FRESH-WATER MUSSELS. IO5 



the clay except one example of the three-ridge, QuadrtUa undidata, and that was in a 

 spot where the clay was mixed with mud and was distinctly softer. Some spikes, Unto 

 gibbosus, were found lying on the blue clay but not embedded; it seemed evident that 

 they were unable to penetrate so tough a bottom. 



The character of the soil has an effect upon the amount of materials carried in 

 suspension in the water. If the amount is too great, as over soft mud or over extremely 

 fine sand, under some conditions the mussel becomes smothered, or having no chance 

 to feed, is starved. Too much decomposing organic matter in the soil is said to cause 

 enough acidity to attack and erode the shell. For several reasons, therefore, areas of 

 rapid silt deposition, or soft-mud bottoms, are quite unfavorable to mussels. Mussels 

 are usually found in rivers in places where the bottom is swept clean by the current, 

 even though in flood time the water may be heavily laden with silt in suspension. 



The selection by some species of bottoms of gravel, pebbles, and bowlders as most 

 favored habitats can readily be understood from the foregoing remarks; but there are 

 still other favorable features of rough bottoms. The very stability of the larger-sized 

 materials protects the bottom from washing, and may save the mussels from being 

 smothered or carried away. It is of advantage to mussels to be surrounded by numerous 

 other animals, especially by the smaller ones, which furnish attraction to fishes and thus 

 promote the' reproduction of the mussel. Many of these small animals live attached to 

 stones, thus giving added value to gravelly and rocky bottom. In gravels, too, the 

 youngest mussels may be protected through inaccessibility to enemies, and as they 

 grow older the resemblance to small pieces of stone among which they lie may be the 

 cause of escape from enemies. As previously indicated (p. 97), where bowlder rock or 

 cobblestone bottoms occur in regions of rapids, mussels are commonly found abundantly 

 and occur over the entire river. 



The following table (6) embodies the experience of several observers regarding the 

 preferences exhibited by 62 common species of fresh-water mussels for bottoms of differ- 

 ent characters. In view of the intergradation of the several types of bottom and the 

 almost unlimited variety of mixtures of sand, gravel, mud, and clay, the classification of 

 the bottoms for the purpose of a table must of necessity be rough, and the characterization 

 of mixed bottoms may in some cases be affected by the personal equation of the observer. 

 Young mussels may have bottom requirements somewhat different from those of adults. 



EXPLANATION OF TABLE 6. 



The letters refer to the experience of the several observers (including the present authors and three 

 previous ^Titers), as follows: A, Baker (1898); B, Call (1900); C, Clark; D, Howard; E, Scammon (1906); 

 F, Shira; G, Coker. 



The use of large capitals indicates that, according to the observer whose letter is in large capitals, 

 a certain type of bottom is preferred by the particular species of mussel. Wherever a small capital is 

 used, the observer corresponding to the letter has indicated the type of bottom as favorable for the 

 particular species of mussel, but not necessarily preferred to other favorable bottom.'^ 



The observations of Shira refer largely to lake conditions (Lake Pepin, Lake Pokegama, and Caddo 

 Lake). 



The observations of Coker refer primarily to shallow rivers (Grand River, Mich.). 



The habitats indicated by Howard are based chiefly on the observed preferences of juvenile mussels 

 in rivers, streams, ponds, and slues to the exclusion of true lakes. 



