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4. COMPOSITION OF THE Bottom FauNA 
The proportion of Mollusca to associated animals in the lake col- 
_lections of 1914—1915 did not run so uniformly high as in the river 
series of 1915. The Mollusca percentages are highest in the open water 
of the deeper lakes of Classes I and IJ, where they run from 84 to 96%. 
In the weedy zones (1—6 feet) of the deeper lakes the Mollusca per- 
centages were noticeably lower (77%). In the shallower weedy lakes 
of Classes III and IV the insects and small Crustacea are much more 
abundant relatively, and the Mollusca ratios drop to 36 and 50% 
Per CENT. Motitusca BY WEIGHT (TO ToTAL WEIGHT OF COLLECTIONS), 
Lakes, 1914—1915 
Peer vgs Poet, |. ate 
open water | no vegetation vegetation 
| / 
Class I | 84.1 | 89.8 17.8 
Class II | 96.8 ee. 17.5 
Class III | BA ace 50.7 
Class IV 36.4 
Class V | 79.7 
The snail fauna of the lakes, like the insect fauna, presents in the 
average somewhat greater variety than that of the river. Viviparidae 
made up the largest percentage of the Mollusca totals in the deeper lakes 
of Classes and II. In the shallower weedy lakes and in the dead timber 
areas the ratios of Viviparidae were lower. The smaller snail fauna 
(smaller Gastropoda, Sphaeriidae) less rarely than in the river consisted 
almost exclusively of Sphaeriidae—the Valvatidae and Amnicolidae 
being well represented in most of the lakes studied, and exceeding 
Sphaeriidae in some cases, in the shallower weedier lakes, both in num- 
bers and weight. 
Further details of the composition of the nie bottom-fauna are 
shown in the detail tables at the end. 
Per CENT. OF VIVIPARIDAE, BY WEIGHT, TO ToTAL WEIGHT OF ALL MOoLLuUScA, 
LaAKEs, 1914—1915 
cone ee 1-6 ft., 1 Bi, 
open a ae no vegetation vegetation 
Class I 56% | 85% 86% 
Class II 99% pores 88% 
Class IIT ooce uses 62% 
Class IV Poti | areca 61% 
Class V 11% 
