GENERA OF SHELLS. 53 



to foreign bodies, to which is attached the inte- 

 rior muscle of the animal. 



The operculum of the anomia has been absurdly 

 mistaken for a third valve. The perforated flat valve 

 is usually considered the lower one, because it is that 

 which rests on the bodies to which it is attached. 



A. ephippium, patellaris, cepa, electrica, pyriforinis, for- 

 nicata, membranacea, squamula, lens. 



Sect. III. — Ligament either wanting or un- 

 known, OR REPRESENTED BY A TENDINOUS CORD, 

 WHICH SUPPORTS THE SHELL. 



RUDISTA. 



Ligament, hinge and animal unknoicn, shell very unequi- 

 valve ; no distinct beaks. 



SpHiERULiTEs. Shell incquivalve, orbicular, 

 globose, somewhat depressed above, echinated 

 externally with large, subangular, horizontal 

 scales ; upper valve smallest, rather flat, oper- 

 cular ; internal surface with two unequal, sub- 

 conical, curved and prominent tuberosities ; 

 lower valve largest, rather ventricose, external 

 margin with radiated scales ; cavity obliquely 

 conical, forming at one side by the folding of 

 the margin, a crest or projecting keel ; internal 

 side of the cavity transversely striate. Hinge 

 unknown. Fossil. 



Radiolites. Shell inequivalve, externally 

 striated ; striae longitudinal, radiating ; lower 

 valve turbinated, and largest ; the upper con- 



