CHELONIA. 765 



centre of the superior border of the first and second costals, and close to the superior 

 posterior angle of the third costal. In the fourth costal, the upper is more convex 

 than the lower half, but there is no distinct nodosity. In the adult, these nodosities 

 are distinctly visible as swellings in these portions of these shields ; and as the 

 second, third and fourth vertebrals are rather concave than otherwise, although they 

 are markedly convex in the young and in males, these nodosities almost simulate a 

 costal ridge in the adult, as they constitute an obscure longitudinal eminence which 

 is separated from the vertebral ridge by an intervening shallow concavity. In the 

 young, the vertebral ridge is most pronounced at the hinder border of the areolae 

 which are strongly transversely developed, slightly posterior to the centres of the 

 plates, the outlines of which they retain, their surfaces being finely roughened, and 

 the areolae surrounded by lines concentric to their forms. 



The ridge is most defined in the first vertebral, and in the young embraces the 

 nuchal. In the male, and young females of the size of the ascertained males, the 

 ridge terminates rather abruptly close to the posterior margin of each plate in a 

 kind of nodosity ; but in the large adult females these entirely disappear, and the 

 ridge is rather depressed at the junction of the plates. In the young, the vertebral 

 plates are relatively much broader than in the adults, the breadth of the second 

 vertebral plate, which is about the same breadth as the third, equalling the distance 

 between its external angle and the margin of the shell, while, in the adult female, 

 the breadth of that plate is less than one-haLf of that distance, whilst in the males 

 it is considerably more. In the young, the nuchal is triangular and broader at 

 its base than long, while in the adult that shield preserves the same form. In 

 the former stage, the first vertebral is almost quadrangular in some, while in others 

 it is narrower in front than behind, with the margins nearly straight, and is con- 

 siderably broader than long. In the adult, this shield is sometimes as broad as 

 long, while in others it is longer than broad. Its lateral margins are sometimes 

 nearly parallel, whilst in others they converge anteriorly as straight lines, while 

 occasionally their anterior halves are concave and their posterior halves convex, so 

 that the shield is nearly bell-shaped. In the majority of adults, the second, third, 

 and fourth vertebrals are longer than broad, with rectangular borders, the costal 

 margins of the third and fourth plates being more or less sinuous, while other indi- 

 viduals have the first and second vertebrals broader than long, and the third and 

 fourth as long as broad, besides other intermediate individual variations. The general 

 characters, however, of the first, second, third and fourth vertebrals are, that they 

 are more or less quadrangular or oblong, generally longer than broad, with nearly 

 rectangular margins and of nearly equal breadth anteriorly and posteriorly, with the 

 exception of the posterior margin of the fourth vertebral, which is less than 

 two-thhds the breadth of its anterior border. In the young, the first, fourth 

 and fifth vertebrals are of nearly equal breadth, but, in the adult, the last men- 

 tioned shield is by far the broadest. It is contracted at its anterior extremity, 

 and its costal margin is convex, and it articulates broadly with the eleventh 

 marginal. 



