

LYMN^IMi OF NORTH AMERICA. 21 



Pisidium and Planorbis resisted the effect better than Lymncca. At 

 Rochester, N. Y., the sewage has been discharged into the Genesee 

 River for the past ten or fifteen years and at the present time is of the 

 consistency of dirty, greasy dish water, yet Galba catascopium and 

 Planorbis trivolvis live and thrive by thousands in this seemingly un- 

 favorable environment. (PI. LVIII, fig. 2.) 2 The writer's observa- 

 tions have been that chemicals and oil are deadly to molluscan life, 

 while sewage does not materially affect them. 



Recently Dr. A. E. Ortmann has made a detailed study of this 

 subject in a study of the waters of western Pennsylvania. He found 

 that sewage, excepting in a highly concentrated form, had little effect 

 on the fresh-water fauna, excepting that it rather tended to increase 

 than decrease certain forms of life. On the other hand, the chemicals 

 discharged from the coal mines, oil wells and other industrial plants 

 proved fatal, as is most clearly shown on the map accompanying Dr. 

 Ortmann's paper, where large areas of the Allegheny, Ohio and Mo- 

 nongahela rivers have been rendered unfit for life. 3 



b. VARIATION OF THE LYMNAEID SHELL IN DIFFERENT ENVIRONMENTS. 



The variation in the form of the shell in Lymnaea may be due to 

 many causes ; that relating to the environment only will be here con- 

 sidered. The effect of heat and cold has already been noted (page 28). 

 Differences in environment have a marked effect upon the form of 

 the shell. In many cases the variation is individual, but in not a few 

 it is common to all the individuals inhabiting certain areas, and hence 

 produces, oftentimes, characteristics of specific or racial value. Those 

 Lymnseas which inhabit alkaline waters always produce a peculiar shell, 

 as, for example, the Polyrhytis utahensis of Call, inhabiting Lake Utah, 

 which has developed a series of longitudinal ribs. Galba palustris, when 

 occupying a saline or alkaline water habitat, produces a shell on the 

 average of from eleven to fifteen per cent smaller than in a fresh-water 

 habitat, showing that salinity produces a dwarfing of the shell. 



Many of the peculiar species of fresh-water pulmonates, which 

 formerly lived in the waters of the Quaternary Lake Lahontan and 

 Lake Bonneville, were produced by the struggle of the mollusks to 

 become adapted to a changing environment. These fresh-water de- 

 posits show the gradual changes which took place in the mollusk fauna, 

 as the conditions became more and more severe, until, at the top of 



2 Since writing' the above the sewage in the Genesee River has become 

 (1910) of such a highly concentrated form that the mollusks have all disap- 

 peared in the river for a mile or two below the point of the discharge of sewage 

 into the river. 



s See Ortmann, pp. 97-98; also Irrigation Papers, no. 186, 219, 226. 



