1 82 THE AMERICAN NATURALIST. [Vol. XXXVII 



It is in a comparison of ratios in shells of different ages, such 

 as Bumpus made, that the effect of erosion would be most seri- 

 ously felt. He found an increase of ventricosity in six localities 

 and a decrease in seven. Doubtless the former would have been 

 very much diminished and the latter augmented in the absence 

 of erosion. 



Weight and bulk are also affected by the combination of 

 erosion without and deposition within. Color is the only 

 remaining test, and that is rather an unsatisfactory measure of 

 variability in this species. 



Various explanations have been offered to account for the 

 erosion of molluscan shells, which according to Cook ('95) is 

 much more common among fresh-water forms than in those 

 inhabiting the sea. Many years ago Lewis ('59) pointed out 

 that the shells in a stream near Mohawk, N. Y., below the point 

 where alkaline wastes were discharged into the water were much 

 eroded, while above the point of pollution they were quite healthy 

 and free from defect. Jeffries ('65) quotes the suggestion of 

 J. R. Grove that the patches and irregular hollows on the surface 

 of L. litorca may be due to electrolytic action arising from lack 

 of homogeneity in the shell substance. Schrubsole ('86) found 

 that certain English fresh waters containing from one half to 

 four grains of lime per gallon had a strong erosive action on 

 shells, while no such action was noticed in water containing eight 

 and one third grains. 



That filamentous plants penetrate many shells of moll uses and 

 brachiopods, as well as numerous corals and other forms, was 

 pointed out by Kolliker ('59). Later Bornet and Flahault ('89) 

 summarizing the results of previous observers, described and 

 figured eight genera of alga? and two fungi having the shell- 

 boring habit, and they pointed out that these plants play an 

 important part in the destruction of shells, especially in quiet 

 waters. Recently, Duerden ('02) has drawn attention to the 

 action of algae in the disintegration of corals in the West Indies. 

 We have not been able to discover however, any evidence of the 

 presence of algae or other plants upon the shells of Littorina 

 collected during the winter months, and have no explanation to 

 offer as to the cause of the erosion that we have observed. 



