662 THE AMERICAN NATURALIST. [Vol. XXXVIII. 



logs and the soil is a rich, black loam. Collected by the writer. 

 The shells studied are not of uniform size, although they are 

 nearly all adult measuring from twelve to twenty-four millimeters 

 in diameter. The young shells of this species always have a flat 

 or nearly flat spire, and as they would materially affect the results 

 they were excluded. 



In Pyramidula altetnata we find a good example of variation 

 caused by individual environment. The species lives for the 

 most part under started bark, in crevices and under flat-lying 

 tree trunks ; hence its shell varies with its abode. For example, 

 a specimen living between the " started " bark of a tree and the 

 tree trunk, the space being very narrow, measured 23x11 mill., 

 the height being about 48 per cent, of the width, while another 

 specimen living under a fallen tree trunk measured 15x13 mill., 

 the height being about 87 per cent, of the width, or 39 per cent, 

 more than the first example. These specimens were from the 

 same locality and from adjacent trees. Their habit of crowding 

 mto narrow crevices and between the bark and the tree trunk 

 has caused this species to become one of the most variable of 

 land shells as regards the form of the shell. 1 



On account of the extreme variability it was thought that a 

 quantitative study of material from several rather widely sepa- 

 rated localities would produce results of some interest. This 

 species has a wide geographic range, being found throughout 

 the eastern and central parts of the United States and Canada. 

 Its western limit is said to be Minnesota. 



HI. Method of Obtaining Quantitative Data. 



To obtain a variation index the diameter and altitude of the 

 shell was measured in millimeters and the altitude divided by 

 the diameter ; t. e., The per cents, obtained in this way 



provide the shell index. In the diagrams the individuals or 

 classes having the same per cent, are indicated on the horizontal 

 Ime and the number of specimens in these classes (the frequen- 

 cies) are indicated on the vertical line 



