CHELONURA SERPENTINA. 141 



is surmounted with a prominent knob or tubercle; those of the vertebral range 

 are placed at the centre of the posterior border of each plate, except the fifth, 

 which is very prominent, and occupies the middle of the plate; these knobs 

 present the appearance of a tuberculated carina, more or less prominent, along 

 the vertebral line; each lateral plate has in like manner a knob or tubercle 

 still more developed, but differently situated, here they occupy the superior and 

 posterior corner of the plate; these tubercles make a lateral carina, which gives 

 the shell a tricarinated appearance, differing, however, in different specimens, 

 as the tubercles are more or less elevated. From each tubercle, lateral as 

 well as vertebral, run, like radii, ridges and depressions, which make the 

 shell beautifully radiated; these ridges are very distinct near the tubercles, but 

 become less so as they approach the margin of the plates, which all have their 

 lateral and anterior borders distinctly marked with four or five concentric ridges 

 and depressions; the fifth vertebral plate alone has all its margins, posterior as 

 well as lateral and anterior, thus marked, for in this plate the tubercle stands 

 nearly in the centre. The marginal plates are twenty-five in number; the nuchal, 

 or intermediate, is oblong-quadrilateral, with its posterior border waving, but 

 concave in the centre for receiving the anterior point of the anterior vertebral 

 plate; the first pair of marginal plates are elongated, narrow and pentagonal; the 

 second, third, fourth, fifth, sixth, seventh, eighth, ninth and tenth, are elongated, 

 quadrilateral, those in front narrow, but gradually increasing in breadth to the 

 tenth, which is largest of all, and has its posterior and external angle projecting in 

 a strong point; the eleventh marginal plates are very irregularly pentagonal, each 

 with a similar strong point projecting backwards; the twelfth are in like manner 

 very irregularly pentagonal, with their longest borders in front, and their internal 

 borders very narrow, where they join each other, while their posterior external 

 and posterior internal margins terminate in a strong pointed process; these points 

 and notches give a deeply serrated border to the shell: the marginal plates are 

 generally smooth to the ninth pair, which have concentric striae on their superior 

 borders; the tenth and eleventh have similar striee, as well as additional ones on 

 their external and internal margins. 



