

32 THE CHICAGO ACADEMY OF SCIENCES. 
deep or shallow. Specimens of Galba emarginata mighelsi from Aroos- 
took County, Maine, exhibit similar fluctuations, as do also examples 
from some of the lakes of Michigan and Wisconsin. Cooke* cites 
examples of the European Lymnea peregra and Lymnea stagnalis 
from the salt marshes near the sea of Aral which show the effect of 
a changing environment. These lakes are salt for several months 
of the year and comparatively fresh during the remainder of the 
year. The effect is to dwarf and otherwise modify the form of the 
shell. 
In many localities in which the habitat consists of a rapidly flow- 
ing river or a wave beaten, rocky shore, the spire is shortened up and 
the aperture is greatly increased in size. This increase in the size of 
the aperture is in direct response to the character of the environment, 
the roughness of the water causing the animal to develop a larger 
foot, and hence, a larger aperture, in order to overcome the pushing 
and pulling power of the water; in other words, a larger foot surface 
is essential in this kind of a habitat, for the preservation of the species. 
A change of habitat is thought by some conchologists to produce 
some species. Hazay, for example, records Lymnea peregra from 
the ova of ovata, and ovata from the ova of peregra, by placing one 
species in running water and the other in still water.* 
Dr. Lewis many years ago? asserted that Galba palustris in the 
Erie Canal became Galba catascopium when transferred to the Gen- 
esee River. . 
Variation in Lymnza may be summed up as due to one or more 
of the following external causes : 
Quality and quantity of food. 
Station in stagnant or quiet water. 
Station in rapid current or on wave beaten shore. 
Temperature. 
Chemical nature of fluid medium. 
The foregoing remarks clearly indicate that the environment plays 
a notable part in the variation of the Lymnzas and hence in the for- 
mation of species, and it seems almost needless to state that when 
collections are made great care should be exercised in carefully noting 
the ecological data.* 
c. LOCOMOTION. 
Locomotion in Lymnza is accomplished by three principal meth- 
ods. These may be called gliding, hunching and thread spinning. 
1Cooke, op. cit., p. 93. 
Amer, Journ. Conch., IV, pp. 2-4, 1868. 
3See Semper’s Animal Life, p. 439, for a discussion of variation under 
various conditions. 
4Mollusca, p. 85. 
