xii PHYLUM CHORDATA 443 



of the trunk, usually ten in number, are immovably united 

 together. Each of the neural spines, from the second to 

 the ninth inclusively, is expanded into a flat plate, and the 

 row of neural plates (Fig. 266, 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 cara- 

 pace on either side of the row of neural plates. The cara- 

 pace is made up of the neural and costal plates supplemented 

 by a row of marginal plates (Figs. 265 and 266, M) running 

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



Fig. 266. — Chelone midas. Transverse section of skeleton. C, costal plate; C, 

 centrum; M, marginal plate; P, lateral element of plastron; R, rib; I', expanded 

 neural plate. (After Huxley.) 



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 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. 



