FRESH-WATER MUSSELS. 



97 



below that barrier. Wilson and Clark (1914) found only 4 species of mussels in the 

 Cumberland River above the Cumberland Falls (one of these probably planted), while 19 

 species were taken in the pool immediately below the falls, but in this case the conditions 

 prevailing in the river above the falls appeared distinctly unfavorable for fresh-water 

 mussels. An impassable dam formed after mussels were generally distributed throughout 

 a stream would have little significance with reference to the distribution of mussels the 

 hosts of which subsequently thrived both above and below the dam. The effect, how- 

 ever, of a dam in changing a region of rapids into a pool, might cause the mussel fauna of 

 swift waters to give place to a fauna of slack-water habitat. 



Studies of rivers in cross section indicate that there may be quite definite distribu- 

 tion of life with reference to the banks. Shelf ord (191 3) has discussed a horizontal 

 arrangement of animals that is best illustrated in the cross sections of cur\^es where there 

 is a horizontal gradation in rate of current and in size of material in the bed of the stream. 

 In the strong current only the coarsest materials are dropped, while the finest silt is 

 deposited where the flow is most retarded. The depth of water is doubtless one factor 

 governing the horizontal distribution of mussels, but the nature of the bottom material 

 is of first importance. Howard (Survey of Andalusia Chute, Mississippi River, report in 

 preparation) found in a branch of the Mississippi, following a comparatively straight 

 course (not on rapids) and averaging 1,200 feet in width, that mussels were uniformly 

 restricted to a border 200 feet from the shore line. (See table below.) Some mussels 

 were found almost anywhere along this border, but occurring in beds at points where the 

 channel touched the shore and where bottom conditions were favorable; depth seemed 

 to be a minor factor as affecting the distribution. 



The following table (3) indicates the results of a sample series of unit hauls taken at 

 stated distances from the water's edge and so represents the distribution in a cross 

 section of the river. It is not typical because of the narrowness of the bed on the left 

 bank, but it illustrates in a general way the distribution found throughout the survey. 

 Table 3. — Distribution op Mussei,s in Andalusia Chute, Mississippi River. 



Where there are rapids with bowlder or cobblestone bottom across a river of this 

 size, it is known that mussels are not limited to such a border but are found at all points 

 across the stream. 



LAKES. 



The mussels from some lakes are large and heavy-shelled, while in others they are 

 small, thin-shelled, and stunted. These extremes represent the varied conditions 

 which lakes present in respect to mussel life. 



Lakes that have a free circulation of water seem to be favorable; such are those 

 that are interposed in the course of a river. A favorable feature in such cases, no doubt, 

 is the direct and free connection with streams that are well supplied with mussels. 

 Examples are Lakes Pepin and Pokegama, Minn., and the former is noteworthy for the 

 abundance of mussels produced. Though at first sight Lake Pepin might be considered 



