134 PROCEEDINGS OF THE ACADEMY OF [Feb;, 



Height. Width. Index. 



Garrett's woods colony, average of 28 46 36 .78 



Somerset colonies, average of 30 53 42 .795 



Ridge near Lincoln colony, average of 16 54 42 .775 



, Fig. 2 shows the comparison of the forms from the Garrett's woods 

 colony and the Somerset colony; the former represented by the black 

 dots, the latter by the crosses. As will be seen, there is a gradation 

 in size from one to the other, but the two groups do not mingle; 

 the index is about the same in the two colonies. The size of the 

 shell is given by the millimeter scales, the vertical scale on the left 

 giving the height, while the horizontal scale below gives the width. 

 The frequency curves for width are plotted at the top and those for 

 height on the right. It will be noticed that there is no overlapping 

 in the widths, which run from 34^ to 39 mm. for the forms from 

 Garrett's woods and from 39 to 44^ mm. in the Somerset forms. But 

 in the heights there is a certain amount of overlapping, as the frequency 

 curves show. 



Fig. 3. — P. jamaicensis. Garrett's Woods and Ridge near Lincoln Colonies. 



