xii PHYLUM CHORDATA 441 



row of neural plates (Fig. 250, V), thus formed constitutes 

 the median portion of the carapace. The ribs are likewise 

 immovable; a short distance from its origin each passes 

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

 plates uniting by their edges form a large part of the 

 carapace on either side of the row of neunl plates. The 

 carapace is made up of the neural and costal plates supple- 

 mented by a row of marginal plates (Fig. 249, AT, and 250, M) 

 running along the edge, and nuchal (Nu) and pygal (Py) 



FIG. 250. Chelone midas. Transverse section of skeleton. C. costal plate; C 1 . 

 centrum ; M. marginal plate ; P. lateral element of plastron ; R. rib J V. ex- 

 panded neural plate. (After Huxley.) 



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 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 or hyposternum, 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 

 Crocodilia (Fig. 248, Sta\ and similar ossifications occur also 

 in the Monitors and in Hatteria. The paired elements 



