404 



CHELOXIA. 



attached to the side of the nuchal, tlie last marginal is in the 

 middle line and attached to the last pygal. Of these plates the 

 nuchal, pygal, and marginal used to be considered as purely der- 

 mal structures, the neui'als and costals benig regarded as expan- 

 sions of the internal skeleton. 



The 'plastron (Fig. 226) or ventral part of the shell is 

 attached directly or by ligament to the marginals and is quite 

 separate from the internal skeleton. It consists, like the rest 

 of the shell, of purely dermal bones, some of which lie behind 

 and some in front of the umbilicus. It is therefore partly a 

 thoracic and partly an abdominal structure. In the turtle it con- 

 sists of nine pieces — a median entoplastron (interclavicle), and four 

 Tpaired pieces, the epi pi astro {davicles), the kyoplastra, the hypo- 



plastra and the xiphiplas- 

 tra (Fig. 226). It is sup- 

 posed that the entoplas- 

 tron and epiplastra 

 correspond to the inter- 

 clavicle and clavicle of 

 other forms. In some 

 Chelonia the pieces of the 

 plastron are in contact 

 by their whole margin 

 and form a continuous 

 plate {Tesiudinidae, etc.). 

 The shell is covered externally by the horny epidermal 

 shields. These are applied to the outer surface of both 

 the carapace and plastron. They are regularly arranged 

 but by no means correspond with the subjacent bony 

 pieces, on the surface of which they leave sutural markings. 

 There is some variation in their arrangement, but typically there 

 is on the dorsal surface, a median row of five shields {vertebral), two 

 lateral rows of four shields {costal), and a marginal row of twenty- 

 four or twentj'-five shields {marginals) of which the anterior 

 median is called nuchal, and the posterior pygal or supracaudal. 

 On the ventral surface are six pairs of shields, called, from before 

 backwards, the g^ilar, humeral, pectoral, abdominal, femoral, and 

 anal. In front of tlie gulars, there is usually a paired or un- 

 paired intergular, and ventral to the marginal shields, between 

 them and the shields of the ventral surface there is a variable 



FiCr. 225. — Transverse section of the skeleton of 

 Chelone midas in the dorsal region (from Huxley). 

 C centrum; V neural plate; C costal plate; "/J 

 rib ; M marginal plate ; P lateral element of the 

 plastron. 



