1912. 



NATURAL SCIENCES OF PHILADELPHIA. 



177 



organs. In the snail the shell at once presents itself. So far we 

 have considered the shell alone. 



Adams, 1900, made a careful study of the fresh-water prosobranch 

 mollusk Io taken from many stations on the same watershed. He 

 measured the height of the spine, the length of the aperture and 

 width of the shell. In the present study three characters were also 

 measured. These were chosen so that when their ratios were plotted 

 a picture of the average shape of 

 the shells of the colony from which 

 the collections were made would re- 

 sult. 



The length of. the spire of the shell 

 may be expressed by the ratio — 

 altitude to length of aperture. See 



AT) m 



fig. 1, where ^ this ratio. The 

 width of the shell may be expressed 

 roughly in terms of a ratio, length 

 of the aperture to the width. The 



AD 



former ratio g^ we will refer to as 

 the ratio, and the latter ratio we will 

 refer to as the index. 



Using the measurements furnished 

 by Baker, 1911, which are prob- 

 ably measurements of extreme in- 

 dividuals, calculating their index 

 and ratio and plotting them, using 

 the index as an ordinate and the 

 ratio as the abscissa, when these Fig. 1. 



are enclosed by a line, then we 



have a rough picture of the shape of the shell. Fig. 2 shows such 

 a picture and the dark oval represents Lymnwa columella as it occurs 

 about Philadelphia as compared with some forms taken from Baker's 

 measurements. 



The writer made a number of collections of Lymncea columella 

 from some ponds and streams about Philadelphia, and also examined 

 several series in the collection of the Academy of Natural Sciences of 

 Philadelphia. 4 The three characters referred to above were measured 

 with proportional dividers, the index and ratio were calculated and 

 plotted in groups. Fig. 3 shows a number of these collections. The 



4 The writer wishes to thank Dr. H. A. Pilsbry, of the Academy of Natural 

 tSciences, for many courtesies. 



