the Shell o/Neiitina fluviatilis. 371 



tlie larger specimens were examined. Of the Barton series 

 159, or 55 °/q, were females ; of the Wye specimens 42, or 



(1) Measurements. — It is not very easy to select dimensions 

 of Neritinu which are capable of precise and relatively easy 

 determination, and at the same time comparable with those 

 of shells of more conventional shape ^. In an enquiry such 

 as the present the former consideration seems to be the more 

 important, and we have accordingly measured three dimen- 

 sions of each shell, wiiich are at any rate objective if their 

 nomenclature is open to question. \\'\\\\ the shell resting on 

 a flab surface, mouth downwards, we measure (a) length, 

 (/3) width, and (y) height. The two former are parallel to 

 the surface on which the shell rests, the latter at right 

 angles; the essential- points are shown in the accompanying 

 sketch. The measurements were made witli sliding callipers 

 reading with a vernier to 0*1 mm. 



The results of \hQ Barton i'eries are summarized in Table I., 

 which sliows the mean figures classified in eight groups, 

 according to the length of the shell. All the dimensions are 

 in millinnitres. 



It appears in the first place that there is no substantial 

 difference in size between males and females. The males 

 range from 4*7 to 11"3 mm. long, the females from 5'2 to 

 11*6 mm. Of the larger specimens (8 mm. long and up- 

 wards) rather more are females tlian males, but the difference 

 is wtdl within the possible chance discrepancy. In the various 

 size groups (Table II.) the percentage of males varies from 

 32 to 52 ; but here again the differences are not significant. 

 The question would be simple if we had some structural 

 criterion of sexual maturity ; without this, however, there is 

 no reason for thinking that the two sexes differ in size. 



Differences in shape, which are shown by rrfeasureraents of 

 this sort, are more appreciable if the data are expressed in a 

 relative form, as in Table III. The widths and heights are 

 here shown as percentages of tiie length. It will be seen 

 that the small shtlls are relatively wider and, to a less 

 extent, higher than the larger specimens, but there is no 

 evidence of any sexual difference. 



^lagnified camera-lucida drawings of the opercula were 

 made in a number of the larger specimens of each sex, but 

 no diflercnce in size or shape could be found. 



* In many cases the apex ia eroded, and in some of the larger speci- 

 mens has in this way disappeared, rendering it impossible to take the 

 apex as a lixed point for any dimension. 



