Manchester Memoirs, Vol. lix. (191 5), No. 4. 3 



The collection showed remarkable variation in the 

 form of individual shells, a variation in form which will 

 be appreciated by a consideration of PL. i, A — F> and 

 one which, in the past, would have led in all probability 

 to the institution of several distinct species. 



On PL 1. ^ and 5 are a pair representing the 

 extremes of variation in breadth as compared with length. 

 Both shells are of approximately the same length, but A 

 is a very broad type and B is a very narrow type. 



Similarly C and D are a pair representing the ex- 

 tremes of variation in depth, in individuals of equal 

 breadth, C representing the very thick type and D the 

 very thin variety, and again E and F represent the 

 extremes of variation in depth in shells of equal length, 

 E being the thick type and F the thin one. 



The present paper gives the results of an investigation 

 pursued with the object of ascertaining : — 



1. The limits of variation. 



2. The distribution of variation, i.e., whether the varia- 

 tion was distributed normally throughout, or whether on 

 the contrary there was a predominance of certain types 

 of variation, indicating either, the presence of more than 

 one species, or, the incipient splitting up of one species 

 into two or more. 



3. Any ontogenetic modification in the variations, i.e., 

 whether, during growth, there was any change in the 

 range or distribution of the variations. 



The variable character best suited for a precise inves- 

 tigation, was the variation in shell form, a variation 



Length Length 



indicated by the ratios ~- and -rr — ~~. For each 



7 Hreadth Depth 



shell the measurements length (A), breadth (/>), and 



depth (D) were obtained with the micrometer calipers, 



