SEXUAL DIFFERENCES IN THE SHELL OF CYCLOSTOMA 



ELEGANS. 



By a. E. boycott. 



(Read before the Society, June 9th, 1909). 



It is stated^ that in Cydostoma elegans the last whorl " is more tumid 

 and voluminous in female than in male individuals." I recently 

 collected a number of specimens at Ashtead (Surrey) for the further 

 examination of this matter. Most of them were taken actually in 

 copula. The specimens were separated into males and females by 

 dissection and the shells then examined. It is clearly necessary to 

 obtain some numerical expression of tumidity and voluminousness, 

 and I have sought to do this by ascertaining the relation between 

 height and breadth according to methods described elsewhere." The 

 altitude is measured from the apex to the most distant part of the 

 peristome along a line parallel with the columella. In the present 

 series I have taken two diameters : one (diameter a) at right angles 

 to the axis of the shell from the outer edge of the peristome to the 

 furthest point on the last whorl ; the other (diameter /?) is the greatest 

 diameter obliquely across the last whorl, excluding the peristome. 

 Diameter a might be influenced by variations in the degree to which 

 the mouth of the shell projects ; diameter /3 is not influenced by this, 

 and though there are some points of uncertainty, I think that one or 

 other or both of these measurements give a fair measure of the fatness 

 of the last whorl. The ratio between these figures and the altitude 

 gives one then a measurement of the tumidity : ^ d'ia"meter L°° &'ves co- 

 efificient A, " diLmet^r g" gives coefficient B. These coefficients are 

 expressions of tumidity which are independent of variations in the 

 absolute figures for altitude and diameter. The lower the coefficient 

 the greater the tumidity. 



Of the 89 specimens examined, 45 were males and 44 females. The 

 average maximum and minimum measurements (in millimetres) may 

 be summarised as follows : — ■' 



1 J. W. Taylor, Men. L. & F. W. Moll. Brit. Isles, vol. i, 1S94, p. 26. 



2 Mollusca of Herefordshire (Woolhope Club), 1900, p. 19: Science 

 ;6i. 



3 The individual measurements may be taken as correct to o"2 mm. 



