as belonging to two or three different species may be only ecological varia- 

 tions of one species. As a general rule, clams from acid or neutral waters 

 have thinner shells than those of the same species in alkaline or limestone 

 regions. Individuals in small streams and in rapid water are not as wide 

 proportionally as those in large or slowly moving streams. Another variation 

 which has frequently pu2;2,led taxonomists is found in some of the normally 

 knobby or pustulate clams, which in swift headwaters may develop quite smooth 

 shells. The color of the nacre or mother-of-pearl lining also varies decidedly. 

 In some localities all shells of one species may be pink or reddish inside and in 

 other localities white, while sometimes both red and white nacred varieties may 

 occur in the same place. Age and sex may also cause marked differences. 

 Female shells are often more swollen and rounded posteriorly than the males. 

 Old shells are usually duller colored than the young, and may lose the rays 

 altogether. The ratio of length to depth may also vary with the age of the 

 clam. 



About 1890 began the first serious attempts to utilize the heavy-shelled 

 varieties of clams from the Mississippi valley for pearl buttons, knife handles 

 and other novelties. By 1910 a twenty million dollar industry had developed, 

 thousands of tons of clams were gathered annually, and button factories sprang 

 up wherever shells were available. Few of these factories were large and, when 

 the local supply was exhausted, they were moved to other localities. Many of 

 them cut, or rather sawed, out the "blanks'' or discs, and sent them to other 

 points to be finished. The almost unregulated fishing, the improvement of 

 collecting methods, the digging of drainage ditches and the straightening of 

 streams, and the increased pollution of rivers by sewage and factory wastes 

 quickly led to the almost complete extinction of many of the most useful species 

 of clams and the resultant decline of the industry. Importation of marine shells 

 and the introduction of new synthetic plastics now seem to have rung the knell 

 of the fresh-water, pearl button industry. 



An added inducement to clammers was the possibility of finding a valuable 

 pearl, for fresh-water mussels produce pearls hardly inferior to those of tropic 

 seas. An especially fine pink or green pearl might bring enough money to 

 enable the finder to have a never-to-he-forgotten spree. As a matter of fact, 

 the annual crop of pearls from the Mississippi valley is estimated at about 

 $300,000.00 Most of them, however, are irregular in si2,e and are termed 

 slugs, bringing to the finder only a few dollars. 



Studies have shown that almost any foreign body, such as a cestode, nema- 

 tode, or even a grain of sand, may become the nucleus of a pearl. Layers of 

 nacre are deposited over this nucleus by the mantle. Those developing in the 

 soft parts near the beak cavity are usually the most symmetrical and therefore 

 the most valuable. Many centuries ago the Chinese discovered that it was 

 possible to induce pearl formation by inserting some object which might serve 



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