1 88 BULLETIN OF THE BUREAU OF FISHERIES. 



the geographical distribution of the Unionidse throughout the Mississippi Valley under 

 the direction of the Bureau of Fisheries during the past four or five years. 



While working in the neighborhood of La Crosse, we made a careful investigation of 

 the west channel of the river at this locality, with a view to determining whether places 

 of this nature presented favorable conditions for experimental rearing of young mussels. 

 As is usually the case with the accessory channels of the river in this region, the west 

 channel at La Crosse is dammed across its head for the purpose of confining the water 

 in the main channel, and, although at high-water stages of the river the dam is sub- 

 merged, during the greater part of the year the volume of water in the channel is greatly 

 reduced and the current retarded. These dams, however, are never tight, and a greater 

 or less quantity of water constantly seeps through them. A thorough study of this 

 channel showed that it contained very few mussels indeed, and of those species that 

 were found living in small numbers under these conditions, the majority belonged to 

 Lampsilis, ventricosa being by far the most abundant form. Whenever a channel of 

 the river is dammed, the slackening of the current causes an enormous sedimentation to 

 take place, and in these "sloughs," as such obstructed channels are called, sand and 

 mud bars and shoals have been formed to an extent varying with the length of time since 

 the dam above them was built. The more sluggish species of mussels, like the quadrulas, 

 are especially ill adapted to these conditions and are frequently buried and destroyed 

 by the deposits of silt in the river, an occurrence of which we found abundant evidence. 

 With the more actively moving and burrowing species, as those of Lampsilis, the case 

 is different, for apparently they may adjust themselves more readily and by their far 

 greater ability to move from place to place they may avoid the danger of being buried. 

 We found little evidence that the quadrulas, for example, move about at all, while, on 

 the contrary, the tracks of slowly wandering individuals belonging to the species of 

 Lampsilis were everywhere conspicuous on the sandy bottoms of the shallow sloughs. 



An interesting case of the destruction of mussel beds in situ by sedimentation is 

 shown in figure 70, plate xvii, which is a photograph taken on the bank of a slough, near 

 Muscatine, Iowa, which was exposed by a gully washed out by rains and cut directly 

 through an extinct mussel bed. The photograph shows the surface of the cut where the 

 mussels are exposed as they lie embedded in the muddy bank. The bed is buried under 

 about a foot of mud, and it is interesting to note that the valves of the mussels are closed 

 and lying together in pairs. The latter fact proves conclusively that this is not an old 

 shell heap, for the valves of the shells would be found scattered and separated in that 

 event, but a mussel bed which had once existed in the river near the bank. It was 

 probably buried under the deposits of sand and mud which followed the building of the 

 dam across the head of the slough. An investigation of the species represented in the 

 bed showed that they all belonged to Quadrula, being chiefly cbena, pustulosa, and irigona, 

 while not a single individual belonging to Lampsilis could be found in it. It is probable, 

 as already stated, that it is the sluggish species, like those of Quadrula, that are the prin- 

 cipal sufferers in catastrophies of this nature, and are caught and smothered in the process 

 of sedimentation, while the propensity to wander possessed by the more active species 



