308 THOMAS J. HEADLEE. 



the fact that heavy beds were found in different temperatures, 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 mud bottoms in ten feet none were taken in any case. 

 Food supply cannot be effective, for it is about equally abundant 

 throughout the zone. The food consists of diatoms and other 

 low algae forms and one-celled animals. 



It seems to us that there are three factors which control the 

 distribution of the mussels in Winona lake --wave action, charac- 

 ter of the bottom, and enemies. 



The first factor is active only in water less than three feet deep. 

 As U. hi (coins and A. grandis appear in this region they are sub- 

 jected to this agency. Specimens of both A. grandis and the 

 dark form of U. Inteoliis 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. Inteolus 

 moving from the water's edge out into deeper parts. These facts 

 point to the conclusion that the two first mentioned forms are, in 

 general, prevented from occupying shallow water by wave action, 

 but that the light form of U. liiteolits, being very active and having 

 a relatively thick shell, can well occupy this region. U. glans, 

 (J. fabalis, U. snbrostratiis, U. rubiginosits, A. grandis and A. c den- 

 tula, if washed ashore would be unable to get back, and the shells 

 of the last two would quickly be broken through by wave action. 

 Those forms which occur in this region, especially those to which 

 washing ashore is fatal, habitually bury themselves so deeply in 

 the mud that only the siphonal tip of the shell projects and thus 

 they are protected from the dragging action of the waves. 



The character of the bottom applies throughout the mussel 

 zone and is by far the most important factor. The bottom in the 

 weed patches differs from that in the deeper parts of the lake 

 in being less soft. The sandy and gravelly bottom affords 

 firm foothold and allows the mussel to assume that position which 

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

 pure black mud allows it to sink so far as to be smothered. 



