Viviparus contectus (Millet, 1813) is the species with the pointed apex. The aperture is somewhat pointed toward the apex, the whorls are convex and the umbilicus is clearly visible. Viviparus viviparus (L., 1758), the other Dutch species, lacks the above mentioned characters, or they are less distinct. Only animals collected in the Netherlands were used. Dark transversal rings on the whorls indicate where growth has stopped for some time. It has been possible to divide these rings into two groups, viz. the darker (winter rings) and the thinner, lighter ones (intermediate rings). That the winter rings are actually formed in winter is shown by the egg-capsules of a leech, deposited in the suture, winter rings are noted by the following facts: 1. The enclosed egg-capsules of a leech. 2. The regular distance between them. 3. In winter and spring they coincide with the aperture. 4. The three colour-bands stop just a little distance before them. 5. They are always preceded by an intermediate ring. N.B., not each winter is marked by a single winter ring. Many times we can see two or more rings close together. In these cases the wintering period has been interrupted by one or more periods of feeding and movement. In my opinion the presence of more winter rings in one winter and of the intermediate rings, has made authors hesitate to calculate the age of molluscs by winter rings. However, I have calculated the groups of winter rings of one winter as representing one winter. Also the concentrical rings on the operculum may be used for age determination. With the calculated height, width and the number of winter rings it is possible to calculate the minimum and maximum height of the shells of one age group (graphs 1a & 1b). The graphs 2a & 2b represent the relation between the average width and the height. An ideal broken line is drawn between the points of the average width and a definite height and a curve exactly through the same points. In these graphs we find a break, possibly indicating the moment at which the animals became sexually mature. With the aid of the graphs and observations we can draw the following conclusions: 1. V. contectus grows faster in both height and width than V. viviparus. 2. The relation height-width differs between the two species. 3. However, both species may show the same ratio in their second year but not in the following. 4. Winter rings and intermediate rings are discernable in both species. 5. V. contectus reaches generally an age of 7 years, sometimes more (10 years). 6. V. viviparus reaches generally an age of 6 years, sometimes more (11 years). 7. In both species growth in height exceeds growth in width in spring; in summer the width increases more rapidly. 8. The relation height-width changes when the animals become sexually mature. 9. Differences in the relation height-width have not been found between the ♀♀ and the ♂♂. 10. The ♀♀ live about half a year longer than the ♂♂. 11. The maximum number of embryos for V. contectus is 55 and for V. viviparus 65. 12. At birth the operculum is bowl-shaped in both species. 13. The embryos of V. contectus have a sharply pointed spire. Those of V. viviparus are flatly coiled. 14. Occasionally abnormal eggs are found between the normal ones.