The author communicates the capture of 4 specimens of Planorbis corneus (L.) var. pyrrholeuca Oldh. in a narrow shallow ditch near Leiden, Netherlands. This variety differs from the typical form by possessing a white shell and a reddish animal. According to Boettger, the animal is not entirely albinistic, as Oldham supposed, but it possesses a smaller amount of pigment than the typical form, which is due to a genetic factor; the degree of pigmentation is influenced by temperature also. Boettger also mentions a true albinistic variety with body and eyes entirely unpigmented and a white shell. For this variety the name erythraea nov. var. is proposed. Boettger’s interpretation of the results of his cross-breedings introduces a new element in genetics, which neither seems probable, nor is satisfactory on all points. According to him the colour of animal and shell is controlled by one series of 3 alleles: strongly-, slightly-, unpigmented. A „Kontrollefaktor” in recessive form would restrict the pigmentation to the animal, causing a white shell; in dominant form it would have no influence on the pigmentation. This means partial cryptomery of the multiple factor, the latter only being visible in the shell if the „Kontrollefaktor” is present in dominant form. Consequently there ought to exist three colours of the shell and three of the animal: Strongly-, slightly- and unpigmented. Boettger does not realize this, for he mentions three colours of the animal, but only two of the shell: strongly- and unpigmented. All of his animals which ought to have a slightly pigmented shell have a strongly pigmented one. The real interpretation is a different one: A series of three factors D1>D2>D3 is responsible for the colour of the body only; moreover a pair of alleles H>h provides for the colour of the shell; the latter being cryptomeric and only visible if D1 or D2 also is present. The mechanism of the cryptomery is discussed and a more probable explanation of the influence of temperature on the pigmentation is given. The reasons are discussed why var. pyrrholeuca is so extremely rare in nature; the specimens mentioned being the first ones that ever have been found. Hitherto it has only artificially been raised by Oldham, by crossing the varieties albina Moq. Tand. and rubra Oldham. These reasons are: It is recessive for two characters and the two parentvarieties albina and rubra, which both are rare, will very seldom be present in the same population, and if so, will be a very small minority against the typical form. Consequently they have but a small chance to copulate. If the population is saturated, as populations of Planorbis corneus usually are, averagely from each pair of parent-snails but two of the offspring will grow to maturity (F1-generation). These two ought to copulate one with the other and not with specimens of other varieties to give rise to an F2-generation in which pyrrholeucas may occur. But of this F2-generation 1/16 of the specimens will be pyrrholeucas, whereas averagely only two of them will survive; thus the chance being very small that these two or one of them are pyrrholeucas. And if so, this rare variety soon will vanish again in the next generations by crossing with dominant varieties. In unsaturated populations, however, a pair of parent-snails will raise more than two young ones to maturity. In these circumstances more than two specimens of the F1- and F2-generations will survive and the chance of some of them being pyrrholeucas will be greater. In fact the population near Leiden mentioned above was unsaturated, as most specimens had perished the year before, when the biotope had been deprived from the greater part of its vegetation and detritus by peasants. The following rule, which does not only apply to Planorbis corneus, but must be of general application, can be formulated: In a population consisting of several genotypes of one species, specimens which are homozygous recessive for two or more characters will have a greater chance to occur when the population is unsaturated than when it is saturated.