I02 BULLETIN OF THE BUREAU OP FISHERIES. 



and protective conditions; stability of soil is important for the establishment of the 

 juveniles, for otherwise they will be overwhelmed. For some species objects for attach- 

 ment, to which the b)'ssus of the juvenile may be fastened, may also be necessary. 

 Most of the varieties of bottom soil encountered are composed of one of the following 

 materials, or of mixtures of two or more of them: Silt, mud, marl, clay, sand, gravel, 

 pebbles, cobbles, bowlders, and ledge rock. 



In rivers, sandy bottoms are regions of change comparable to sand-dune areas on 

 land where immobile forms are killed. Sand bottoms occur extensively in many rivers 

 and they may be veritable deserts. Rivers like the Missouri are devoid of mussels for 

 hundreds of miles partly because of a preponderance of bottom of shifting sand. Mussels 

 when found on sand bars in rivers are in transit seeking more stable conditions. Although 

 comprising regions of instability in rivers where decided currents prevail, bottoms of 

 sand may offer more favorable conditions in lakes where they furnish a permanent 

 habitat for mussels. 



A greater variety of bottoms favorable for mussels, as well as a more indiscriminate 

 disposition of them, prevails in rivers than in the other bodies of water considered. In 

 many lakes there is a more definite sorting of materials, leading especially to a segregation 

 of the finest sediment in the deeper portions of the lake to form a bottom that is very 

 soft and generally unsuitable for the Unionidae; mussels possessing much mass would 

 sink too deeply and have the gills too much clogged with silt to survive (Headlee and 

 Simonton, 1904, p. 176). Where such conditions prevail the mussels are found near 

 shore. 



Headlee (1906, p. 315) summarizes observations and experiments in certain lakes 

 of Indiana in the following words: 



The work of 1903 and 1904 shows conclusively that the mussels of Winona, Pike, and Center Lakes 

 can not exist on the fine black mud bottom — they become choked with mud and apparently smother — 

 and that the light-weight forms and the forms exposing great surface in proportion to weight can rest 

 on top of comparatively soft mud and can, therefore, live farthest out on the deep-water edge of the 

 bed. Because the mussels can not occupy any region where the pure black mud is present, they are 

 confined by it to isolated beds and narrow bands of shore line. 



I believe that the whole evidence of the distributional and experimental work of 1903 and 1904 

 points clearly to the character of the bottom as the great basal influence in the distribution of mussels in 

 small lakes generally. 



The species he dealt with were the fat mucket, Lampsilis luleola, Larnpsilis sub- 

 roslrata, Qiiadrula rubiginosa, Anodonta grandis, and other small species with light shells. 

 While his conclusion accords generally with the observations of the writers in other 

 waters, the exclusion of mussels from mud bottoms can not be taken as an invariable 

 rule. In the Grand River, at Grand Rapids, Mich., for example, one of the authors has 

 observed such a heavy -shelled mussel as the three-ridge, Quadrula undulata, living in 

 considerable numbers along with the light floater (Anodonta) in very soft mud. Also, 

 in Mississippi Slough, in the Wisconsin lowlands along the Mississippi River opposite 

 Homer, Minn., the blue-point, Qtmdrula plicata, the pimple-back, 0. pustulosa, and the 

 pig-toe, Q. undata, have been found in considerable numbers on a soft-mud bottom 

 along with the heel-splitters, Symphynota complanata and Lampsilis alata, and the 

 slopbucket, Anodonta corpidenta. 



