Clausilia rugosa has often been regarded as a form of C. bidentata because of close similarities in shell form. However, in genital anatomy rugosa closely resembles parvula in having a long slender loop of penial epiphallus, a character in which both of these differ from all other Clausiia species including C. bidentata. Mapping of the ranges of parvula and rugosa has disclosed that they never coexist; studies of shell variation reveal that they are connected through the occurrence of populations with intermediate shells. They are therefore regarded as conspecific and should be known as C. rugosa rugosa Draparnaud, 1801 and C. rugosa parvula Féussac, 1807. In southern France C. bidentata coexists locally with varied populations of C. rugosa, without any evidence of hybridisation. On the basis of anatomical study of large numbers of populations the geographical ranges, patterns of shell variation and habitat preferences are described for C. rugosa and C. bidentata. It is suggested that self-fertilisation may account for maintenance of some of the local population variants in these species, but studies of population genetic structure are needed to test this. Explanations are offered for the adaptive significance of regional variations in shell size, shape and ribbing. The geographical ranges, habitat preferences and distributional history of all species of Clausilia (Clausilia) are compared. Ecological factors involved in the wider range expansion of C. bidentata than of C. rugosa during Post-glacial (and several Pleistocene interglacials) are discussed.