No. 453-] 



PYRAMIDULA ALTERNATA. 



667 



Geographically the western shells show a wider range of vari- 

 ability than do the eastern shells. With one exception (Auburn 

 79^) the eastern shells have the per cent, of spire elevation 

 between 44 and 68, while those from the west range between 46 

 and 87, the per cents, between 68 and 81 being numerous. 



One of the most noticeable features in the curves as plotted 

 in this paper is their tendency to assume a multimodal form. 

 This is clearly shown in the individual diagrams, but stands out 

 prominently in the comparison of the localities (Fig. 4). This 

 is, of course, indicative of great variability. 



From the study of these three lots of shells we may conclude 

 that the western alternata has a higher shell, on the average, 

 than does the eastern form, and that it shows a much larger 

 amount of variation in spire elevation. It would be very inter- 

 esting to have curves plotted from other localities, east and west, 

 to ascertain whether their results would coincide with the con- 

 clusions of the present paper. 



BIBLIOGRAPHY.i 



Baker, Frank C. 



Rib Variation in Cardium. Amer. Nat. XXXVII, 4S>-48«. •9°3- 

 3AKER, Frank C. ' • d S " II 



On Some Variations of Cardium edule apparentiy correlated to the Con- 

 ditions of Life. Phil. Trans. Roy. Soc. London, 180, B, 297, 

 1889. 



BiGELow, R. P. and Rathburn, Eleanor P. 



On the Shell of Littorina litorea as Material for the Study of Variation. 

 Amer. Nat, XXVII, 171, 1903- 

 BuMPus, H. C. 



The Variations and Mutations of the Introduced Littorina. Zoological 

 Bulletin, I, 247, 1898. 



the variation of the^MoUusca, from a biometric standpoint; the list is not 



