An illustrated overview of the type series of nominal taxa of the gastropod genus Theodoxus (Neritidae) inhabiting the Ponto-Caspian region is presented. It is made on the basis of a revision of large European malacological collections and includes descriptions of the type specimens of 16 taxa of different rank (species, variety, morph). Apart of these, the “type” specimens of four taxa with unavailable names have been traced in ZIN collection; we illustrate them to show the variability of Ponto-Caspian Theodoxus. In most cases, the photographs of the type specimens are published for the first time. We provide a taxonomic analysis of all studied type series, with comments on the nomenclature, systematic position and taxonomic rank of all nominal taxa of Theodoxus recorded from the Ponto-Caspian area.

The morphological variability of the bivalve mollusc Flexopecten glaber was studied based on the analysis of 306 specimens from the Kazach’ya Bay (Sevastopol, Crimea) of the Black Sea. The sampling of mollusks was carried out with the help of skuba equipment in June–September of 2017–2018. For F. glaber, the age and size characteristics are given and the main trends in their ratios with the shell growth are considered. For the first time, quantitative indicators of thickness of the shell valves, their weight and height of the radial ribs are presented and discussed. A wide intraspecific polymorphism of the shell is shown in a relatively narrow range of habitat conditions. The obtained data expand the understanding of the morphological variability of F. glaber in the Black Sea and provide new information on the functional morphology of the species.

Cellular composition and morphology of components of photoreceptor apparatus of the retinae of camera eyes of some species of terrestrial gastropod pulmonate molluscs were studied and its evolutionary transformations were traced. It was demonstrated that all examined characteristics of photoreceptor apparatus of the most investigated species were normal for terrestrial pulmonates, while those of one species were special. Evolutionary transformations of photoreceptor apparatus of terrestrial pulmonate molluscs due to habitat shift were quite significant and concerned of its cellular composition and morphology of photoreceptor cells.

A new deep-water polyplacophoran, Stenosemus nitens sp. nov., from Papua New Guinean waters is described. It differs from other species of the genus Stenosemus by having a sculptureless, smooth and shiny surface of the tegmentum, smooth dorsal spicules with only a few small transverse wrinkles on top, and unusual bends of the front margin of the intermediate valves. Within the genus the new species belongs to a limited group of coloured chitons.

The phenotypic composition of the introduced colonies of Cepaea nemoralis in five settlements of Ivano-Frankivsk and Lviv regions of Ukraine, studied from May to July 2019, is described. The largest colonies were found in Bohorodchany (Ivano-Frankivsk region) and Lviv. In most cases, there was a pronounced dominance (from 40% or more) of the individuals with unbanded and five-banded shells – pink, less often yellow. Among the snails with five-banded shells, phenotypes with the fusion of two or more bands prevailed, among which in Lviv the phenotype (12)3(45) most often occurred, and in Bohorodchany (123)(45). Molluscs with brown shells were found in only one of the studied colonies. In Bohorodchany a differentiation of the phenotypic structure of the C. nemoralis colonies separated by a highway, was noticed. The available data on the present distribution in Western Ukraine of two introduced species of the genus Cepaea: C. nemoralis and C. hortensis are analyzed.