No. 5-] 



THE INTRODUCED LITTORINA. 



249 



geographical limits, and at least two are admitted to be " per- 

 haps monstrous rather than varietal forms." 



Of the 3000 British shells which I have examined, 1000 

 are from Tenby, Pembrokeshire, southwest coast of Wales, 

 1000 from South Kincardineshire, east coast of Scotland, and 

 1000 from the Humber District on the east coast of England 

 -three localities widely separated geographically, and charac- 

 terized by different geological formations. Ten thousand 

 American shells have been collected from the ten stations 

 indicated on the annexed map, 1000 shells being considered 

 sufficient to represent any given locality. 



The First Test of Variability. 



A most superficial examination of the complement of shells 

 (1000) from any locality shows that certain individuals are rel- 

 atively longer and others are relatively shorter (ventricose) 



FIG. 2. Diagram of shell to illustrate the points at which measurements were taken. 



than the majority of their companions. The ratio of breadth 

 to length, then, is variable, and may be exactly measured and 

 mathematically expressed. A shell which has a breadth equal 

 to eight-tenths the height may be indicated by 80/0; one whose 

 breadth is nine-tenths the height, by 90^0 ; and one whose 

 breadth equals the height, by 100/0. The distance from A to 

 B, Fig. 2, is taken as the height, and from C to D as the 

 breadth of the shell. These distances were selected because 

 they were more easily measured than the actual diameters. A 

 measuring instrument gave the ratio of breadth to height, i.e., 

 the index of stature, without the labor of computation. 



