84 THE FRESHWATER MUSSEL. 



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 im- 

 pression, 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. The 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 paths of shifting of the muscles are slight triangular 

 depressions, converging 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 around it form the whole valve, 

 adductor and other muscles are present in the same 

 relative positions as in the adult : during the growth 

 of the shell by addition of new matter round its 

 margin the muscles gradually shift outwards, in- 

 creasing in size as they do so. 



C. The Structure of the Shell. 



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

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



1. The periostracum or outer layer is thin, horny, and 

 uncaloified. To it the brownish-green colour of 



