78 THE FRESHWATER MUSSKL 



B. The Inner Surface of the Shell. 



Wedge open the valves of the shell with the handle of a 

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

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

 can he clearly seen. Note the mantle-lobes or flaps liming the 

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

 from the shell. 



Identify the positions of the adductor muscles by com- 

 parison with fig. 26, and detach with a scalpel first the 

 anterior, then the posterior adductor muscle from the right 

 val/ve, keeping 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 has 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 gradu- 

 ally 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 of the 



shell, to which the adductor and other muscles are 

 attached. 



a. The anterior adductor impression is a large oval 

 slightly depressed area, with its long axis ver- 

 tical, close to the anterior end of the shell, and 

 nearer the dorsal than the ventral edge. 



