312 THOMAS J. HEADLEE. 



almost perfect agreement with those of Winona. There are, 

 however, two noteworthy differences very few mussels occur 

 in water deeper than twelve feet, although our dredge scraped 

 over gravel in seventeen feet ; very few are to be found in water 

 less than three feet deep. I have no explanation to offer for the 

 first variation ; the second is undoubtedly due to human 

 agency. 



The mussel zone in Center Lake, therefore, extends over 

 gravel, sand, and white marl banks in from three to twelve feet 

 of water. The deep-water edge is determined by the character 

 of the bottom, modified locally, doubtless, by some as yet 

 unknown factor, while the shoreward edge is determined by the 

 ravages of small boys. Neither the muskrat nor the action of 

 the waves is effective here for the first is not present and the 

 second is not powerful enough. That the shoreward limitation 

 is due to man is shown by the fact that only those individuals of 

 the deep-burrowing species which best exemplify this tendency, 

 are able to survive in the area from three feet to the water's edge. 

 So rigid is this selection that we looked long and carefully before 

 we found any mussels at all in this region. We found the light 

 colored form of U. luteoh/s, U. rubiginosns, and U. subrostratus , 

 and in every case the mussel was buried so deeply that only the 

 extreme tip of the shell projected. Even this was frequently 

 covered with a spongy growth, which still more effectually 

 concealed it. 



Pike Lake, in size, falls between Center and Winona and in 

 depth it does not exceed the former. The bottom conditions 

 resemble those of Winona and considerable diversity of beach 

 obtains. All the species found there, except the Corbiculidse, 

 which we did not seek, were taken here. Everything seemed 

 favorable for a large bivalve fauna, yet mussels are almost as 

 scarce as in Center. I can give no complete explanation for this 

 condition, although here, too, human agency is certainly a very 

 important factor. 



The mussel zone extends from a depth of four feet to fifteen 

 feet on the sandy and gravelly beaches ; on gray marl beaches 

 its shoreward edge extends into shallower water. The deep 

 water edge is determined by the character of the bottom, the 



