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 of Limnaea.

2. In L. palustris the 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 in L. palustris the ratio L/l (at least) is subject to considerable local variation, a fact which detracts from its value as a species constant.

1. The complacent attitude that fluke is no longer a disease of importance is attacked on the grounds that the recent low incidence of the disease is largely due to a temporary scarcity of the intermediary, Limnaea truncatnla.

2. A description is given of methods of making contacts, taking samples, and recording observations in a survey of typical habitats of L. truncatula in England and Wales.

3. Twenty-two such habitats are described in some detail.

4. These form the basis of a brief discussion on factors contributing to the ideal habitat, knowledge of which is desirable for the elaboration of new methods of control.

5. A provisional map showing the distribution of liver fluke in England and Wales is appended.