Whorl number, shape, and size of shell in some pulmonate faunas

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Whorl number (w) is only partly related to the height (h) and maximum diameter (d) of the shell. The distribution of h with respect to d in most faunas falls into two groups, with high-spired shells in the upper distribution (U) and equidimensional to planorboid ones in the lower (L). In French, Floridan, south-west North American and Javanese pulmonate land-snail faunas, there is little or no relation in L between h, d, or h/d and w except that w is always low for very small shells. In contrast, the U group is highly structured with respect to w, which at high values of h/d increases very rapidly with increase in h, but falls rapidly to a rather uniform value irrespective of h as h/d decreases. It is suggested that for very slender high-spired shells a high number of whorls is necessary for mechanical strength, and for very small shells in both distributions there is a limit on the possible reduction of whorl cross-section.