EXOSKELETON 



15 



plate which extends to the margin of the carapace and is fused to the 

 rib below. The margin of the carapace is formed by a series (11 

 to 12) of marginal plates, passing in front and behind into the 

 nuchal and pygal. Outside of this osseous carapace are cuticular 

 horny plates which correspond to neurals and costals neither in 

 number or position. 



The plastron normally consists of nine bones (also with cuticular 

 plates) : an anterior unpaired entoplastron (fig. 13, 5) succeeded by a 

 pair each of epi-, hyo-, hypo- and xiphiplastra. Occasionally 

 (Dermochelydae, Cinosternidae) the entoplastron is lacking, while 

 some have a mesoplastron intercalated between hypo- and hyo- 

 plastra of either side. 



A B 



Fig. 13. — Carapace (A) of Chelopus insculplus (Parker, 'oi) and plastron of 

 Trionyx (B). c, costals; en, endoplastron; ep, epiplastron; hy, hyoplastron; hyp, hypo- 

 plastron; m, marginals; n, nuchal; p, pygal; v, vertebrals; xp, xiphi plastron. 



From this typical (not primitive) condition many variations occur. Usually 

 the costals of either side extend to the marginals, but in some sea turtles and in 

 Trionychidae the costals do not extend as far as do the ribs, leaving gaps between 

 them and the marginals. In still others the costals are narrower, leaving gaps 

 from marginals nearly to the vertebral column, while in some Trionychidae (which 

 lack cuticular scales) marginals may be absent. The Dermochelydae afiford a 

 problem. The plastral bones are small, forming a ring with a gap in the centre, 

 marginals are lacking and costals are reduced. In the skin overlying these last 

 are numbers of polygonal plates which articulate with each other, and are 

 arranged in longitudinal rows. The plates in seven of these rows are larger 

 than the others and each plate is keeled. Some Emyidae have the anterior 

 and posterior plastral plates hinged to the others and are capable of being closed 

 against the carapace (box tortoises). 



