50 CONCHIFEKA. 



" British Mollusca ; " but have mainly followed 

 Mr. S. P. Woodward's valuable " Manual of the 

 Mollusca." 



CONCHIFERA. 



Molluscous animals enveloped in a shell com- 

 posed of carbonate of lime, forming two valves, 

 connected by a joint, and applied the one to the 

 right, the other to the left side ; mantle two- 

 leaved, more or less open ventrally, generally with 

 two orifices behind ; gills four-leaved ; no head ; 

 mouth placed at the angle of the gills, furnished 

 with lips and palps ; sexes distinct ; young under- 

 going a metamorphosis. 



Pallial line sinuous Smupallialia. 



Pallial line simple Integropallialia. 



Order I. SINUPALLIALIA. 



M antle closed : 



Foot short, truncate; shell gaping 



at both ends Pholadida. 



Foot small, finger-shaped; shell 



gaping in front Gastrochtenadtf. 



Foot small, tongue- shaped; shell 

 closed, or gaping behind. 



Shell thin, pearly within . . . Anatinada. 

 Shell thick, opaque, dull . . . Myadce. 



Mantle open in front : 



Foot cylindrical; shell very broad, 



gaping at both ends Solenidte. 



Foot compressed, tongue-shaped; 

 shell closed, or nearly so. 



Siphons separate Tellinada. 



Siphons united. 



Ligament internal .... Mactradae. 

 Ligament external . . . Veneridae. 



