THE SHELL. 83 



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

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

 can he 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 ivith fig. 26, 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, 

 Clotting through the ligament along the hinge-line. Glean the 

 detached valve, and examine it. 



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



round, the edge. The hnes 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 con- 



tinuous with the posterior border of the anterior 

 adductor impression. 



g2 



