THE LIFE OF THE UNIO 



U 



Fig. 4. Interior view of Quadrula heros, or "Wash-' 



board" shell. 



U., Umboes, or Beak; L., Ligament ; B. H., Back Hinge; 



T. H., Tooth Hinge; A. A. S., Anterior Adductor 



Scar; P. A. S., Posterior Adductor Scar; 



A. E., Anterior End; P. E., Posterior 



End; P. L., Pallial Line; S., Stains. 



end of the shell is called the posterior end. The teeth 

 are near the anterior end of the shell and are for the 

 purpose of holding the shells together so they will not 

 become misplaced in case of accident. The hinges are 

 for the same purpose as the teeth and are rather long, 

 elevated, nacreous formations at the top of the shell near 

 the ligament. At each end of the shell there are rough 

 scars where the adductor muscles are joined. One is 

 called the anterior adductor scar and the other is called 

 the posterior adductor scar. The pallial line is the name 

 given to the rather distinct and slightly indented line near 



