CLASSIFICATION 33 



cuticular surface of the animal, but are distinct from 

 the real shells, which in the Teredo and Gastrochana 

 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.r.) 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.L), 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 



