CLASSIFICATION 33 
cuticular surface of the animal, but are distinct from 
the real shells, which in the Tevedo and Gastrochena 
are within the tube and free, while in Brechites they 
are built into its wall. 
The points of attachment of the adductor muscles 
to the shell are marked by well-defined scars 
(Plate XVI., Figs. 1-3, a.a. and p~.a.). Close to the 
posterior adductor scar, that of the muscle for with- 
drawing the foot (Plate XVI., Figs. 1-3, p.7.) is found, 
and minor scars occur nearer the umbones of the 
valves to which other small muscles are attached. 
Running from one adductor scar to the other, at a 
short distance from the ventral margin of the shell, 
is a shallow groove, the “ pallial line” (Plate XVI., 
Figs. 1-3, p./.), which marks the attachment of the 
mantle edge. In those molluscs that have large 
retractile siphons, room has to be made for these, 
and the pallial line, instead of forming a continuous 
curve, takes, shortly before it reaches the posterior 
scar, a sharp bend backwards towards the centre of 
the valve, forming a bay or sinus, known as the 
pallial sinus” (Plate XVI., Fig. 2). The size of 
this sinus naturally corresponds to that of the 
siphons. 
In some Bivalves, such as the Common Mussel of 
the seashore, the posterior portion of the body is 
more developed than the anterior. This develop- 
ment tends to bring the two adductor muscles and 
the hinge into a line, and in proportion to render 
3 
