24 THE CHICAGO ACADEMY OF SCIENCES. 



still may sometimes be sufficient to wash heavy driftwood upon the 

 beach. In the outer half the wave action is but slightly less than on 

 the lake itself, and no shells are found. In the inner or upper half, 

 Linuicca stagnalis lilliancc, Limncea cmarginata and Physa sayii Tap. 

 live along both sides, where the bottom is rock, but not across the ends. 

 They live only on a rock substratum, which may be either horizontal 

 or vertical, and in water up to 45 cm. in depth. The larger species, 

 Limncea stagnalis lilliana, is more abundant in the deeper water, and 

 only the smaller species live at a depth less than 15 cm. They then 

 prefer the vertical walls to the horizontal or flat bottom. 



"Conglomerate Bay is a rocky inlet similar to the one just de- 

 scribed and about 1.6 km. long. Being wider at its mouth than Tonkin 

 Bay, the force of the wave action is felt farther up the bay. Near 

 the end the waves have little effect, as is evidenced by a sandy beach, 

 almost without driftwood. At the upper end of this bay, along the 

 north side, Limncea emarginata and Physa sayii Tap. are found in 

 water 15-45 cm. deep, in the deeper water on the tops of flat rocks, 

 in the shallower water, also on the vertical sides and in small crevices. 

 They never occur on the sand or gravel deposited around the rocks, 

 as is frequently the case near the sand beach at the upper end of the 

 bay. The distance to which they extend from shore is greatest opposite 

 the concavities of the shore line and least opposite the small rocky 

 headlands. Their distribution in both Tonkin Bay and Conglomerate 

 Bay seems to be regulated mostly by the wave action, since they seek 

 the most protected places and avoid the shallow water where the waves 

 would strike them most, and do not live on loose or small rocks, gravel 

 or sand which w T ould easily be dislodged. The fact that the smaller 

 shells are found at the least depth, while the larger Limncea cmarginata 

 inhabits the deeper water, would indicate that the small size of the 

 former renders them less easily dislodged by the waves. Again, their 

 greater abundance on the north side suggests the possibility of a light 

 relation. 



''Siskowit Lake, with its rock}' shores and large area, offers essen- 

 tially the same condition as Lake Superior itself, and the shells have 

 the same general distribution. Along the very gently sloping rocky 

 shore near the outlet Limncea sanctcemarice occurs in abundance, always 

 at a depth of 10-40 cm. Along the south side of a large island near 

 the south shore, where they are sheltered from waves in every direc- 

 tion, the same species is abundant. They live on rocks in the full sun 

 in water 10-45 cm. deep, with the optimum depth at 20-25 cm. They 

 may occur on the tops or sides of rocks, but never on the sand between 



