226 Zoological Society. 



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 ; external 

 hranchice shortened ; mantle tenta- 

 cular ; labial tentacles large Donax. 



Foot small ; external branchice short- 

 ened ; edge of the mantle simple ; 



tentacles small Psammohia. 



Foot moderate ; external branchice as 

 long as the internal; tentacles 

 large ; margin of the mantle en- 

 tire Tellina. 



Foot small ; branchice equal ; mantle 



tentacular Amphidesma. 



Tubes more or less united ; foot various. 

 Branchice united medianly. 



Tubes small, partially divided ; foot 



very long, obtuse Cyclas. 



Tubes small, united to the ex- 

 tremity ; foot very long and 



pointed Mactra. 



Tubes large, foot short and promi- 

 nent behind Yenerupis. 



Branchice disunited medianly. 



Foot lanceolate, prominent behind ; 



tubes small, united Cytherea. 



Foot securiform ; tubes larger and 



more or less distinct Venus. 



Branchice produced into, or attached to, the 

 lower tube ; tubes always united. 

 Mantle only open inferiorly for the protru- 

 sion of the foot. 

 Tubes small ; lips long. 



Foot small ; branchice of each side 



united into one Pandora. 



Foot larger ; branchicB separate .... Corbula. . 

 Tubes long ; lips small. 



Foot not byssiferous ; tubes large and 



coriaceous Mya. 



Foot byssiferous ; tubes moderate . . Hiatella. 

 Mantle open anteriorly. 



Foot long, club-shaped ; tubes short . . Solen. 

 Foot very short, rounded. 



Two distinct adductor muscles, the an- 

 terior one situated below a reflect- 

 ed portion of the mantle uniting 

 the beaks instead of a cartilage ; 

 tentacles large Pholas. 



