Manchester Memoirs, Vol. Ixv. (1921), No. 10 



X. Variation of Sphaeria. 



III. — Sphcerium pallidum, Gray. 



By W. E. Alkins, M.Sc, and J. Harwood, M.Sc. 



(Read January 25th, 1921. Received for publication May 10th, 1921). 



Introduction. In the two preceding papers (1, 2) of the 

 present series an account has been given of the results of the 

 statistical investigation of a series of two hundred Sphcerium 

 lacustre (Miiller) from a North Staffordshire station and of a 

 series of five hundred Sph. corneum (Linn£) from the Ashton 

 and Guide Bridge Canal, near Dukinfield station. It is now 

 possible to give similar results for the third of the British 

 species of the genus, and it is hoped that it will be possible 

 to complete the series by an investigation of the one remaining 

 species — Sph. fivicola (Leach). 



Material. The shells used were taken in November 1920 

 by Mr. W. Cartwright, B.Sc, and one of the writers (J. H.) 

 from the Ashton and Guide Bridge Canal, near Dukinfield 

 station ; this, as may be remembered, is the station which 

 furnished the series of Sph. corneum for the previous paper. 

 The conditions under which the shells occur there was 

 described at some length in dealing with that species (2, 1), 

 and the fact that the conditions were apparently highly 

 favourable to the bivalves, though perhaps not so favourable 

 to gastropods — which were present only in much smaller 

 numbers — was noticed. 



It is interesting to be able to give results for the two 

 species based upon series of shells taken from the same station 

 at a short interval of time, for it is probable at least that the 

 influence of varying environment will thus be eliminated. 



Rather more than five hundred shells of the species were 

 collected ; all shells brought up by the scoop were taken with 

 the exception of the smallest. Of these five hundred were 

 measured, the method of measuring, position of the measured 

 axes, and method of treatment of the data so obtained, being 

 exactly as described in the case of Sph. lacustre (1, 2). 



September 30th, 1021. 



