I. STATEMENT OF THE PROBLEM. 



Modern statistical methods in zoological study give us a means of expressing 

 variation quantitatively. Such methods are a prerequisite to the discovery of the 

 actual causes of variation. It is frequently stated that an important cause of the 

 difference in variability of two lots of animals from different regions is some sort of 

 unlikeness in the physiographic conditions. The present study tests that asser- 

 tion by comparing the variability of two lots of Pecten shells of close relationship 

 from localities having very different geological histories and present physiographic 

 conditions. 



II. MATERIAL AND METHODS. 



The material used was of two lots. One came from Dunedin, on the Gulf Coast, 

 fifteen miles west of Tampa, Florida; the other came from San Diego, California. 

 The Dunedin shells were obtained for me by Mr. J. H. Holmes of Dunedin in the 

 summer of 1900. He says in his letter, dated August 2, 1900: "These shells were 

 picked up on the flats commencing off Dunedin and going three-quarters of a mile 

 south toward Clearwater; all were found in this locality." They were all in pairs. 

 The shells from San Diego were received by me from a dealer there who states that 

 they were collected on Coronado Beach. 



The Pecten shells from Dunedin have been variously classified and named by 

 systematists. The commoner names have been: Pecten gibbus Linn, and Pecten 

 dislocatus Say. The shells are closely related to the northern P. irradians and con- 

 nected with it by intergrading forms so that Dall ('98, p. 746) has given the name 

 Pecten (Plagioctenium) gibbus to all these forms and included the Dunedin form- 

 unit under the name Pecten gibbus var. dislocatus, or, better, var. gibbus. With 

 reference to the well-known city and bay from near which these shells came they will 

 henceforth in this paper be called merely the "Tampa shells." 



The shells from San Diego agree with specimens at the Field Columbian Museum 

 labelled Pecten (Plagioctenium) ventricosus Say, also from San Diego. The species 

 has also been named P. tumidus and P. circularis. It occurs both living and fossil 

 (Pleistocene) south of Santa Barbara, Cal. These shells will henceforth be known 

 as the "San Diego shells." 



123 



