489 
TAXONOMY. 
It has been urged by some taxonomists that the ecological study 
of nature has no bearing upon the subject of taxonomy, and that 
little, if any, aid can be secured from this subject in unraveling the 
mysteries of specific differences. This opinion, however, does not 
appear to be based on the facts which have been gathered from this 
source. On the contrary, the study of ecology has proven an aid of 
great value in drawing specific and varietal lines and in ascertaining 
the true character and value of taxonomic characters; and it is 
quite logical that this should be the case, because the environment 
reacts upon the organism, causing the latter to be modified to fit the 
environmental conditions. This is especially true of fresh-water 
mollusks, which respond to every change of habitat. 
Two interesting facts bearing upon the taxonomy of the fresh- 
water pulmonates have been discovered during the field work in 
connection with the present study. In 1821 Thomas Say described 
a large, long-spired Physa as Physa gyrina (Pl. XXV, Fig. 9-13). 
In 1866, G. W. Tryon described a small, short-spired Physa as Physa 
oleacea (Pl. XXV, Fig. 14-17). It was noted as the field work 
progressed, that these two species were always associated, and the 
fact soon became apparent that one was but the immature stage of 
the other, or, in other words, that oleacea was the half-grown shell 
of gyrina. To confirm this theory, a large gyrina was broken down 
until it became a perfect oleacea. Gyrina has five whorls, while 
oleacea has a trifle more than four. The evidence seems conclusive. 
A Lymnea attaining the size of an inch or more, lives in the 
intermittent or summer-dry ponds. It has been called both 
Lymnea palustris and Lymnaea reflexa. It differs from palustris in 
having a narrower shell, and it is notable for developing within the 
outer lip a heavy rib or varix. It also appears to live exclusively 
in this type of habitat, the heavy varices being caused by the periodic 
formation of an epiphragm during the time when the pond is dry. 
As many as three of these may be found in a year. This habitat 
indicates that the age of the Lymnzas (as well as of certain Physas 
which inhabit such an environment) can not be ascertained by the 
number of these varices on the shell. These varices have been ob- 
served in a young shell six or seven millimeters in length, and as 
many as six of them have been counted on a shell thirty millimeters 
inlength. It is believed that the formation of these varices is due to 
