44 GENERA OF SHELLS. 



The pinna? are all sea shells, generally very large, 

 thin in proportion to their size, often brittle ; its frac- 

 ture exhibits delicate transverse stria3 like gypsum. 



P. rudis, flabellum, semi-nuda, angustana, nobilis, squa- 

 mosa, marginata, muricata, pectinata, saccata, varicosa, dol- 

 abrata, ingens, vexillum, -nigrina, subquadrivalvis. 



MALLEACEA. 



Ligament marginal, sublinear, either interrupted ly crenu- 

 lations or serial teeth, or wholly simple ; shell subincqui- 

 valve, foliated. 



The mallacea differ from the mytilacea by the folia- 

 ted texture of the shell, and by being irregular and in- 

 equi valve. The ligament also is not perfectly internal. 



CRENATULA. Shell subequivalve, flattened, 

 foliate, somewhat irregular ; no peculiar aper- 

 ture for the byssus. Hinge lateral, linear, 

 marginal, crenulate ; crenulations in regular 

 series, callous, somewhat excavated, receiving 

 the ligament. 



The hinge of the crenatula a good deal resembles 

 that of the perna, but presents a row of callous and 

 rather concave indentations, which receive the liga- 

 ment, whereas the perna has a row of linear, parallel, 

 truncated teeth, articulating with those of the opposite 

 valve, the ligament being inserted in the interstices. 

 The crenatulaB are rare shells, generally thin, some- 

 times almost membranous and brittle. 



C. avicularis, modiolaris, nigrina, bicostalis, viridis, myti- 

 loides, phasianoptera. 



PERNA. Shell subequivalve, flattened, some- 

 what deformed, texture lamellar ; hinge linear, 



