222 KEY. 



MARINE BIVALVES. 



Valves somewhat circular, joined by one large muscle . . XXI 



Valves dark, mussel-shaped XXII 



Valves white, circular XXII 



Valves cordate, no pallial sinus XXIII 



Valves rough, attached to rocks XXIII 



Valves mostly white, strong, sculptured ; with pallial 



sinus XXIII 



Valves mostly thin, smooth; ligament internal XXIII 



Similar, but with ligament external, sinus deep . . XXIII, XXIV 



Valves somewhat razor-shaped XXIV 



Hinge tooth spoon-shaped XXV 



Valves thin, inflated, irregular XXIV 



Valves with dissimilar ends, borers XXV 



Brachiopod shells XXV 



FRESH-WATER BIVALVES. 



Small and nearly circular XXII 



Larger and oblong XXII 



MARINE MULTIVALVES. 



Shell composed of eight plates XV 



LAND SHELLS. 



Shell very thin, slender XVIII 



Shell minute, cylindrical XVIII 



Shell spiral, varying from flattened to globose. . . . XVIII — XX 

 Shell rudimentary, often concealed under the mantle of 



the slug-like animal XX 



