374 



On the Shell of Neritina fluviatilis. 



is some suggestion that the females may be rather larger 

 than the males, but the difference is inconclusive. Tiie 

 length, width, height ratios are given in Table V. These 



Table V. 



show that tlie Barton shells are relatively wider and higher 

 than the AVye specimens, but they fail to bring out any 

 difference between the male and female shells in the latter 

 series. 



(2) There are, of course, many differences in shape and 

 other characters which would not be brought out by the 

 simple measurements which we have made. Careful com- 

 parison and inspection has, however, failed to reveal any 

 constant sexual difference in shape, texture, colour, or any 

 other feature. In the Wye series many (54 °\^ of the 

 females and relatively few (18 °/q) of the males showed a 

 ragged notch or notches on the upper lip, i. e. on the snail's 

 right-hand side as it crawls, and close to the generative 

 oritice. This was, however, quite wanting in the Barton 

 series, so that less stress can be laid upon it. It may, indeed, 

 be artefact ; if so, it possibly indicates that in the rapid 

 currents of the Wye the females cling more closely to the 

 stones than the males, and are more liable to damage during 

 collection. 



We do not know the reproJuctory habits of Neritina nor 

 how the eggs are laid. There may, indeed, be no reason 

 for supposing that there might be sexual characters in the 

 shell ; the male generative organs are roughly about the 



