3^4 JOURNAL OF COts'CiiOI.OGY, VOL. 12, NO. 12, OCtOlsfeR, igo^. 



It appears from these figures that the diameter in the females is 

 about o"5 mm. greater than in the males, but if the striking difference 

 in altitude is taken into account, it is found that this does not mean 

 that the females are more tumid than the males. The coefificients are 

 in fact a trifle higher in the females, indicating that they are, if any- 

 thing, rather slimmer than the males. 



On this the point immediately arises that there may be some 

 relation between the tumidity coefficients and the absolute altitude. 

 Grouping the shells to the nearest half millimetres of altitude, we 

 obtain the following average coef^cients : — 



These results. indicate pretty clearly that there is a definite relation 

 loetween tumidity and size in the sense that the larger shells are rather 

 less tumid. The results for coefficient A among the smaller shells 

 (male) are irregular. This is due to the fact that the smallest ones 

 have no fully developed peristome ; when this appears the diameter 

 is thereby suddenly increased and the coefficient falls. If, then, the 

 females are larger, they will tend on that account to be slimmer. Is 

 this the whole explanation of the slightly greater tumidity of the males 

 which appears in the crude averages ? The material is not sufficiently 

 extensive to enable a very absolute conclusion to be drawn, but the 

 answer seems to be in the affirmative, for comparing males and 

 females of equal altitude (i3'o and i3"5 mm.), there is no appreciable 

 sexual difference in the coefficients. The conclusion is therefore that 

 males and females are equally fat. 



It is possible that these results apply only to the particular batch 

 of Cyclostoma examined. They appear, however, to be rather 

 particularly significant, since practically all the individuals concerned 

 were actually observed to be sexually active towards one another. 

 Taylor's figures {loc. cit.) give, however, altogether different measure- 

 ments,^ and I should be glad to examine batches of Cyclostonia from 

 other localities. It is extremely important to avoid bias in collecting 

 such material. Without having any idea that the females were larger 

 than the males, I took i8 shells out of the box, as I thought, purely at 



I Approximate measurements give for the male i3"i x ii'5 X io"2 with coefficients of 114 

 and 128 ; for the female 13*2 X i2"5 X ii'5 and 105 and 115. 



