Manchester Memoirs, Vol. Ixv. (1921), No. 9 



IX. Variation of Sphaeria. 



II. — Sphcerium corneum (Linne). 

 By W. E. Alkins, M.Sc, and Maurice Cook, M.Sc. 



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



Introduction. 



In the first paper (1) of the present series one of the authors 

 gave an account of the results of the measurement of a series 

 of two hundred Sphcerium lacustre (Miiller) from a small pond 

 near The Three Lows, North Staffordshire. It was then 

 stated that it was hoped that a similar investigation might be 

 carried out in the case of each of the other three British species 

 of the genus Sphcerium Scopoli. In the present paper are 

 given the results of an inquiry along elementary statistical 

 lines into the variation of the commonest species of the group 

 — Sph. corneum (Linne). 



Material. The shells used were taken in March 1920 from 

 the Ashton and Guide Bridge Canal, near Dukinfield Station, 

 where they occurred in enormous numbers, associated with 

 Sph. pallidum, Gray, Bythinia tentaculata (Linne), and 

 Planorbis albus (Miiller). The bottom of the canal consisted 

 of rather fine ashes ; the canal was remarkably free from 

 vegetable matter, while the water was warmed to a slight 

 extent by the hot-water discharge from an adjacent mill. The 

 conditions appeared to be highly favourable to the bivalves, 

 for the number of Sph. pallidum was only slightly inferior to 

 that of Sph. corneum. On the other hand, the gastropods 

 were by comparison very scarce ; they were present on the 

 walls rather than on the bottom of the canal, while the Sphceria 

 were, with the exception of a few juvenile examples, on the 

 bottom ; a few dozen B. tentaculata were seen, but the record 

 of PI. albus is based on a single specimen. 



Rather more than five hundred shells of the species under 

 discussion were collected, all shells brought up by the scoop 

 being taken, with the exception of the verv smallest. Of 

 these, five hundred were measured ; the measurement was 

 carried out exactly as in the case of Sph. lacustre (2), the three 



September 30 th, 102 1. 



