485 
small zonitoids, the pupoids and Philomycus seem to be the least 
widely distributed in this limited area. The last was observed in 
but one habitat. 
The typical molluscan societies and their habitat relations 
may be summed up as follows. 
In swamp with Typha or Iris. 
Succinea retusa, Succinea avara, Agriolimax campestris. 
On low ground subject to overflow. 
Agriolimax campestris, Polygyra thyroides, Polygyra fraterna, Pyra- 
midula alternata, Zonitoides arboreus, Vitrea hammonis. 
On higher grounds, raised above overflow. 
Succinea ovalis, Agriolimax campestris, Polygyra albolabris, Philo- 
mycus carolinensis. 
On dry ground. 
Strobilops virgo, Helicodiscus parallelus, Vitrea 1ndentata, Euconulus 
julvus, Bifidarta contracta, B. pentodon. 
Living under “‘started ”’ bark, etc. 
Zonitoides, Vitrea, Strobilops, Helicodiscus, Vertigo, Euconulus, Bifidaria 
and Carychium. Pyramidula is frequently found under “started ”’ 
bark, and Polygyra albolabris haunts holes and large crevices in dry 
weather. 
Table II (fluviatile species) is also of interest. Station XXI, 
the East Branch of the Chicago River, yields nine species. The 
highest number of species from any other habitat is seven, which 
number was observed in Stations IV and XI. Both are summer- 
dry ponds. Physa gyrina is the most abundant species, occurring 
in all but six of the fluviatile habitats. The naiads are the least 
abundant, occurring only in the East Branch of the Chicago River. 
The habitat relations of the molluscan societies may be sum- 
marized as follows. 
Found in all varieties of habitat. 
Physa gyrina. 
(4) 
