FRESH-WATER MUSSELS. 



133 



ABNORMALITIES IN GROWTH OF SHELL. 



Seriously malformed mussels are not infrequently found, and peculiar interest 

 attaches to these because shellers generally entertain the belief that a mussel wth de- 

 formed shell is most likely to contain a pearl. It seems possible that this belief is not 

 without some foundation. Pearls probably occur more frequently in parasitized mus- 

 sels, and many of the obser\'ed malformations are undoubtedly due to parasites. 



A few distomids upon the mantle of Anodontas along or near the dorsal fold evidently 

 cause rusty stains in the nacre, abnormal growths on the inner surface of the shell, de- 

 formities of the hinge teeth, and dark or poorly formed pearls. Another parasite which 

 infests the reproductive or- 

 gans may almost completely 

 destroy the gonads of the fe- 

 male mussel, and in such case 

 the female may develop a 

 shell in the form of a male or 

 in a form intermediate be- 

 tween that of the male and 

 the female. There is evi- 

 dence that parasites found 

 encysted in the margin of the 

 mantle may give rise to stains 

 on the nacre at the margin of 

 the shell, that others cause 

 the not unfamiliar steely or 

 leaden-colored margins of 

 shells, while some produce a 

 pitting of the inner surface 

 of the shell. 



One of the most common 

 and serious defects of other- 

 wise valuable commercial 

 shells is the presence of yel- 

 low and brown spots or bluish or greenish splotches in the nacre. Regardless of the 

 texture of the shell, the partially or wholly discolored buttons must be given a ven,- low 

 grade. The spots are not always found upon the surface but ma)' lie deep within the 

 nacre, to be brought out in the finished button by the processes of shaping and polishing. 

 Spotted shells are most common in certain rivers or parts of rivers, particularly where 

 the current is sluggish as in partly inclosed sloughs. Some of these discolorations are 

 often observed to be associated with a parasitized condition of the mussels, but it is 

 not probable that the spots are always due to parasites. The U. S. Bureau of Stand- 

 ards, in connection with an investigation of the bleaching of discolored shells, has found 

 that the dark-yellow and bro%\Ti spots are mud fixed by the nitrogenous organic layer 

 which binds together the calcium carbonate, and that the pale-yellow color is apparently 

 due to an organic coloring matter in the organic layers. That bureau also reports that 

 the color of the pink shells is due to an organic coloring which is not confined to the 

 organic layer but permeates the whole shell. 



Fig. 6. — A shell of the pocketbook, LampsUis -ientriccsa, which was recovered after 

 having been measured and confined in a wire cage in the Mississippi River for 

 two years, four and a half months. The line a, an interruption ring, marks the 

 size at the time of measuring. The lines b and c evidently correspond to the two 

 periods of -n-inter intervening. The inconspicuous sign of a winter interruption 

 preceding the date of measurement does not appear in the drawing. Natural 

 size. (After Lefevre and Curtis.) 



