14 



With two adductor muscles. Dimyaria, Lam. 



Mantle without separate orifices or tubes. 



Foot slender, byssiferous ; tentacles fixed . . Avicula* 



Foot thick, rounded, with a callosity A)-ca. 



Foot compressed, securiform Pectunculus. 



Foot oval below, its margin tentacular, ten- 

 tacles volute Nucula. 



Foot large, pointed anteriorly, bent at an 



angle Trigonia.'* 



Mantle with a distinct anal orifice. 

 Foot small, byssiferous. 



Anterior muscle small ; retractile muscles 

 of the foot numerous ; byssus large. 



Byssus di\'ided to its base Mytilus. 



Byssus with a common corneous cen- 

 tre Modiola. 



Anus furnished with a long ligulate 



valve Pinna* 



Muscles equal ; two pairs of retractile 



muscles only ; byssus rudimentary .... Lithodomus. 

 Foot large, not byssiferous Unio. 



Mantle with a superior and inferior orifice ; not 

 elongated into tubes. 



Mantle widely open Cardium. 



Mantle closed around the foot or byssus. 

 Foot short and discal, byssiferous ; an- 

 terior muscle small Tridacna.* 



Foot small, cj'lindrical, bent at an angle; 



lips fohated Chama* 



Foot small, sharp ; lips simple Isocardia* 



Mantle with two produced tubes, or siphons. 

 Branchia not produced into the lower tube. 



Mantle closed around the foot Loripes.* 



Mantle open. 



Tubes disunited ; foot lanceolate. 



Foot large, rather falciform ; external 

 branchicE shortened ; mantle tenta- 

 cular ; labial tentacles large Donax. 



Foot small ; external branchicE short- 

 ened ; edge of the mantle simple ; 



tentacles small Psammobia. 



Foot moderate ; external branchia as 

 long as the internal; tentacles 

 large ; margin of the mantle en- 

 tire Tellina. 



Foot small; branchia equal; mantle 



tentacular Amphidesma. 



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