. Bulletin of the United States Fish Commission. Fisheries -- United States; Fish-culture -- United States. 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 distomid
. Bulletin of the United States Fish Commission. Fisheries -- United States; Fish-culture -- United States. 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 discolorati
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