116 Wisconsin Academy of Sciences, Arts, and Letters. 
which, after flowing for a distance of about two miles, enters Lake 
Winnebago. The waters of this lake find outfiow by way of the 
lower Fox Kiver which empties into Green Bay of Lake Michigan. 
The change in the physical environment from the comparatively 
quiet waters of a river to the rough and turbulent waters of a 
lake, has been strikingly refiected in at least one group of animals, 
the mollusks, the lake species being mostly different from the river 
species. As would be expected. Lake Butte des Morts, which is 
but the widening out of the lower part of the Fox River, shows 
on the whole less change from the river type than does Lake Win¬ 
nebago, which is larger, with all the features of a true lake. 
As has been shown elsewhere (Baker, 1916, 1918) the larger 
part of the macrofauna of a lake is found within the two meter 
contour (about six feet) and consists of animals in intimate con¬ 
tact with the substratum of the lake—the bottom and the plants 
growing on the bottom—where are found the optimum conditions 
for successful continuance of life—food and oxygen. This is the 
littoral (or eulittoral) area comparable to the great areas of the 
sea shore which teem with life. 
The shallowness of Lake Winnebago, with a maximum depth of 
6.38 meters, makes an ecological division of the lake into depth 
zones impossible ; there is no deep area, or even a true aphytal area, 
as in the deeper lakes, such as Lake Mendota and Green Lake. An 
area possibly comparable to the aphytal region of the deeper lakes, 
where large aquatic plants are absent, may be represented in Lake 
Winnebago by the central area of maximum depth, although this 
may equally well be placed in the sublittoral region. The littoral 
(eulittoral) area extends from the shoreline to a depth of three 
meters, forming a narrow shelf bordering the shores of the lake; 
life is most abundant in this region. The sublittoral (or aphytal) 
area includes the remainder of the lake bottom. In this region the 
bottom consists of soft, black mud with a considerable amount of 
organic material. 
Nearly all of the bottom of the lake is covered with a detritus 
which is made up of small fragments of plants, minute pieces of 
wood, crustacean skeletons, caddis-worm cases, and fragments of 
molluscan shells. This material is absent only where the waves 
are strong enough to wash it away. The plant fragments probably 
constitute an important source of food for the bottom-feeding 
forms. 
