15 



Tubes more or less united ; foot various. 

 • BranchitB 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 Venerupis. 



Branchics disunited medianly. 



Foot lanceolate, prominent behind ; 



tubes small, united Cytherea. 



Foot securiform ; tubes larger and 



more or less distinct Venus. 



Branchia 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 ; branchia of each side 



united into one Pandora. 



Foot larger ; hranchia 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 museles, the an- 

 terior one situated below a reflect- 

 ed portion of the mantle uniting 

 the beaks instead of a cartilage ; 



tentacles Isirge Pholas. 



Body very elongated ; adductor mus- 

 eles united ; end of the mantle vdth 

 two calcareous pieces; tentacles 

 small ; no cartilage nor reflected 

 portion of the mantle Teredo. 



For the anatomy of the several genera marked in the above table 

 ■with an (*), the author acknowledges himself indebted either to 

 Cuvier, Poli, or M. de Blainville. 



He refers occasionally to other genera, besides those enumerated, 

 as included in the groups distinguished by the characters given 

 above. 



