78 THE FEESHWATER MUSSEL 



b. The anterior retractor impressiou is small, and con- 



tinuous with the posterior border of the anterior 

 adductor impression. 



c. The protractor impression is a separate small 



impression, about ^ inch behind the lower part 

 of the anterior adductor impression. 



d. The posterior adductor impression is near the 



dorsal edge and posterior end of the shell, below 

 and behind the hinder end of the ligament. It is 

 rather larger than the anterior adductor impres- 

 sion, and is oval in shape, with the longer axis 

 horizontal. 



e. The posterior retractor impression is small, and 



continuous with the upper and anterior edge of 

 the posterior adductor impression. 



f. Th6 pallial line, or line of attachment of the mantle 



to the shell, runs nearly parallel to the ventral 

 edge of the shell, and about half an inch from it. 

 It connects the hinder border of the posterior 

 adductor impression with the ventral edge of that 

 of the anterior adductor. 



5. The lines of shifting of the muscles are faint lines con- 

 verging from the margins of the several muscular 

 impressions towards the umbo. They mark the 

 boundaries of the paths along which the muscles have 

 shifted during the growth of the shell. In the young 

 state, when the umbo and the part immediately round 

 it form the whole valve, adductor and other muscles 

 are present in the same relative positions as in the 

 adult ; and as the shell grows by addition of new 

 matter round its margin the muscles gradually shift 

 outwards, increasing in size as they do so. 



C. The Structure of the Shell. 



If the shell be broken across, and the cut edge examined 

 with a pocket lens, it is found to consist of three layers. 



