6 



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 is 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 lie just beneath the hinge line. 

 On the dorsal margin of the posterior adductor is a small 

 oval scar (Bet.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 

 {Bet. 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 



