BOYCOTT: THE RADULA OF* HYALINIA. il^ 



number of individuals. The range and significance of the variation 

 will be dealt with more fully in another communication. 



Tarle II. 



These data show the increase of size which the radula undergoes 

 as the animal grows larger, compared with the growth of the shell. 

 There is no quality of the radula which is likely to be rationally 

 associated with an increase of the diameter or other linear dimension 

 of the shell. If any, the association may be reasonably assumed to 

 be with the bulk of the animal. Some indication of this is afforded 

 by the volume of the shell. This has been calculated in Table II. 

 on the assumption^ that the shell is a regular cone, the base being 

 the mean of the major and minor diameters and the height the mean 

 of the major and minor altitudes." To ascertain thef size of individual 

 teeth is an excessively laborious and not very accurate procedure ; for 

 the present, therefore, I give the only available summary of the rela- 

 tive sizes of teeth by calculating, from the number of teeth and the 

 area of the radula, the general mean area occupied by one tooth. 

 This method assumes, of course, that the teeth are equally spaced on 

 the basement membrane at different stages of growth, a point which 

 requires investigation. 



A better notion of the relative importance of the various changes 

 in determining the difference between a grown-up and an immature 

 radula is obtained if the data are expressed as relative to an arbitrary 

 index. This has been done in Table III., where the data of group I. 

 are taken as 100, and the figures for the other groups reduced to 

 this standard. It appears from this that the size (area) of the radula 

 and the total number of teeth increase much more slowly than the 

 size of the shell ; in other words, as is well known, that young snails 

 have relatively large radulae, no doubt in association with the desira- 



1 Which gives results about io% too small in T. nemoralis and T. hortensis, where I have 

 tested it against direct measurements of volume by weighing in air and water. 



2 The volume is then (-diam. maj. + diam. min.-j \^_^^^ 'i^t,m^T_±^X^rum, Throughout 



the present series of helvetica the major diameter, minor diameter, major altitude, and minor 

 altitude are in the ratios loo : 89 : 53 : 35. 



