54 GENERA OF SHELLS. 



vex, or depressed conic, opercular. Hinge 

 unknown. Fossil. 



Calceola. Shell inequivalve, triangular, tur- 

 binated, flattened beneath ; the larger valve hol- 

 lovi'ed or hood-shaped, obliquely truncated at 

 the aperture ; cardinal margin straight, trans- 

 verse, a small notch or indentation in the mid- 

 dle ; superior margin arched ; the small valve 

 flat, semi-orbicular, opercular, having a tuber- 

 cle on each side of the cardinal margin, and in 

 the middle a pit with a small lamina. Fossil. 



BiRosTRiTES. Shell inequivalve, two horned, 

 disc of the valves obliquely elevated conically, 

 unequal, obliquely diverging, nearly straight, 

 horn-shaped, the one enveloping the other at 

 base. Fossil. 



DisciNA. Shell inequivalve, rounded, oval, 

 somewhat depressed ; valves nearly equal in 

 size, each having a very distinct, central, orbi- 

 cular disc ; disc of the superior valve not per- 

 forated, rising into a mamillary protuberance ; 

 that of the other valve very white, with a trans- 

 verse slit. 



The identity of discina and orbicula has been so 

 satisfactorily proved, that the former is now given up. 



D. ostreoides. 



Crania. Shell inequivalve, suborbicular , 

 inferior valve nearly flat, perforated on its in- 

 ternal face by three unequal, oblique holes ; 



