The conchological variability of Sphaerium corneum (Linnaeus, 1758) from Polish waters is manifested by the presence of two types of populations. All the features mentioned in the diagnosis show great variability. For instance the shell outline oscillates from oval to triangular or trapeziform. In some populations all the margins of the shells are much curved, in others the ventral margin is straight, joining the anterior and posterior ones at sharp angles. The variability of the shell colour is considerable, much like in other bivalve freshwater molluscs. The asymmetry of the shells, mentioned in the description of S. corneum “2nd type” is already distinctly marked in the appearance of the young.
The regression lines w/l (l) (width: length depending on length), w/h (l) (width, height depending on length), and w(l) (width depending on length) show that the first mentioned relation reflects more pronouncedly the differences between the manners of growth of the populations discussed. The regressive slope b (y = a + bx) is the best index of these differences. The values of that index change in the material studied from 1.5 (Ina river) to 7.28 (forest bogs – Makruty). The changes of the non-metric features presented in the diagnosis are parallel to the changes in the shell proportions.