MARINE MOLLUSCS 



193 



Examining the shell from the exterior we observe that each 

 valve has a nucleus (the umbo) close to the hinge, round which 

 are usually a number of more or less distinct concentric lines, 

 extending to the lower or ventral margin. This nucleus represents 

 the whole shell of the young mollusc, and the lines are the lines 

 of growth, each one marking the extreme limit of the valve at a 

 particular period of the animal's existence. Further it will be 

 observed that the lines of growth are often wider apart in some 

 directions than in others, thus denoting the unequal rate of growth 

 that determined the form of the adult shell. 



The shell of a bivalve is often made up of two very distinct 

 layers, the outer one called the prismatic layer because, when 

 examined microscopically, it is seen to consist of minute vertical 

 prisms of calcareous matter; and the inner one presenting a 

 beautiful pearly iridescence, due to the fact that it is made up of 

 a number of extremely thin and finely waved layers of calcareous 

 substance that have the power of decomposing light. This latter 

 layer is secreted by the whole surface of the mantle that lies in 

 contact with it, while the outer, prismatic portion of the shell is 

 formed only by the free edge of the mantle ; and we often find 

 a distinct line (ihepallial line), some little distance from the ventral 

 margin that marks the junction of the muscle of the mantle with 

 the shell. The shape of this 

 line is a very important 

 feature of the shell, since it 

 is of great value in the de- 

 termination of relationships. 



Further, the inner sur- 

 face of each valve is marked 

 by the impressions or scars 

 of other muscles, the num- 

 ber and position of which 

 vary considerably in diffe- 

 rent species. They include 

 the adductor muscle or 

 muscles (one or two in 

 number) that pull the valve 



together; the muscle or muscles that withdraw the foot, called 

 the retractor pedis, and the protractor pedis that pulls the foot 

 out. Not only are these scars often very distinct in themselves, 

 but we may frequently observe lines running tangentially from 







FIG. 128. INTERIOR OF BIVALVE 

 SHELL, SHOWING MUSCULAR SCARS 

 AND PALLIAL LINE 



