SOME INDIAN FRESHWATER SHELLS. 95 



One Indian species, namely Unio favidens, is especially so noted. In Great 

 Britain a few of these pearl fisheries still exist in parts of Ireland and Scot- 

 land, the mussel producing these being Unio margartifer. We read that the 

 Tay fishery was at one time very productive, and in three years pearls were 

 obtained to the value of £10,000. Mother-of-pearl is rarely if ever obtained 

 now from freshwater mussels, this industry having subsided owing to the 

 large numbers of pearl oysters (Meleagrina mar gar lifer a) used for that purpose. 

 Nevertheless many Indian species of Unio are especially rich in brilliant nacre. 



In the descriptions of the following shells I have purposely avoided many 

 technical terms which are so often puzzling to a beginner, but this has not 

 always been possible, hence the following explanations have been added for 

 the benefit of those who are not experienced in " shell lore " by which a 

 shell is described. 



Take a shell of any snail and observe : 



(1) An aperture or mouth. 



(2) The Up bordering the mouth. 



(3) The nucleus or apex of the cone is the oldest part at the top of the 



spire. 



(4) Lines of arowth are transverse ridges parallel to the mouth and indicate 

 successive positions of the mouth during the growth of the shell. 



Take a shell of a Unio and observe — 

 Outer surface — 



(1) The shape. 



(2) The umbo, the oldest part of the valve of each shell lying close to 

 the hinge line and about a \ of the length of the shell from its anterior 

 end. 



(3) Lines of growth are concentric curves on the surface of the shell mark- 

 ing growth stages. Each line was at one time the margin of the shell. 



(4) The ligament connecting the two valves together along the mid- 

 dorsal line. 



Inner surface — I. — 



(1) The white pearly appearance. Here lines of growth are visible 

 but not so readily distinguished as on the outside. 



(2) The teeth. Two shelly ridges just under the ligament, the Hinge teeth. 

 Two shorter projections at the posterior end, the Posterior teeth. 



II. — Muscular Impressions. 



These are slight depressions on the surface and mark the place of attach- 

 ment of the animal to the shell. 



(1) Anterior adductor impression. — Is a large oval depressed area close to 



the anterior end of the shell and near the dorsal edge. 



(2) Anterior retractor impression.-Sma.il and continuous with the posterior 



border of the anterior adductor impression. 



(3) Protractor impression. — A small impression behind the lower part ot 



anterior adductor impression. 



