CHELOXURASERPEXTINA. 23 



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 tubcrculated 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 

 in appearance, however, in different specimens, as the tubercles are more or less 

 elevated; from each tubercle, lateral as well as vertebral, run, like radii from a 

 centre to a circumference, ridges and depressions, which make the shell beauti- 

 fully 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-three in number; the nuchal or 

 intermediate is oblong quadrilateral, w ith its posterior border waving, but concave 

 in the centre for receiving the anterior point of the anterior vertebral plate; the 

 first pair of marginals 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 plate is 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 strise, as well as additional ones on 

 their external and internal margins. 



The sternum is cruciform, narrow, rather rounded anteriorly and pointed pos- 



