XI.] THE FRESH- WATER MUSSEL. 121 



d. The markings on the shell. 



a. External markings. The outside of the shell 

 is greenish brown, and on it are seen a number 

 of concentric lines generally parallel to the 

 margin of the shell, and more numerous to- 

 wards the ventral edge. 



j3. Internal markings. The interior of the valve 

 is white and iridescent : on it are seen, near the 

 dorsal border, two oval marks, the anterior and 

 posterior adductor impressions. 



Joining the two adductor impressions is a 

 curved line, the pallial impression, which marks 

 where the muscles of the edge of the mantle 

 were fixed to the shell. 



In front of the posterior adductor impression 

 is seen a small mark, indicating where the 

 posterior retractor muscle was fixed. 



Extending from each adductor impression 

 towards the umbo is a fainter, gradually taper- 

 ing impression, which may be followed into 

 the cavity of the umbo, and indicates the 

 successive attachments of the adductor muscles, 

 as the animal has increased in size. 



13. In the breeding season, examine the contents of the 

 testis for spermatozoa, and those of the ovary for 

 ova. Note the micropyle of the latter. If the outer 

 gill appear to be thick and distended, it will be 

 found full of the larvae of the Anodon,-rGlocliidium. 

 Note the characters of their shells and the entangled 

 filaments, or byssus, with-which they are provided. 



