il6 JOURNAL OF CONCHOLOGY, VOL. I4, NO. 7, JULY, I914. 



regard to the nomenclature of the teeth in any one row, there is one 

 central tooth, on either side three lateral teeth (which are called 

 laterals whatever their shape), and the rest are marginals. The 

 number of marginals is complicated by the presence or absence of a 

 minute terminal denticle, and by the variation there may be in differ- 

 ent parts or on the two sides of the same radula. It is somewhat 

 characteristic of grown-up helvetica that the terminal tooth is generally 

 of good size ; very small teeth are more frequent in young specimens 

 and in other Hyalinia. No arbitrary rule will surmount the difficulty 

 of these small denticles ; I have always counted them as each equiva- 

 lent to one marginal tooth. Where the number of marginals in some 

 parts of the radula is e.g. lo and in other parts e.g. ii, the single 

 expression io\ is used. The total number of teeth is ascertained by 

 multiplying the average number in a row {i.e. number of marginals 

 X 2 -f 7) by the number of rows, not by individual enumeration. The 

 length and width of the radula were determined from camera lucida 

 outlines. I have relatively less confidence in these results, as the 

 apparent sizes appear to be influenced by the details of preparation 

 and mounting. The characters of the individual teeth were mostly 

 examined at about the tenth row from the posterior end where they 

 have reached their full development but have not become worn. 



Table I. 



From these figures it is evident that the growth of helvetica from 

 3 to 8 mm. in diameter of shell is accompanied by («), an increase 

 in the number of rows; {b\ an increase in the number of teeth in 

 each row ; and consequently {c), an increase in the total number of 

 teeth on the radula. It is equally evident that at all stages the range 

 of variation is considerable. Thus specimens with 38 or 39 rows 

 might occur in all the groups, and similarly ten marginals is a figure 

 common to every size examined; a radula with approximately 1,000 

 teeth can be found in each group. At the same time, this variability 

 is not enough to mask the fact of a progressive and pretty regular 

 increase in the number of teeth when the average figures are dealt 

 with, even when these are derived from such a comparatively small 



