Baker—The Fauna of The Lake Winnebago Region, 131 
Table 5 shows the relation of the number of species of Mollusca 
to the depth of the water. The greatest variety is found at depths 
not exceeding two meters. 
Table 6 shows the relation of the number of species of Mollusca 
to the character of the bottom in Lake Winnebago. The maximum 
number was found on sand bottom. 
Table 7 shows the variety of the molluscan faunas of three lakes, 
namely, Oneida Lake, New York; Maxinkuckee Lake, Indiana; and 
Lake Winnebago, Wisconsin. 
One of the interesting features brought out by the study of the 
molluscan fauna of the Lake Winnebago region is the difference 
in size and shape between the Unionidae of the Fox River and 
those of the lakes, a difference which appears to be comparable to 
that noted by Grier (1919) between the Naiads of Lake Erie and 
the upper drainage of the Ohio River. Grier states that “if we put 
a shell in the lake environment we may expect it will change its 
morphological features, not at random, but in a distinct, deter¬ 
minate, or orthogenetic direction. ’ ’ This change in the morphology 
of shells that have migrated from a river to a lake is strikingly 
shown in the Lake Winnebago fauna, and a study of the two areas 
by the methods of Grier would produce the same results as attained 
by the study of the Lake Erie shells. It is a significant fact that 
the same varietal forms inhabit both Lake Winnebago and Lake 
Erie, indicating that the law holds good under similar conditions 
in widely separated areas. Dr. Sterki notes that almost all of the 
Pisidium of the lake are small and slight, some even depauperate, 
and Sphaerium and Musculium are similarly affected. The gastro¬ 
pods of the lake are in the main different from those of the river. 
Thus the entire molluscan fauna is affected by the same law of 
variation produced by river and lake environment, clearly indi¬ 
cating that ecological station plays a large part in the evolution of 
species. A study of the tables and of the systematic list which fol¬ 
lows will bring out additional features of this ecological character¬ 
istic. Just what factors have been potent in producing these 
changes does not seem to be definitely known. It is probable that 
variation in food supply, in the chemical character of the fluid 
medium in which they live, as well as in the general physical 
environment, plays a large part in these changes of form. 
