2 W. E. Alkins — Variation of Sphceria 



The only other mollusca which occur in the pond, to the 

 writer's knowledge, are Limncea truncatula (Muller) and 

 Pisidia. 1 The abundance of the Sphceria will be appreciated 

 when it is stated that practically three hundred specimens were 

 taken with one scoop in under two hours, all from the outer 

 five or six feet of the pond. 



All the living shells taken were collected, cleaned, and 

 dried, care being taken that the valves were tightly closed. 

 There was a number of casualties during the process, and 

 also during that of measurement, and the number of perfect 

 shells was reduced by these to just over two hundred ; of these, 

 two hundred were measured, with the results given below. 



Measurement. The length, width, and thickness of each 

 shell were determined by means of an optician's sliding gauge 

 provided with a vernier capable of reading to one-tenth of a 

 millimetre. The position of the three axes may be defined as 

 follows : — 



Length : From the umbones to the ventral margin, in a 

 direction perpendicular to the latter ; 



Width : From the anterior to the posterior extremity of 

 the shell, parallel with the ventral margin (I.e. perpendicular 

 to the length axis) ; 



Thickness : The greatest thickness of the shell from the 

 (outer) surface of one valve to that of the other, perpendicular 

 to the axes of length and width. 



Measurements were made to the nearest one-tenth of one 



millimetre. Each dimension was noted for each shell, and 



from the figures thus obtained the values of the ratios 



Width , Thickness . , , . . 



T ic and — r *r~ were calculated. An investigation 



Length Length b 



of the data on elementary statistical lines gave the results 

 which are discussed in the following pages. 



Results. 



(a) Length, Width, and Thickness Distribution. The 

 distribution of length, width, and thickness is shewn in 

 Table I below : — 



1 See Appendix. 



