THE NAUTILUS. 133 



The problem of food may have some influence in the produc- 

 tion of these forms. The discrepancies in size of the whorls 

 may be connected with periods of scarcity and abundance of 

 food. With the exception of the small muddy bays above men- 

 tioned, the bottom is very free from weeds and algae, the usual 

 food of this gastropod. It is indeed remarkable that the lake 

 can support so large a molluscan fauna as it does. Of plankton 

 there is none. The gastropods are not abundant, but certain of 

 the Unionidae in the bays occur in thousands. 



The temperature of the water and range in depth of the gas- 

 tropods are not sufficiently distinctive to be an important factor 

 in this connection. The average water temperature is not much 

 colder than it would be in the Ottawa valley. 



SUMMARY. 



To summarize the results of this study it would appear that 

 the specimens of P. campanulatus from Blue Sea Lake show con- 

 siderable variation as follows: 



a. Progressively in an increasing deflection upwards of the 

 extremity of the last whorl and aperture from the general plane 

 of the whorls. 



b. In degree of elevation and obliquity of the spire. 



c. In size and shape of the whorls, which vary from rounded 

 to sharply carinate. 



d. In presence in one specimen of well-marked revolving 

 lines. 



e. In the flare and obliquity of the aperture. 



f. In presence or absence of color-banding. 



Of these the first only is regularly progressive, and the latter 

 deviations bear no relation either to it or to each other. To the 

 writer, bottom environment, wave action, and food conditions 

 appear to be the main factors in producing such a series of 

 forms as have been above described. Such conditions are favor- 

 able for the development of new varieties and species. 



Note. The writer wishes to gratefully acknowledge the as- 

 sistance and helpful criticism received from Dr. E. M. Kindle 

 of the Geological Survey of Canada, and from Mr. Frank C. 

 Baker. 



