175 



least, is shown liy the fact that heavy beds were foiuul iu different 

 temiieratiires, and by the fact that temperature variation in the mussel 

 zone did not amount to more than two degrees: oxygen is not important, 

 for the supply of oxygen throughout the mussel zone varies very little; 

 pressure can have but little to do with it. for we found specimens on a 

 sandy bottom in twenty-two feet of water, while on dark marl bottoms 

 in ten feet none Avere taken in any case. Food supply can not be effec- 

 tive, for it is about equally abundant throtighout the zone. The food 

 consists principally of diatoms; secondarily of low algoe forms, and one- 

 celled animals. 



It seems to us that there are three causes which control the distribil' 

 tiou of mussels as it appeared in Winona Lake— wave action, character 

 of the bottom and enemies. 



The hrst cause applies only in water less than three feet deep. 

 As U. luteolus and A. grandis appear in this region they are subjected 

 to this agency. Specimens of both A. grandis and the dark form of U. 

 luteolus have been found washed ashore after a storm, and scores of these 

 shells appear along the shore line. Under similar conditions we have 

 seen the light form of U. luteolus moving from the water's edge out 

 into deeper parts; these facts point to the conclusion tliat the two first 

 mentioned forms are prevented from occupying shallow water by wave 

 action, but that the light form of U. luteolus. being very active and 

 having a tliick shell, can well occupy this region. Not only is washing 

 ashore fatal to A. grandis. but wave action quickly action quickly wears 

 away the shell and leaves the animal open to attack. Unio glans, fabalis, 

 edentula, and subrostratus are very light and slow moving; U. rubigiU" 

 osus is heavy and clumsy, like the dark form of luteolus; the first three, if 

 washed ashore, would be unable to get back, and their shells would be 

 unable to resist the wearing action of the waves, while the last men- 

 tioned form could resist Avave Avearing but Avould be unable to get back 

 if washed ashore. 



The character of the bottom applies throughout the mussel zone. 

 The bottom in the weed patches differs from that iu the deeper parts 

 of the lake in being slightly less soft. The sandy and gravelly bottom 

 affords tirm foothold and alloAvs the mussel to assume that position 

 which enables it to get the best supply of food and oxygen, while 

 the pure marl allows it to sink so far as to be smothered. Even if 

 the animal does not sink entirely under, the overlying sediment is suf- 



