THE SHELL. 91 



scalpel or other instrument, so that the ventral edges of the 

 valves are about half an inch apart, and the adductor muscles 

 can be clearly seen. Note the mantle-lobes or flaps lining the 

 shell-valves, and with a scalpel separate the right mantle-lobe 

 from the shell. 



Identify the positions of the adductor muscles by com- 

 parison with fig. 28, and detach with a scalpel first the anterior 

 then the posterior adductor muscle from the right valve, keeping 

 the edge of the knife close to the shell. As soon as this is done 

 the valves will gape open about half or three-quarters of an 

 inch, owing to the action of the ligament. Bend back the right 

 valve of the shell, and remove it completely, cutting through the 

 ligament along the hinge-line. Clean the detached valve, and 

 examine it. 



1. The inner surface lias a white pearly appearance, except 



round the edge. The lines of growth are visible, 

 but not nearly so plainly as on the outside. 



2. The hinge is a smooth longitudinal ridge extending 



backwards from the umbo along the dorsal edge 

 of the shell. When the valves are closed, the 

 two hinges are in contact opposite the umbo, but 

 gradually diverge from each other behind it. 



3. The ligament, which is very strong and elastic, com- 



mences in front of the umbo, and extends back 

 along the straight dorsal edge of the shell, being 

 very narrow in front, but wider behind. 



4. The muscular impressions are slight depressions in the 



surface of the shell at the places of attachment of 

 the adductor and other muscles. 



a. The anterior adductor impression is a large oval 



slightly depressed area, with its long axis vertical, 

 close to the anterior end of the shell, and nearer 

 the dorsal than the ventral edge. 



b. The anterior retractor impression is small, and 



continuous with the posterior border of the 

 anterior adductor impression. 



