GENERA OF SHELLS. 17 



terior, fixed in the cardinal fossa ; sometimes 

 there are more or less prominent lateral teeth ; 

 the shells are generally small. 



A. variegata, doriacilla, lactea, cornea, albella, lucinalis, 

 Boysii, tenuis, flexuosa, prismatica, phaseolina, corbuloides, 

 glabrella, purpurascens, nucleola, physoides, reticulata, 

 transversum, orbiculata, equalis, punctata, radiata, lepida. 



CORBULEA. 



Shell inequivalve ; ligament interior. 



CoRBULA. Shell regular, inequivalve, ine- 

 quilateral, gaping none, or very little. A conic, 

 curved, ascending cardinal tooth on each valve, 

 and a lateral fossa ; no lateral teeth ; ligament 

 interior, fixed in the fossa. 



The corbulse are distinguished from the ungulinee 

 and crassatellae by the inequahty of their valves, and 

 by the strong, prominent cardinal tooth, which charac- 

 terises them. 



C. australis, sulcata, erythrodon, ovalina, taitensis, nucleus, 

 impressa, porcina, semen, — contracta. 



Pandora. Shell regular, inequivalve, ine- 

 quilateral, transversely oblong, superior valve 

 flattened, inferior convex ; two oblong, diverg- 

 ing, unequal cardinal teeth in the upper valve ; 

 two oblong depressions on the other ; ligament 

 interior. 



The pandora is distinguished from placuna by hav- 

 ing two muscular impressions, and from the chamacea 

 by the shell being regular and free. 

 P. rostrata, obtusa, — trilineata. 

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