ORDER VI 



CHELONIA 



191 



the series of neural plates is a large hexagonal plate, the nuchal, which is 

 broader than long, and situated above the first dorsal vertebra. This plate, 

 which is present in all Chelonians, is a cartilage bone, developed simultaneously 

 with the neurals, and has been supposed to represent the modified ribs of the 

 last cervical. Continuing the series of neurals behind are two to four un- 

 paired membrane bones termed the pygal plates or " postneurals," the last of 

 which, in shape and position, forms part of the peripheral series. In addition 

 to the posterior azygous peripheral or pygal, there are eleven (exceptionally 

 twelve) peripheral plates on each side, except in Staurotypus and Cinosternum, 



Fig. 297. 



Chelone rixidas, Latr. Recent ; Atlantic Ocean. A , 

 Dorsal aspect of carapace. B, Section of shell. C, 

 Ventral aspect of plastron, ci — c8, Costal plates ; e, 

 Entoplastron ; cp, Epiplastron ; hyp, Hyoplastron ; 

 hpp, Hypoplastron ; m, Marginals ; 7i, Expanded neural 

 spine ; nu, Xuchal ; p, Plastron ; py, First suprapygal ; 

 )', Ribs ; w, Vertebral centrum ; xp, Xi phi plastron ; 

 1 — 8, Xeiiral plates, followed by two suprapygals and 



pygal- 



which have but ten. The peripherals and pygals owe their origin entirely to 

 dermal ossification. 



There are normally eight pairs of neural plates, but some fossil marine 

 forms have nine or ten. The first neural is constantly the broadest, the last 

 the smallest; in a few forms these plates are of unequal width, alternately 

 widening proximally and narrowing distally. The distal extremity of the rib 

 persists as a free point fitting into a corresponding socket of the peripheral 

 plate. In a number of forms there are persistent fontanelles or vacuities 

 between the neural and peripheral plates, and also in the plastron. 



The plastron arises exclusively from dermal ossification, and is entirely 

 independent of the pectoral arch, with which it has sometimes been homo- 

 logised. It consists usually of nine bones, a median anterior entoplastron (or 

 " interclavicular "), and on each side following this in the order named, an 

 epiplastron, a hyoplastron, a hypoplastron, and a xij^hiplastron. Some 

 authors have regarded the epiplastra as equivalent to the clavicles, the 

 entoplastron to the interclavicle, and the other elements as modifications of 



