie in the wall of the abdomen in the Croco- 

 dilia (Fig. 927, Sta), and similar ossifications occur also in the 

 Monitors and in Hatteria. The 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 consider- 

 able extent only in some Lizards and in Hatteria, and the number 

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

 place is taken by large basi-occipital, basi-sphenoid, 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 inter-orbital septum, the presence of the epipterygoid and the 

 mobility of the quadrate. The last of these features it shares with 

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

 being absent in the Geckos, the Amphisbaenians, and the Chamse- 

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



FIG. 931. Chelone midas. Transverse section of 

 skeleton. C. costal plate ; C 1 . 1 centrum ; M. mar- 

 ginal plate ; P. lateral element of plastron ; R. rib ; 

 T. expanded neural plate. (After Huxley.) 



