a.r 



Fig. 255. — The fresh-water mussel {XJnio). 



The uppermost figure represents the bivalve in motion in the mud 

 with protruded foot (F.) ; note inhalant and exhalant aper- 

 tures. The middle figure shows the inside of the shell (left 

 valve). The lower figure shows the outside (right valve). «., 

 The umbo; L., the ligament ; c./., lateral teeth ; a.a., anterior 

 adductor mark ; a.r., mark of protractor of the foot ; p.h, 

 pallial line ; p.a., posterior adductor mark ; p.r., mark of 

 posterior retractor of the foot; l.g., a line of growth; A., 

 anterior (the blunter end) ; P., posterior ; V ., ventral. 



