258 DREISSENAD^. 



the bottom of the vessel with the posterior extremity 

 of the shell elevated, and the siphons exserted. 



The cup, or swollen part of the umbo in the ven- 

 tricose shell of the young animal, remaining on the 

 top of the shell, is common to several species of the 

 family, which, like many other viviparous Mol- 

 lusca, produce their young of a large size compared 

 with their parent. 



Fam. 2. DREISSENAD^. 



The mantle entirely close all round, except three 

 apertures, one below for the passage of the 

 foot, and two behind ; the upper one a round- 

 ish hole, the lower one produced into a conic 

 siphon with a large reflexed mouth, which is 

 broader within ; anterior extremity of the body 

 bifurcate and included between the segments 

 of the anterior transverse muscle, which is at- 

 tached to the front septum of the shell ; the 

 abdomen depressed; foot small, elongate-conical, 

 with tuft of byssus at the base, and a distinct 

 byssal groove ; the extremities of the gills free 

 throughout their hinder half; anterior adductor 

 muscle small, on a transverse subumbonal plate ; 

 mantle edge double. 



Shell regular, equivalve, inequilateral, subtrigonal, 

 keeled ; beaks terminal, furnished with a septum ; 

 muscular scars three, the central one simple 

 linear ; hinge toothless ; cartilage external, and 

 marginal. Fluviatile. 

 This genus perhaps bears the same relation to 



