ORDER CHELONIA. 



IO85 



logist generally has to deal with what is frequently and conveniently 

 termed the shell, — that is, the carapace and plastron together, — 

 attention must be more particularly directed to its structure in the 

 more typical forms. Commencing with the epidermal skeleton of 

 those forms in which this is fully developed, we find that it con- 

 sists of horny shields, which usually have their edges in apposi- 

 tion, but occasionally overlap (fig. 102 1). On the upper surface 

 there are five vertebral shields in the middle line, on either side of 

 which there are four costals ; while the border has 24 or 25 smaller 

 shields, of which the median anterior one (when present) is termed 

 the nuchal, and the double or single posterior shields are known 



Fig. 1009. — Bones of the plastron of Thalassochelys caretta. Much reduced, es, Epiplastral 

 (clavicle) ; s, Entoplastral (interclavicle) ; /is, Hyoplastral ; ps, Hypoplastral ; xs, Xiphiplas- 

 tral. (After Owen.) 



as the caudals ; the remaining eleven on either side being reckoned 

 as marginals. The position of all these shields is exhibited in 

 fig. 1017. On the ventral aspect of the plastron (fig. 1016) 

 there are usually six pairs of shields, of which the most anterior 

 are termed gulars, the next humerals (postgulars) ; the next pecto- 

 rals ; then the abdominals ; the fe??iorals ; and finally the anals. 

 In some cases, however, there is an additional intergular (very 

 rarely double), which is usually (as in fig. 1012) placed between 

 the gulars, but in some cases may be situated below the gulars, 

 being then surrounded by the gulars, humerals, and pectorals, 

 as in Chelodina. In the extinct Archceochelys, of the We^den, 

 where the intergular occupies the latter position, there is also a 

 series of apparently single interpectoral, interabdominal, and inter- 



