SEA-FISHERIES LABORATORY. 183 
of the posterior adductor muscle, which is often striped 
with brownish yellow and white. This posterior margin is 
much denser than any other part of the shell, and dissolves 
slowly in dilute acid, always remaining after the rest of 
the shell has disappeared; the organic matrix 1s more 
abundant here than at any other part. 
The scars indicating the attachment of the muscles 
show plainly on the dry shell (fig. 10). The posterior 
adductor scar (Add.p'.) is large and usually pigmented. 
The anterior adductor scar (Add.a'.) is rather smaller, 
and is always unpigmented. Both approach very near the 
margin of the shell, and he just beneath the hinge line. 
On the dorsal margin of the posterior adductor is a small 
oval scar (Het.p’.), sometimes not very obvious. ‘This 
indicates the place of attachment of the posterior retractor 
of the foot. ‘Two scars are to be seen in a similar position 
over the anterior adductor scar. The more dorsal of these 
(Ret.a’.) is the scar of attachment of the anterior retractor 
pedis. The other (Pro'.) shows the attachment of the 
protractor pedis. The pallial line indicating the place of 
attachment of the radial series of muscle fibres serving 
for the retraction of the mantle edge runs parallel to the 
ventral shell margin, at a distance of about 8 mm., and is 
slightly indented in correspondence with the notches on 
the margin. There is no siphonal sinus, but at the 
posterior margin of the shell the pallial line becomes 
much broader as the retractor muscle of the mantle edge 
passes into the retractor of the siphons. A small scar 
hidden in the umbo serves for the attachment of a small 
bundle of muscle fibres attached to the dorsal margin of 
the wall of the viscero-pedal mass. 
The shell in the region of the umbones is always thin, 
and the periostracum is worn off in the fully grown 
specimen. Towards the margin, and especially at the 
