60 MICHIGAN SURVEY, 1908. 



water-striders and whirligig beetles is found on the surface, while 

 numerous mussel shells live on the silt or sand bottom. In Sumner Lake 

 and others of limited area the motion of the water has no measurable 

 effect on the biota, and they will therefore be treated under a separate 

 heading. 



The distribution of shells along the shore, particularly species of 

 Limnaea and Physa, is of especial interest. Having relatively low 

 motility they are correspondingly limited in their distribution and the 

 factors governing it are more readily determined. These will perhaps 

 be made clearer by concrete illustrations. 



Tonkin Bay is a small inlet about half a kilometer long, opening 

 to the east upon the lake, and with steep, approximately parallel sides. 

 It is narrowed half way up by two beaches lying perpendicular to its 

 length. By this the wave action on the upper part is reduced, but still 

 may sometimes be sufficient to wash heavy drift-wood 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 Limnaea 

 stagnalis L. (Nos. 50, 54, 57), Limnaea emarginata Say (Nos. 50, 57), 

 and Physa sayii (Tap.) (Nos. 50, 57), 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, Limnaea stagnalis, 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 hori- 

 zontal or flat bottom. 



Conglomerate Bay is a rocky inlet {Fig. 11) similar to the one jvist 

 described 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 

 {Fig. Jf), almost without driftwood. At the upper end of this bay along 

 the north side Limnaea emarginata Say (Nos. 118, 125) andP/iysft sayii 

 Tap. (Nos. 118, 125) are found in water 1.5 — 45 cm. deep, in the deeper 

 water on the tops of flat rocks, in the shallower water, also on the verti- 

 cal 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, avoid the shallow 

 water where the wavies 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 Limnaea 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 essentially 

 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 Limnaea stagnalis occurs in abundance, always at a depth 



