62 ANATOMY OF VERTEBRATES. 



The median pieces, « i to s n, are called the 'neural' plates; 

 the lateral pieces, pi i to pi 8, the ' costal ' plates ; the term 

 ' marginal ' is restricted to those peripheral pieces which form pairs, 

 m 1 to m 12 ; the anterior symmetrical piece, ch, constant in all 

 Chelonia, is called the ' nuchal ' plate ; the posterior symmetrical 

 piece, pij, which is wanting in all the Trionycida, is the ' pygal ' 

 plate. The neural arch, connate with the first neural plate, s j, is 

 supported partly by the centrum of the vertebra to which the first 

 pair of free riljs is articulated, and which, therefore, is reckoned 

 as the first dorsal vertebra : these ribs are small and slender, 

 attached at both their extremities, the outer end abutting against 

 the under part of the first pair of costal plates, wliich they help 

 to sustain. The second to the ninth dorsal vertebras inclusive, 

 being those which are more immediately connected with the neural 

 and costal plates, are the ' vertebrre of the carajjace : ' their 

 characters, though not less artificial than those which distinguish 

 the ' dorsal ' or ' lumbar ' vertebras of other rejitiles, are much 

 more marked and constant. The eighth vertel^ra of the cara- 

 l^ace is succeeded by one, which in some species (e. g. Chelone 

 caouanna) supports a pair of short ribs, in others ( Trionyx) none, 

 and which is therefore reckoned a ' lumbar ' vertebra ; this is 

 followed by two otlier vertebras, with short and thickened ribs, 

 abutting against the iliac bones and representing the ' sacrum,' 

 fig. 51, G : as these three vertebras are not innnediatelv united with 

 the ninth, tenth, and eleventh ' neural plates,' they have less claim 

 than the first dorsal vertebra to be regarded as euterino- into the 

 composition of the carapace. 



The ' plastron,' fig. 53, or floor of the thoracic-abdominal 

 chamber, consists, in all recent Chelonia, of nine pieces. The 

 median and symmetrical piece, .?, is the ' entosternal ; ' the four 

 pairs, counted from liefore backward, are res])cctively, the 

 ' episternals ' (e.s), ' hyostcrnals ' {hs,) ' hyposternals ' {ps), and 

 ' xiphisternals ' {xs). 



In all the Chclonians, save the coriaceous (Splitnyis) and soft 

 turtles ( Trioriyeidce), the outer surface of the carapace is impressed 

 by the horny scutes, commonly called ' tortoise-shell ; ' and these 

 epidermal productions have received definite names in Zoological 

 Treatises, their modifications being found of great use in charac- 

 terising species. In fig. 52, v i is placed on" the first ' vertebral 

 scute ' close to its union with the first and second ' costal scutes • ' 

 and v'2 io V r, indicate the succeedhig ^•crtebral scutes, the outer 

 angles of which are similarly wedged between (lie adj,.inino- pairs 

 of ' costal scutes : ' beyond the costal scutes arc a scries of'^' mar- 



