How to distinguish Oxychilus cellarius (Müller, 1774) easily from Oxychilus draparnaudi (Beck, 1837) (Gastropoda, Stylommatophora, Zonitidae)

Identification of the two most widespread European Oxychilus species, O. cellarius (Müller, 1774) and O. draparnaudi (Beck, 1837) is still mainly based on conchological features of controversial diagnostic value. Despite this, certain anatomical characters, known since Taylor’s (1905-1921) excellent monograph, enable the two species to be readily distinguished. O. cellarius has a cylindrical penis, rather constant in width in the middle portion, whereas that of O. draparnaudi is divided by an abrupt constriction into a usually shorter slender proximal portion and a longer wider distal portion. The two parts communicate through a very slender ‘bottle-neck’, level with constriction. Obviously, apart from other differences in the internal ornamentation of the penis (fewer larger papillae in single rows, papillae sometimes fused to form wavy pleats in O. cellarius), most of the rows of papillae of the proximal penis of O. cellarius are continuous with the pleats of the distal penis, whereas in O. draparnaudi the rows of papillae of the proximal penis stop at the ‘bottle-neck’ and the pleats of the distal penis begin after it, without any continuity with the rows of papillae of the proximal penis.