On the territory of Republic of Armenia a new species of land snails has been found. Illustrated description is presented. Relationship between five close related Transcaucasian species of Armenica is discussed. New species differs from closely related A. unicristata in having more large dark shell with well developed axial striation and character of clausilium.

Based on a study of samples made by the author, 70 species of freshwater bivalves belonging to 6 genera and 2 families are recorded for the Taz River basin (north of Western Siberia). An annotated list of bivalves of the Taz basin is provided, with data on species’ range, their findings in Western Siberia and within the studied area. Some information about bionomics and abundance of bivalves are given. 45 species are for the first time registered in this basin. Most studied molluscan communities are characterized by low species richness; the distribution of species by their occurrence was extremely uneven. 22 species are characterized as rare. The highest species richness of bivalves was found in river channels, rivers and brooks; the lowest – in temporary habitats. The core of the fauna is constituted by species with broadest range (cosmopolitan, trans Holarctic, trans Palearctic) as well as by taxa with European Siberian type of distribution. From the taxonomic point of view, the bivalve fauna of the Taz basin is relatively separated from the faunas of other river basins of Western Siberia and is more similar to the fauna of the Lower Yenisean zoogeographic province (sensu Starobogatov [1986]).

TEM and SEM electron microscopy have been used to study osphradia in 6 species of marine Caenogastropoda. The ultrastructural features of mechanoreceptor cells that perform the Littorina osmoreception function in osphradium organs are presented. Mechanoreception is based on a possible change in the volume of cisterns of microvilli of supporting cells, which can be transmitted by the cilia of nearby mechanoreceptor cells. These cells obviously, have mechanosensory channels on the apical surface. It has been first discovered in predatory molluscs actively searching for food, that single receptor cells with a mobile sensilla consisting of several cilium were joined together. They are located along the groove zone and follow the direction and force of the movement of water along the osphradium lamellae.

A new species Acrotoma (Acrotomina) mallabica sp. nov. from the Western Caucasus is described. The morpho-anatomical relations of the new taxon with closely related species are discussed.

The invasion of the predatory gastropod mollusk Rapana venosa, known as veined rapa whelk, in the Black Sea and its outspread in many regions of the World Ocean have led to complex structural changes in the communities of bivalve mollusks. When capturing a prey, the adult rapa whelk excretes a biotoxin from the hypobranchial gland that causes severe paralysis of the musculature of bivalve mollusks. Under experimental conditions, it was established that the biotoxin, after the short-term exposure of the fertilized eggs of the mussels Mytilus galloprovincialis to it, has a mutagenic effect, causing chromosomal aberrations in the embryos. The acentric groups of chromosomes, single and multiple chromosome bridges, chromosome lags and polyploidy indicate transformation or destruction of threads of the achromatin spindle in the mitosis anaphase. The chromosome fragmentation and emergence of ring chromosomes in the metaphase and anaphase of mitosis of mussel embryos are the consequence of the partial destruction of chromatin. The underdeveloped shell, anomalous development of D-veligers’ velum, and the changed form of the ‘eye spots’ and hepatopancreas of the larvae at the veliconcha stage result from chromosomal mutations during the embryo development. The mechanism of the rapa whelk biotoxin action on the chromosomal apparatus of mussel embryos is discussed.

The extant polyplacophoran tail valve is traditionally considered as a unity despite its clear separation into two distinct regions, which were in relation to the delimiting point – the mucro – termed the antemucronal area for the front part and the postmucronal area for the hindermost region. However, earlier conceptions do exist, which consider the postmucronal area as semiplate, with the antemucronal area as modified “intermediate” plate.

To test the usefulness of the terms “antemucronal” and “postmucronal” in their traditionally sense, three independent attempts were made to demonstrate existing differences between the mucro-separated areas.

Leptochiton rugatus was histologically examined allowing the confirmation of a cardial complex-antemucronal relation. Valve morphology of the brood of Radsia nigrovirescens not only confirms a tegmental development prior the building of the articulamentum but shows that the postmucronal area develops to its final shape before the antemucronal area appears. For the first time it is demonstrated that the antemucronal area of Schizoplax brandtii shows a delayed splitting of the relevant area, characteristic for the conditions found in the intermediate valves of this species only. That leads to the assumption that the underlying valve build processes are of the same nature as in the intermediate valves. Additionally, literature data on valve characters were compiled that show a stronger relationship (61%) of the antemucronal area to the central area of intermediate valves rather than to the merged postmucronal area. In the light of this evaluation it is suggested that the term “tail valve” should be used to demonstrate the merged condition of two independent zones only, the antemucronal area and the postmucronal area.

The monotypic land snail genus Campolaemus, known only from Saint Helena Island, has been classified in the family Hypselostomatidae, a family occurring in the eastern Palaearctic and in the Oriental region. Due to biogeographical reasons and morphological traits, especially the arrangement and morphology of apertural barriers, Campolaemus is moved to the family Streptaxidae, which is a pantropic family, being highly diverse in tropical Africa.

The paper describes a new species of the genus Thermochiton, T. papuaensis sp. nov., found in deep waters off Papua New Guinea. This species differs from T. undocostatus primarily in the dorsal scales, the marginal spicules, sculpture of the jugal area and the shape of the central teeth of radula. There are apparent similarities between the species of the genus Thermochiton, Connexochiton platynomenus, C. kaasi and Ischnochiton crassus. The last species is proposed to be transferred to Connexochiton. Owing to the friable, rusty brown deposits that densely cover the shell and girdle of both specimens of T. papuai, the latter probably lives in areas of high chemical activity.

