124 PROCEEDINGS OF THE AMERICAN ACADEMY. 



lots differ is recognized by many naturalists and has been designated 

 "geographical variation." Hitherto attention has been directed especially 

 toward the geographical variation of land animals, especially mammals 

 and birds. Above all, in America, ever since Allen's work on the 

 subject, has the study of the geographical variation of mammals and birds 

 of North America been pursued with energy. The studies made have, 

 it must be confessed, rarely been undertaken in the spirit of great pre- 

 cision which chai'acterizes modern biometry. Not only is the fact of 

 geographical variation thus recognized, but this class of variations is 

 regarded by many as being more important than any other in the origin 

 of species. Under these circumstances it becomes interesting to examine 

 a case of geographical variation biometrically. 



II. Material. 



The material used in this investigation has been the shells of Pecten 

 opercularis, a common bivalve mollusc. Visiting Europe in the autumn 

 of 1902, with the aim of gathering material for this study, I obtained 

 sufficiently large numbers of this species from four localities, as follows : — 



1. The Firth of Forth, two miles north of Newhaven on the " city 

 of Edinburgh grounds," living in six fathoms (12 metres) of very clear 

 water, on hard, clean shell-bottom. The salt content of the estuary at 

 this point must be much diluted below that of the adjacent North Sea, 

 where it is about 3.4 per cent.* 



2. The Irish Sea, in latitude 54° 18' N and longitude 4° 12' \Y, 

 The salt content of the water in which these shells were living is probably 

 between 3.4 and 3.5 per cent. The bottom is sand and mud. The 

 depth was about 20 metres. The July temperature of the surface of 

 the sea is about 57° F. (14° C.).t 



3. The northwestern part of the English Channel, in the vicinity of 

 the Eddystone Lighthouse, within two miles of the Light to the north, 

 and within five miles to the east and to the west. The depth of the 

 water is 30 to 35 fathoms (60 to 70 metres). The mean density of the 



* Mr. Thomas Devlin of Newhaven kindly arranged for the collection and 

 shipment of these shells and gave me the data concerning the collecting ground. 



t For the collection of shells from the Irish Sea, I am indebted to the Liverpool 

 Marine Biology Committee, whose well-equipped new laboratory I visited in 

 August, 1902. Especial thanks are due Professor Ilerdman, director, and Mr. 

 H. E. Chadwick, curator, for interesting themselves in my search for pectens. 

 Most of the data concerning the place of collection were sent me by Mr. Chadwick. 



