490 
the exigencies of the habitat. The following interesting conclusion 
has gradually been reached as the studies on these Lymnzas pro- 
gressed: the smallest, narrow forms with acute spire are Lymnea 
palustris michiganensits Walker (Pl. XXV; Fig. 8); the larger form 
with more rounded whorls is Lymnea reflexa crystalensis Baker 
(Pl. XXV, Fig. 2-3); and the fully mature form is Lymnea reflexa 
Say (Pl. XXV, Fig. 1). In this instance a study of the ecological 
relations of the Mollusca in the area in question has shown the 
relationship of these three forms,—a relation which probably would 
not be discovered from a few isolated specimens in the study. It 
is also probable that Pilsbry’s Succinea ovalts optima is the old 
or senile stage of ovalis (see Pl. XXV, Fig. 18-20). 
To aid those ecologists who are not intimately acquainted with 
the mollusks and who may desire to use this class of animals in their 
field work, a systematic catalog of the Mollusca of the area is ap- 
pended. Descriptions and figures of the majority of the species 
of mollusks which live in northeastern Illinois will be found in the 
writer's monograph of the ‘‘Mollusks of the Chicago Area,’’* and 
a reference to plate and figure in that work is made for most of the 
species herein recorded. 
This catalog includes two classes, three orders, fourteen families, 
twenty-three genera and thirty-eight species and varieties, all living 
within an area three miles long and one half mile wide. 
* Bull. Nat. Hist. Surv. Chi. Acad. Sci., No. III. 
