168 



equal; hinge with a prominent callosity in each valve, depressed above, show- 

 ing an impression of a conical and obliquely arched pit for the ligament. 



V. lingulata, V. rugosa, V. mytilina, 1 fossil species. 



' hians, ' spongiarum, ' ovata, 



Placuna. Shell free, irregular, flat, subequivalve, hinge internal, having 

 two sharp, longitudinal ribs in one valve, approximate at their base, and 

 diverging in form of a V.; and in the other valve, two impressions, which 

 correspond with the cardinal ribs, to which is attached the ligament. 



P. sella, P. placenta, P. papyracea, 1 fossil species. 



Anomia. Shell inequivalve, irregular, operculated, adhering by the oper- 

 culum, lower valve perforated, generally flattened, having a hole or channel at 

 the beak; the other valve rather larger, concave, entire. Operculum small, 

 elliptical, osseous, fixed to marine substances. 



SECTION III. Either no ligament, or none known; or represented by a ten- 

 dinous cord, which supports the shell. 



This section contains two families — Rudista, and Brachiopoda. 



RUDISTA.— Six Genera. 



Sph^erulites. Shell inequivalve, orbicular, globose, rather depressed 

 above, echinated externally, with large subangular, horizontal scales; upper 

 valve smallest, rather flat, opercular; the internal surface furnished with two 

 unequal, subconical, curved, and prominent tuberosities; lower valve largest, 

 rather ventricose, the external margin radiated with scales; cavity obliquely 

 conical, and forming, on one side, by the folding of the internal margin, a 

 crest or projecting keel; internal side of the cavity transversely striated. 

 Hinge unknown. 



I fossil species. 



Radiolites. Shell inequivalve, externally striated; the striae longitudinal, 

 radiating; lower valve turbinated, and largest; the upper, convex, or depressed, 

 conical, opercular. Hinge unknown. 



3 fossil species. 



