THE RIVER MUSSEL. 149 



and close beside them the smaller but often deeper scars 

 of the retractor muscles. Observe a narrow linear muscle 

 impression, a curved line marked around the free border 

 of the valve, about half an inch from the edge. This is 

 the line along which the mantle was found clinging. It 

 is therefore called the pallial line. 



Observe the hinge teeth between the dorsal edges of the 

 two valves. Open and close the valves, and see how these 

 interlock. They hold the valves together very firmly, 

 when closed. The blunt, irregular, serrated ones between 

 the umbones are the cardinal teeth. The long, narrow, 

 shelf -like folds of pearl beneath the hinge are the lateral 

 teeth. 



Observe that the shell consists of three layers : 



1. An outer layer of horny, brownish, or greenish epi- 

 dermis, which was continuous at the edges of the shell 

 with the border of the mantle. The hinge at the back is 

 a modified and very elastic portion of the epidermis. 



2. An inner, pearly layer. This is seen, under magnifi- 

 cation, to consist of thin, flat layers of pearl, the edges of 

 which appear as fine, sinuous lines. These cause inter- 

 ference in the light waves that fall upon them, and give 

 rise to all the singularly beautiful colors seen within the 

 fresh shells of some Unios. 



3. A darker middle layer, seen, under magnification, to 

 consist of polygonal prisms placed perpendicular to the 

 surface of the shell. 



Dissolve out the mineral matter from a shell by placing 

 it for a time in dilute acid. What properties has the 

 remaining part ? 



Burn out the animal matter from a number of shells. 

 Weigh them before throwing them into the fire and after 

 taking them out, and note the proportional loss. Pick a 

 burned shell to pieces, and note the disposition of layers 

 in its thickened part. 



