ECOLOGICAL INVESTIGATION In the province of Limburg (Netherlands), Ancylus fluviatilis (Müller) is almost restricted to springs and small stony streams in the extreme south-eastern part of the province (see Fig. 1), where it lives on bare, smooth stones without any vegetation or deposit of sand, etc. Its distribution depends not only on the percentage of oxygen in the water, but also on the nature of the stream-bed, which is very important. BIOMETRICAL INVESTIGATION From this investigation the following conclusions can be drawn: with increase in the velocity of the current the size of the shell decreases, the height decreasing proportionally more than the length and width, while the apex moves backwards and becomes more pointed. In the population of Ancylus in the spring of the Landeus, near the village of Mechelen, where the water is barely flowing, shell height increases proportionally to increase in total size.