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.

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.

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.

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.

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 article is devoted to the fauna of Bivalvia (family Sphaeriidae) of waterbodies and watercourses of the eastern slope of the Polar and Pre-Polar Urals. The fauna of freshwater clams of this region remains practically unexplored. This study aimed at identification of the species content of Bivalvia and publication of images of their shells. We analyzed both qualitative and quantitative samples of molluscs as well as fish stomach contents. The identification of the material was based on conchological features, including the hinge structure. In total, 31 sphaeriid species is reported, and the annotated check-list, brief zoogeographic characteristics of the fauna, and shell images are provided. Most species demonstrate a limited spatial distribution within the studied area. The causes of discrepancies between published lists of sphaeriid species are discussed.

Pontophaedusa gregoi sp. nov. is described from the Imereti Region of Georgia. This is the second species of a Tertiary relict genus that represents a very early diverged lineage of the Phaedusinae subfamily. The differences between the shell characters and climatic preferences of the Pontophaedusa species are discussed.

A new genus and species of Cepolidae are described from the Island of Cuba: Plagiosimilis gibarensis gen. et sp. nov. It is classified in Cepolidae based on the following diagnostic characteristics of the family: dart apparatus consisting of a dart sac with a pedunculated gland on its apical side, both covered by a sheath; at the base of the dart apparatus there are two accessory glands; absence of a diverticulum. The specimens from the new genus come from the northern part of the Holguin province in the vicinity of the town of Gibara and live in special habitat dominated by Tillandsia plants. Shell with the rounded last whorl last very globular round, circular umbilicus and strongly reflected lip distinguish conchologically the new genus. The reproductive system and the shell differ from the other genera, although it have a certain similarity with the genera Plagioptycha and Hemitrochus.

A report is given on the genus Rhiostoma (Cyclophoridae) collected in Vietnam during recent fieldwork. Locality data, Vietnam distribution and general distribution are given for each species, including references to previously published records. Five named species are listed, of which a new species is described, Rhiostoma ningbien sp. nov., from Dien Bien, North-western Vietnam.

The family Lyonsiellidae comprises carnivorous bivalve molluscs, distributed worldwide from sublittoral to ultraabyssal depths. One of the key anatomical characters of lyonsiellids is the reduction of the gill filaments in size and number and an increase of amount of muscles and connective tissue in gills, so their ctenidia represent an intermediate stage between eulamellibranch and septibranch conditions. The family Lyonsiellidae presently includes four genera: Lyonsiella, Allogramma, Policordia and Dallicordia. Taxonomy of the family is still problematic at both generic and species levels. The genus Dallicordia, typified by Lyonsiella alaskana was established on the assumption that it has no gills but possesses a muscular septum pierced by ostia, in contrast to the other lyonsiellids that have filamentous gills. Additionally, Lyonsiella uschakovi and Policordia ochotica were tentatively assigned to the new genus. Later P. ochotica was synonymized with P. media. In this paper we show that actually the Dallicordia was typified by an undescribed species misidentified as Lyonsiella alaskana which we describe here as D. bernardi sp. nov. Taxonomic position of the genus is dubious and could be clarified only after additional anatomical data on its type species becomes available. On the basis of examination of the type material of P. ochotica and images of the holotype of P. media, we reinstate the species P. ochotica. For the first time photographs of the holotype of L. uschakovi are provided for this specimen that was previously considered lost. Given the current state of knowledge we propose to consider species L. alaskana, L. uschakovi and P. ochotica in frame of the genus Policordia, for which we provide a list of all 29 species described up to date.

Vietnam with its limestone ranges that are concentrated mainly in northern, north–central and southern parts, and many different limestone islands running along the coastline, has a phylogenetically-diverse land snails fauna dominated by endemic species. The genus Cyclophorus (Cyclophoridae) in Vietnam is studied. Sixty species and subspecies from various habitats of limestone areas throughout Vietnam are confirmed and recorded. New species and subspecies, Cyclophorus implicatus kanhoensis ssp. nov., from Northwestern Vietnam, and Cyclophorus tamdaoensis sp. nov., from Northern Vietnam are described and discussed. In this paper, the Cyclophorus courbeti var. leucostoma Dautzenberg et Fischer, 1905, is elevated to subspecies.

Unio mongolicus Middendorff, 1851 (Bivalvia: Unionidae) was described from Gorbitza mountain stream (in the east of Trans-Baikal Territory, Russia) and the taxonomy has changed over the past century and a half. A specimen of Middendorffinaia mongolica from the Gorbitza stream collected 160 years later is a match with the type illustration of Unio mongolicus from the type locality. Comparison of the shell morphology of the type specimen of U. mongolicus with the modern specimen collected from this location, combined with our analyses of molecular, anatomical and conchological data of M. cf. mongolica species from rivers of the Upper Amur River Basin and the far eastern Russia, revealed that these taxa all belong to a single polymorphic species. The molecular analysis of the COI gene fragment mtDNA of examined Comparatory Species assigned to Middendorffinaia Moskvicheva and Starobogatov, 1973 confirmed they are only intraspecific forms of M. mongolica. Morphological and genetic distances between M. mongolica and Nodularia douglasiae Griffith et Pidgeon, 1833 confirmed their assignment to two independent genera. Differences of morphologically similar species M. mongolica, N. douglasiae and Unio crassus Philipsson in Retzius, 1788 were separated based on conchological characteristics and anatomical features. History of the taxonomic changes for Unio mongolicus is discussed here.