120 Wisconsin Academy of Sciences, Arts, and Letters. 
probably due to the better clinging powers of the latter, many of 
which are especially modified to meet these conditions, as for ex¬ 
ample the flattened bodies of Psephenus and Heptagenia. Three 
groups of associated animals make up 85 per cent of the entire pop¬ 
ulation of this kind of habitat, Amphipoda with 23 per cent, Tri- 
choptera with 38 per cent, and Diptera with 24 per cent. 
Ecological conditions in the boulder habitats are usually rather 
rigorous. Feeding can only be done during periods of comparative 
calm. The snails are all phytophagous, eating the algae. The 
clams may secure their sustenance from the microorganisms that 
are brought to them by the waves and currents. The difference in 
shape and size between the clams of the river and lake may be due 
in part to the greater difficulty of obtaining a sufficient quantity of 
food in this rougher environment. The .same laws govern many 
of the associated animals as regards their food supply. 
Life on Gravel Shores (Table 1). Gravel habitats occupy a 
large part of the shore line around Lake Winnebago. Gravel is 
often found in association with boulders but is more usually found 
occupying the place of the boulders on some exposed shore. It 
varies in character from cobble stones to fine gravel, with which 
there is usually more or less sand, often in spots. The gravel or 
cobble is generally smooth and polished by the continuous wave 
action. This kind of habitat is such that the inhabitants must be 
good dingers or burrowers to escape being destroyed by the shift¬ 
ing character of the environment. Gravel beds may occur from 
shore to a depth of nearly two meters. In one place it was found 
at a depth of 3.4 meters, and subaqueous bars of gravel extend 
lakeward from the points for a long distance. Vegetation consists 
of the same species noted in the boulder habitats, with the addition 
of Potamogeton lucens and Castalia odorata. 
Among the Mollusca the same kind of naiades occur but the num¬ 
ber of species is reduced to six. Sphaerium appears sparingly and 
Pisidium increases in both species and individuals. A number of 
new gastropods appear and several of the boulder species (Pleu- 
rocera, Physa) are less numerous, while Lymnaea has disappeared. 
Amnicola and Yalvata occur in some abundance owing probably to 
the presence of filamentous algae which is attached to the larger 
stones. Three species of gastropods are common and abundant on 
gravel bottom; Amnicola limosa porata, Amnicola emarginata, and 
Valvata tricarinata and its varieties. Habitat number 40 was the 
