DREISSENID^. 45 



Gumfrieston, near Tenby (Smith). A monstrosity, or 

 distortion, of this last variety is also in my cabinet, 

 which is flatter and has a rounded outline above in 

 consequence of the umbonal region not projecting. A 

 young specimen of the same variety is nearly round. 

 This appears to be analogous to the variety rostrata of 

 A, cygnea, and tends to confirm the idea of the two 

 species being distinct. 



The chief points of difference between A. cygnea and 

 A. anatina are, that the shells of the latter species are 

 smaller and comparatively longer; the hinge-line or 

 crest is raised in that species, instead of being straight 

 or parallel to the lower margin ; and the posterior side 

 slopes abruptly instead of (as in ^. cygnea) gradually. 



Family III. DREISSENID^. 



Body nearly rhoraboidal, compressed : mantle closed, except 

 at the posterior side, where it is folded into two orifices, one 

 for respiratory and nutritive, and the other for excretalpiu'poses, 

 besides an opening at the lower or front margin for the passage 

 of the foot. The upper, or excretal, fold is the smallest, and is 

 not much produced : the other fold is extended into a pyra- 

 midal tube, which has a thickened or reflected margin and is 

 fringed with numerous short spine-shaped cirri or tentacles : 

 foot long and tongue-shaped, furnished with a byssal groove. 



Shell equivalve, oblong, triangular, very inequilateral, 

 ventricose, covered with a thick and horny epidermis : beaks 

 placed at the anterior end : ligament internal : inside porcelaic- 

 white : himie furnished with minute cardinal teeth, but some- 

 times toothless ; below the beak in each valve is a triangular 

 shelf or hollow plate (as in the marine genus Crepidula) for the 

 reception of the anterior muscle. 



These characters are also generic, as the family con- 

 tains but one genus. In their general aspect the Dreis- 

 senidce bear a closer resemblance than the last to the 



