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 A. cygnea) gradually. 



Family III. DREISSENIDJE. 



BODY nearly rhomboidal, compressed: mantle closed, except 

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

 for respiratory and nutritive, and the other for excretal pur- 

 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 ex- 

 tended into a pyramidal 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 porcelain- 

 white : hinge 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- 

 senidm bear a closer resemblance than the last to the 



