Larger context for LACK in Corpus USbrown_UKbncw/UK_bncw.txt
Crothers (1973) related the resulting gradation in N. lapillus shell shape to exposure on Pembrokeshire (Fig. 24) shores by the expression: y= 1.214+0.036 X Where Y is a Shell-shape ratio and X the Ballantine (1961) Exposure Grade. r = 0.878 (Crothers, 1983b).
L Ap — shell length divided by aperture length (Fig. 22) is the simplest, most convenient and most objective measure of shell shape in dog- whelks.
Although lacking the intrinsic relevance of " mouth area" ( used by Smith, 1981, for winkles) or of "apical angle" and "whorl ratio" (employed by Kitching, 1976; 1977; Cambridge and Kitching, 1982) this ratio is less affected by the absolute size, thickness, age or condition of the shell and it is undeniably easier to record in the field.
Shells were measured to the nearest 0.1 mm, using a vernier caliper, and the sample mean calculated for collections of 100 shells (later reduced to 50 and then 30 as it became apparent that increasing the number of measurements did not improve the accuracy of the mean) (Fig. 23).
Substituting for X in the expression opposite, it will be seen that mean I, Ap ratios range from 1.25 on shores of Grade 1 (" Extremely Exposed" on Ballantine's scale) to1.50 on "Extremely Sheltered" Grade 8s.