388 THE CHICAGO ACADEMY OF SCIENCES. 



made in 1899 (plate XL, figures 11-12) with those made in 1906, 

 which resemble figures 13 to 15 on the same plate. These figures are 

 of catascopium which live between the upper and middle falls, where 

 the water is badly polluted with chemicals. Strangely enough, the 

 variation is toward a palustris-like shell. 1 In the Erie Canal, at a 

 point where it spreads out forming a lake-like expansion called the 

 "Wide- Waters/' catascopium is abundant along the stony shore, and 

 is usually typical and very uniform (pi. XL, figures 6-10). Some in- 

 teresting variations in the outer lip occur among the shells from this 

 locality. In Seneca Lake, New York, this species lives on a fairly 

 open shore in shallow water. 



The original habitat in the Delaware River, near Philadelphia, is 

 of special interest as it is in tide water, though not saline. The 

 Lymnaeas are subject to the ebb and flow of the tide in the same 

 manner as the Littorinas, Purpuras and similar littoral forms. This 

 ecological condition is interesting because of the fact that during regu- 

 larly recurring periods the Lymnseids are out of the water and are 

 subject to the rays of the sun. It may be that the heavy shell (which 

 is much thicker than in specimens living in quiet, uniform waters, 

 as in the Genesee River at Rochester) serves as a protection against 

 too much drying during these periods of ebb tide. The animals in- 

 habit a wide strip of beach between low water mark and half tide, in 

 company with Physa ancillaria, Planorbis bicarinatus and Goniobasis 

 virginica. 



Mr. Bryant Walker, thus speaks of the habitat of certain forms 

 of catascopium in Pine Lake, near Charlevoix, Michigan 2 (see plate 

 XL, figures 16-22) : 



"The Limnaeidae of Pine Lake, which empties into Lake Michi- 

 gan at Charlevoix, were also extremely interesting. The bottom of 

 the lake is composed almost wholly of marl, except where it has been 

 covered by a thin coating of sand washed in from the shores, and, as 

 a consequence, both plant and animal life exist under very unfavor- 

 able circumstances. The level of the lake seems to have been lowered 

 by the canal made by the U. S. Government to connect it with Lake 

 Michigan, and the former lake terrace is now largely exposed, and, in 

 many places, quite dry. In the numerous pools, however, which are 

 left along the shore, the Limnaa catascopium Say is found in great 

 abundance and almost infinite variety. It varies in shape from the 

 comparatively slender form usually found in the Great Lakes to the 



*A recent visit to the Genesee River (June, 1910) revealed the fact that 

 the river had now become so filled with highly concentrated sewage that all 

 traces of molluscan life had vanished. 



2 Nautilus, IX, p. 4. 



