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. 
Earn. 2. DEEISSENAD^. 
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 
