THE FRESH-WATER MUSSEL. 333 



In front of the posterior adductor impression, is 

 seen a small scar of attachment of the posterior 

 retractor muscle. 



Behind the anterior adductor impression are two 

 others, one opposite its upper, the other opposite 

 its lower end: the former indicates the point of 

 attachment of anterior retractor, the latter of the 

 protractor pedis muscle. 



The adductor and retractor scars are not unfre- 

 quently confluent, and there extends from each 

 into the umbo a fainter tapering impression, in- 

 dicative of the growth of the muscles themselves 

 as the animal has increased in size. 



g. Prepare (as directed for the Crayfish on p. 203) 

 a dried section across the middle of one valve, cut 

 at right angles to its long axis. Mount in Canada 

 balsam and examine under a low power, note 

 a. The epiostracum; a thin uncalcified superficial 

 layer; it is greenish-yellow in colour, and very 

 frequently torn away in the process of grinding. 

 ft. The prismatic layer; composed of elongated 

 columns of calcified substance running parallel 

 with each other. 



y. The nacreous or pearly layer; immediately in- 

 ternal to /?. Its relative thickness is proportion- 

 ate to the age of the animal from which the 

 valve was taken. 



Examine under a high power it is finely 

 granulated and traversed by delicate longi- 

 tudinal striae. 



8. Make a tangential section across the prismatic 

 layer and examine under a low power. The 



