522 DREISSENIDiE. 



an inhabitant of the Caspian and Black seas, at the mouths 

 of rivers, has become naturalised in many of the docks and 

 canals throughout England. There are about fourteen 

 fossil species known, from the Eocene of Britain and 

 Germany. 



Species of Dreissena. 



carinata, Dkr. polymorpha, Pallas. 



Cumingiana, Dkr. strigata, Hinds. 



Sub-gen. praxis, H. and A. Adams. 



Shell with a small lamina affixed to the septum. 



Africana, Van Ben. Morchiana, Dkr. 



Americana, Becluz. Pfeifferi, Dkr. 



cochleata, Kickx. Rossmaessleri, Dkr. 



Domingensis, Becluz. Eiisei, Dkr. 



Kusteri, Dkr. Sallei, Becluz. 

 Gundlacbii, Dkr. 



Genus SEPTIFER, Kecluz. 



Shell equivalve, very inequilateral ; ventral margin con- 

 cave and emarginate for the byssus ; surface of valves 

 covered with an epidermis; beaks terminal, sub-inflexed. 

 Hinge without teeth, furnished with a lamellar septum 

 placed vertically under the beaks ; ligamentiferous fossulse 

 linear, marginal, dorsal, and vertical, with a sub-spongy 

 cretaceous body between the margins. Muscular impres- 

 sions superficial, the anterior small, rounded, the posterior 

 reniform ; pallial impression straight, without any sinus. 



Ryu. Septi ger, March. 



Ex. S. bilocuhuis, Linnaeus, pi. 122, fig. 3, 3, a. 



