98 BULLETIN OF THE BUREAU OF FISHERIES. 



no more than an expansion of the Mississippi River, further observation shows it to be 

 a true lake in many of its characters, as in clearness of the water, depth, growth of vege- 

 tation, and virtual absence of current. In both of the lakes mentioned, a characteristic 

 lacustrine species, the fat mucket, or Lake Pepin mucket, Lampsilis lutcola, which is 

 thin-shelled and worthless in ordinary inclosed lakes, attains so fine a commercial quality 

 of shell as to appear almost as a distinct variety. Caddo Lake, La., which is interpolated 

 in the course of a stream, possesses a rich mussel fauna and has been the scene of active 

 pearl fishery (Shira, 191 3). The small Rice Lake near La Crosse, Wis., which is, in 

 effect, an expansion of a thoroughfare connecting the Black River, near its mouth, with 

 the Mississippi River, also supports a varied and luxuriant mussel fauna. Where lakes 

 are freely connected with rivers, as are those first mentioned, or as are others with short 

 open outlets to the rivers, the lakes and rivers have many species of mussels in common. 



In Lake Pepin, Shira (report in manuscript) found that the distribution of the mus- 

 sels is confined wholly to the shore line and the flats within a maximum depth of 25 feet; 

 no mussels at all were taken in the deep central part of the lake. In certain places the 

 mussels were quite densely distributed, fonning very well-defined beds, but as these 

 beds were generally connected by areas of lesser population, a more or less continuous 

 mussel bed was found to occur on each side of the lake. The largest and most extensive 

 beds were located on a gravel bottom, or a mixture of gravel and sand. Several good 

 though less extensive beds occurred on bottoms containing a considerable percentage 

 of mud. 



The upper end of the lake evidently serves as a settling basin for the silt poured in 

 from the river proper, and for a distance of about 2 miles below the entrance of the river 

 the lake is comparatively shallow with a soft oozy bottom. In this section of the lake 

 very few mussels are found. 



Shira records 32 species of mussels (report in manuscript). Ten of the most abund- 

 ant species with the percentage of occurrence afe given as follows: 



Per cent. 



Fat mucket, Lampsilis luieola 3i-5 



Spike, Unio gibbosus i3- o 



Blue-point, Quadrula plicata 12. 7 



Pig-toe, Quadrula undata 10. o 



Pink heel-splitter, Lampsilis alata 8. 3 



Pocketbook, Lampsilis ventricosa 5. 6 



Slop-bucket, Anodonta corpulenta 5. 5 



Squaw-foot, Strophitus edenhilus 4. 3 



White heel-splitter, Symphynota complanata 2. 3 



Black sand-shell, Lampsilis recta i. 4 



In small lakes of considerable depth and without circulation, except as effected by 

 winds and changes of temperature, animal life generally is absent or greatly restricted 

 in the deeper portions, and mussels, when present, are confined to zones near the shores 

 (Headlee and Simonton, 1 904). Muttkowski (i 91 8) in Lake Mendota found the optimum 

 conditions for mussels at depths of 6 to 9 feet on sand bottom, but there was not an 

 extensive mussel population in the lake as a whole. 



The restriction of mussels to the border zones is indeed generally characteristic 

 of the lakes of the Middle Western States, and even in this environment where the cir- 

 culation effects of wave action may be felt, the mussels are stunted in growth. In their 

 report on the mussel fauna of Lake Maxinkuckee, Evermann and Clark (1918, p. 251), 



