THE FRESH-WATER MUSSEL. 



115 



tlie shiny knobs — the umbones— is an elastic band which 

 lies between the two valves, and joins them together, 

 and hence it is called a ligament (Figs. 107, 108). This 



Fig. 108. 

 Umbo. Ligament. 



Front 

 Muscle. 



Edge of Mantle. 



elastic band is always on the stretch when the adductor 

 muscles close the valves, or folding-doors of the shell ; 

 but, when these muscles are cut, the band springs back 

 or shortens itself ; thus the shells of dead mussels al- 

 ways stand open. The outside of the shell is covered 

 by a kind of a skin, of a greenish-brown color, and this 

 skin prevents the water from wetting the lime, of which 

 the shell is made. All over the shell you can see lines, 

 or arches, and these increase in size and number toward 

 the lower, or ventral, edge (Fig. 107). The upper lines 

 seem to overlap the lower, and the shell becomes thin- 

 ner at the ventral side. The inside of the shell is white 



