
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, 
Limnea stagnalis lilliane, Limnea emarginata and Physa sayw 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, 
Limnea stagnalis lilliane, 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, Limnea 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 would easily be dislodged. The fact that the smaller 
shells are found at the least depth, while the larger Limne@a emarginata 
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 rocky 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 Limnea sanctemarie 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 
