Abstract
1. In differentiating between species of snails various ratios of shell dimensions are frequently used as biological constants descriptive of the species. Some of the ratios commonly used are: length to breadth (L/B), length to length of body-whorl (L/W), length to length of shell aperture (L/l), and length of shell aperture to its breadth (1/b). Mean and extreme values of these ratios are given for small samples of five species ofLimnaea.2. InL. palustristhe least variable of these simple ratios is L/W and the next best is L/l. Statistics are given for the distributions of these two ratios.3. Data collected by Mozley are used to show that inL. palustristhe ratio L/l (at least) is subject to considerable local variation, a fact which detracts from its value as a species constant.

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