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Gastroch.ena. Shell bivalve, equivalve, rather wedge-shaped, gaping 

 very much; the anterior aperture large, oval, oblique; scarcely any aperture 

 posteriorly. Hinge linear, marginal, without teeth. 



G. cuneiformis, G. mytiloides, G. modiolina. 



SOLENACEA.— Three Genera. 



SJicll without accessary pieces, and gaping only at the lateral extremities. 

 Ligament external. 



Solen. Shell bivalve, equivalve, transversely elongated, gaping at both 

 sides; beaks very small, not projecting; cardinal teeth small, the number va- 

 riable, sometimes nine; rarely diverging; more rarely inserted in pits. Lig- 

 ament external. 



S. vagina, 

 ' corneus, 

 ' vaginoicles, 

 ' siliqua, 

 ' ensis, 



S. ambiguus, 

 ' cultellus, 

 ' planus, 

 ' minutiis, 

 ' legumen, 

 ' Dombeii, 



S. javanicus, 

 ' caribseus, 

 ' antiquatus, 

 ' constrictus, 

 ' coarctatus, 



S. strigilatus, 

 ' radiatus, 

 ' violaceus, 

 ' rostratus, 

 ' orbiculatus. 



Panop^ea. Shell equivalve, transverse, unequally gaping at the sides; 

 one cardinal conical tooth in each valve, and near it a short, compressed, 

 ascending callosity, not projecting outwards, 

 longest side of the shell, fixed to the callosities 



Ligament exterior, on the 



P. Aldrovandi. 



Glycymeris. Shell transverse, gaping much on each side; hinge callous, 

 without teeth; nymphae projecting outside. Ligament external. 

 G. margaritacea, G. siliqua. 



MYARIA.— Two Genera. 



Ligament internal; having one large spoon-shaped tooth in each valve, or 

 in one only, to the cavity of which the ligament is attached. The shell gapes 

 at one or both sides. 



Mya. Shell bivalve, transverse, gaping at each end; having a large car- 

 dinal tooth in the left valve, broadly compressed, rather rounded, and pro- 



