299 



Fam.3. DREISSENADiE. 



Shell regular, equivalve, inequilateral ; beaks ter- 

 minal, furnished with a septum ; muscular scars 

 three, the central one simple and linear, hinge 

 toothless. 



Animal ; — mantle entirely close all round, except 

 three apertures ; one below for the passage of the 

 foot, and two behind ; the upper one a roundish 

 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 ante- 

 rior transverse muscle, which is attached to the 

 front septum of the shell ; the abdomen depressed 

 foot elongate-conical, with a tuft of byssus at the 

 base, and a distinct byssal groove ; the extremities 

 of the gills free throughout their hinder half. 



A group of Dreissena tolymorpha, on an oil shell, 

 a, The lower siphon ; a*, magnified : b, upper siphon ; c, the byssus. 



This genus perhaps bears the same relation to 

 o 6 



