166 THE FRESH-WATER MUSSEL [CH. VI 



The inner surface of each valve is marked by certain 

 muscular impressions. In the anterior region is a large 

 shallow pit due to the attachment of the anterior adductor 

 muscle, one of those which close the shell, and of the (so 

 called) anterior retractor muscle of the foot. Posterior to 

 this impression and about level with its ventral end is a 

 smaller pit to which is fastened the (so called) protractor 

 muscle of the foot. In the posterior portion of the shell is 

 the large posterior adductor impression, and immediately 

 dorsal and in front of it the smaller impression of the (so 

 called) posterior retractor of the foot. Diverging from the 

 region of the umbo towards the final position of these 

 several chief muscles are faint tracks which mark the lines 

 along which the muscular attachments have shifted during 

 the growth of the shell. The details of the shifting process 

 are not known. Parallel to the ventral border of the shell 

 there runs the " pallial line " caused by the insertion of 

 numerous muscle fibres lying in the mantle (pallial 

 muscles) : the line extends from the anterior to the 

 posterior adductor impressions. In addition to these well- 

 defined markings there are others less easy to detect ar- 

 ranged in two groups, the one near the anterior and the 

 other near the posterior limit of the ligament. To these 

 small impressions are attached groups of minor retractor 

 (or better, depressor) muscles. The universally accepted 

 terms of " retractor, or protractor, pedis " as applied to the 

 above-mentioned muscles are unfortunate. I have no 

 doubt that, so far from drawing the foot backward or 

 forward, their real function is to drag the shell and there- 

 fore the whole animal forward or backward, the foot being 



