114 NEW ZEALAND NATURE-STUDY BOOK 
the shell, and which can be made the means of a good deal 
of useful and interesting investigation. 
As the animal increases in size, and the shell becomes too 
small to cover the soft parts, additions are made to the 
outer margin. ‘These, too, frequently leave well-defined 
markings on the outer surface. 
The following are amongst the points to be noticed about 
the shell of a bivalve Mollusc. The top of the valve is 
called the beak (d) 
and is generally 
turned towards the 
front part of the 
animal. In forms 
where this is the 
case, we can readily 
distinguish between 
right and left valves, 
for we have only to 
hold the shell, with 
its beak pointing 
away from us, and 
Fig. 75.—Left Valve of Mollusc. 
a, front; b, back; c, lower margin; d, beak; e, teeth ; 
, pallial or mantle line; g, break (sinus) in mantle a 
EP line; h, adductor muscle impressions. the right and left 
valve will correspond 
in position with the right and left side of the body. 
That portion of the shell where the beaks are situated, 
and where the valves are united, is called the dorsal or 
upper side, and the opposite margin along which the shell 
opens is known as the ventral side or base. The length 
of a shell is measured from its front to its back margin, and 
its width from the dorsal margin to the base, The inside 
markings include the adductor muscle impressions 
and the pallial line, which may be entire or may contain 
a sinus or bay (see figure 75). 
