2i4 



THE RADULA OF HYALINlA. 



I. 



By Prof. A. E. BOYCOTT, F.R.S. 



(Read before the Society, February tith, 1914). 



Plate 3. 



Before one begins to use the characters of the radula in conjunction 

 with other malacological and conchological features as an aid in the 

 definition of species, it is obvious that one ought to know (i), the 

 changes which are associated with growth in specimens from a single 

 locahty ; (2), the amount of variation which occurs in specimens of 

 the same size from a single locality ; and (3), the variation between 

 examples from different localities. There are many subsidiary points, 

 such as whether the radula is related to age or size, but these three 

 enquiries are fundamental before we can properly discover (4), the 

 difference between different species. I purpose to take up these 

 matters in order, using, in the first place, Hyalinia helvetica as material. 

 " The most likely way," wrote the immortal Stephen Hales not far 

 short of two hundred years ago, " to get any insight into the nature 

 " of those parts of the creation which come within our observation 

 " must in all reason be to number, weigh and measure." A pretty 

 firm conviction that the ingenious minister of Teddington is right 

 has led to the subject being treated in a quantitative or numerical 

 manner whenever such has been possible. The superiority of quanti- 

 tative over qualitative information is even now not fully recognised ; 

 but I would here only remark that the application of methods of 

 measurement to the shells and other parts of snails is an almost 

 unexplored field of immense possibilities which I would commend to 

 the notice of students of the moUusca. 



I. — The Growth of the Radula in Hyalinia helvetica. 

 The snails whose radula are dealt with in the present communi- 

 cation were collected at Banstead, in Surrey, in 191 1 and 1912, from 

 a small nettle-bed in a hedge-bank, measuring about 4 by 2 yards. 

 By laying down brick-bats and pieces of wood and cardboard, and 

 visiting the place about once a week over a period of some fifteen 

 months, considerable numbers oi H. helvetica were obtained in various 

 stages of growth ; there is no difficulty in recognising quite young 

 specimens, as the black edge to the mantle is present from the first. 

 They were collected at all seasons of the year, for helvetica hardly 

 hibernates in that locality,^ and may, I think, be regarded as all 



I See Taylor's Monograph, vol. iii., p. 50. 



