Manchester Memoirs, Vol. Ixv. (192 1), No. 8 



VIII. Variation of Sphaeria. 



I. — Sphcerium lacustre (Miiller). 

 By W. E. Alkins, M.Sc 



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



Introduction. The writer hopes that ; some studies of 

 variation in the British species of the genus Sphcerium may 

 be of interest. It is hoped that it will be found possible to 

 obtain each of the four species in sufficient numbers for the 

 purpose of measurement, when at least one series of each will 

 be studied. In the case of Sph. corneum (Linne), and Sph. 

 pallidum, Gray, no difficulty is anticipated, but in the case of 

 Sph. rivicola (Leach) it is doubtful whether the species is at 

 present available in the required quantity in any sufficiently 

 restricted locality in the Manchester district. 



The present paper contains the results of an investigation 

 of a rather small series of Sph. lacustre (Miiller). 



Material. The shells were collected by Mr. W. Cartwright 

 and the writer in February, 1920, from a small pond, perhaps 

 thirty feet in diameter, near the Three Lows in North Stafford- 

 shire, at an altitude of about 1,050 feet above O.D. This 

 pond is situated on Pendleside shales, a few hundred yards 

 south of the boundary of the carboniferous limestone ; while 

 there is no possibility of the pond receiving- drainage water 

 from the limestone — for it lies almost on the watershed, the 

 ground sloping away to north and to south — the water must 

 be fairly calcareous, since small pieces of limestone are 

 frequent on the bottom. The floor consists mainly of a 

 slightly clayey mud, which is so finely divided that it only 

 settles very slowly from water. There is practically no 

 vegetation to be seen, except a little grass ; microscopic plants 

 are no doubt present. Decaying sycamore leaves are present 

 in great numbers. 



September 30th, 192 1. 



