62 ANATOMY OF VERTEBRATES. 



The median pieces, s 1 to s n, are called the 'neural' plates; 

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

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

 >/i i to /// 12; the anterior symmetrical piece, ch, constant in all 

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

 piece, pt/, which is wanting in all the Trionycidce, is the ' pygal ' 

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

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

 pair of free ribs 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, which they help 

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

 being those which are more immediately connected with the neural 

 and costal plates, are the ' vertebrae of the carapace : ' their 

 characters, though not less artificial than those which distinguish 

 the ' dorsal ' or ' lumbar ' vertebrae of other reptiles, are much 

 more marked and constant. The eighth vertebra of the cara- 

 pace is succeeded by one, which in some species (e. g. Clielone 

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

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

 followed by two other vertebrae, with short and thickened ribs, 

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

 fig. 51, G : as these three vertebrae are not immediately united with 

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

 than the first dorsal vertebra to be regarded as entering 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, s, is the ' entosternal ; ' the four 

 pairs, counted from before backward, are respectively, the 

 ( episternals ' (es), ' hyosternals ' (hs,) ' hyposternals ' (/«), and 

 e xiphisternal ' (.rs). 



In all the Chelonians, save the coriaceous (Sphargis) and soft 

 turtles ( Trionycidai), 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 l is placed on the first ' vertebral 

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

 and v 2 to v 5 indicate the succeeding vertebral scutes, the outer 

 angles of which are similarly wedged between the adjoining pairs 

 of ' costal scutes : ' beyond the costal scutes are a series of ' mar- 



