M 



With two adductor museles. Dimyaria, Lam. 



Mantle witliout separate orifices or tubes. 



Foot slender, byssiferous ; tentacles fixed . . Avicula* 



Foot thick, rounded, with a callosity Arca. 



Foot compressed, securiform Pectunculus. 



Foot oval below, its margin tentacular, ten- 

 tacles volute Nucula. 



Foot large, pointed anteriorly, bent at an 



angle Trigonia* 



Mantle ■vvith a distinct anai orifice. 

 Foot small, byssiferous. 



Anterior musele small ; retractile museles 

 of the foot numerous ; byssus large. 



Byssus divided to its base Mytilus. 



Byssus with a common corneous cen- 

 tre Modiola. 



Anus furnished with a long ligulate 



valve Pinna* 



Museles equal ; two pairs of retractile 



museles only ; byssus rudimentary .... Lithodomus. 

 Foot large, not byssiferous Unio. 



Mantle \vith a superior and iriferior orifice ; not 

 elongated into tubes. 



Mantle ■vvidely open Cardium. 



Mantle closed around the foot or byssus. 

 Foot short and discal, byssiferous ; an- 

 terior musele small Tridacna.*^ 



Foot smaU, eylindrieal, bent at an angle; 



lips foliated Chamą.* 



Foot small, sharp ; lips simple Isocardia* 



Mantle with two produced tubes, or siphons. 

 Branchice not produced into the lower tube. 



Mantle closed around the foot Loripes* 



Mantle open. 



Tubes disunited ; foot lanceolate. 



Foot large, rather falciform ; extemal 

 branchiee shortened ; mantle tenta- 

 cular ; labial tentacles large Donax. 



Foot small ; extemal branchiee short- 

 ened ; edge of the mantle simple ; 



tentacles small Psarnmobia. 



Foot moderate ; external branchiee as 

 long as the internaJ ; tentacles 

 large ; margin of the mantle en- 



tire Tellina. 



Foot small ; branchia equal ; mantle 



tentacular Amphidesma. 



