96 BULLETIN OF THE BUREAU OF FISHERIES. 



Grand Rapids and the mouth at Grand Haven. It might be thought that this was due 

 to the fact that there would be fewer obstacles to the passage of mussels downstream 

 than to their distribution in an upstream direction. It seems sufficient, however, to 

 assume that the unequal distribution is due rather to the greater variety of conditions of 

 depth and of fish associates presented in the lower portion of the river. Shallow water 

 only is found in the upper river, except as artificial pools have been formed in recent 

 years by the construction of dams, while in the lower river deep water prevails in its 

 channel and all lesser depths are found between the channel and the shores. The very 

 breadth of the lower part of the river affords also a greater area for fish and mussels. 



A difference of up-river and down-river habitat is presented by the distribution of two 

 closely related species, the three-ridge, Quadrula undulaia, and the blue-point, Quadrula 

 plicata; the former, a more compressed and rougher form, is found in the more rapid 

 waters of upstream habitats, while the latter, being thicker and less ridged, occurs in the 

 deeper waters of the lower parts of a river system." (See Clark and Wilson, 191 2, and 

 Wilson and Clark, 191 2.) 



In some rivers mussels are almost entirely lacking for long distances, as in the main 

 course of the Missouri River for hundreds of miles above its mouth, where the absence 

 of mussels is apparently due to the rapidly shifting bottom of sand. The Red River, 

 with its heavy load of silt and its habit of suddenly cutting into its banks and changing 

 its course, is manifestly unsuited for mussels, and examination of its bottom in many 

 places by Isely (1914) and Howard revealed extremely few mussels (Pl.VII, fig. 2). There 

 is also a virtual absence of mussels in the Mississippi River, except close alongshore, below 

 the mouth of the Missouri River. Examination of the Musselshell River in Montana by 

 J. B. Southall in 1919 revealed the presence in numbers of only a single species of mussel, 

 and this a species {Lampsilis luteola) characteristic of lakes, which lived in the portions 

 of the river deep enough to remain as isolated pools during the periods of dry weather. 

 In the east fork of the Chicago River, Baker (1910) found only 3 species, and these were 

 mussels characteristic of pond habitats, which were able to survive the dry seasons in 

 the small ponds left isolated in the deeper parts of the river channel. 



The James River, in North and South Dakota, though having very few fish, was found 

 to possess a comparatively varied and abundant mussel fauna in the still waters between 

 shallow riffles ; but there was evidence that the mussels were derived from fish infected 

 in other waters, that ascended the stream in times of flood (Coker and Southall, 1915). 



The suitability of any section of a stream for the growth of mussels arises from a 

 diversity of causes, including the nature of the rock or soil through which the stream is 

 flowing, the character of the drainage waters entering the river at or above the section, 

 the gradient of the stream bed with its effect upon depth and currents, and the species 

 of fish which frequent the region. 



Barriers in the course of a stream such as natural falls, or artificial dams, if impassa- 

 ble to fish, may have an effect upon the distribution of mussels. Wilson and Danglade 

 (1914) found no mussels of the genuS Quadrula above the Falls of St. Anthony in the 

 Mississippi River, although several species of this genus are very common in the river 



a Ortmann C1920) has definitely shown, for certain species, that; "(i) The more obese (swollen) form is found farther down 

 in the large rivers, and passes gradually, in the upstream direction, into a less obese (compressed) form in the headwaters; (2) 

 with the decrease in obesity often an increase in size (length) is correlated; (3) a few shells which have, in the larger rivers, a pe- 

 culiar sculpture of large tubercles, lose these tubercles in the headwaters." He ascertains also that these laws do not apply to all 

 species. 



