Previous taxonomic and molecular studies demonstrated the existence of two distinct species of the freshwater pearl mussel genus Margaritifera in the Kamchatka–Sakhalin–Kuril–Japan region, but problems remained as to their identification. To solve them, shells of nominal types and other voucher specimens were reexamined morphologically in detail and compared with Japanese populations using multivariate analyses. The results reconfirmed the importance of the shape of the dorsal edge of the anterior adductor muscle scar, and demonstrated the usefulness of the width of pseudocardinal teeth as a distinguishing character. Based on the results the previously accepted classification was revised: M. laevis (Haas, 1910) (Japanese name: Kawa-shinju-gai) is characterized by a rounded dorsal edge of the anterior adductor muscle scar and thin pseudocardinal teeth, and M. kurilensis (Zatravkin & Starobogatov, 1984) (Japanese name: Kogata-kawa-shinju-gai) is characterized by an angulate dorsal edge of the anterior adductor muscle scar and robust pseudocardinal teeth. Margaritifera middendorffi (Rosén, 1926), a name often applied to the latter species, is here considered a junior synonym of M. laevis, and M. togakushiensis Kondo & Kobayashi, 2005, often synonymized with M. middendorffi, is shown to be a junior synonym of M. kurilensis, judged from the morphology of the primary types.

Limopsis belcheri (Adams & Reeve, 1850) was based on a Samarang Expedition specimen from "off Cape of Good Hope, South Africa, 120 fathoms", but Adams (1863) revised the type locality to "Korean Islands" without specifying a reason. The placement of East Asian limopsids such as Limopsis tajimae G. B. Sowerby III, 1914, L. uwadokoi Oyama, 1951, and L. tokaiensis Yokoyama, 1910 in synonymy with L. belcheri by recent workers has caused confusion. This study reexamines the figured syntype of L. belcheri and corrects its synonymy, focusing on Recent specimens. Limopsis belcheri is confirmed to be a distinct species from L. tajimae, L. uwadokoi and L. tokaiensis, distinguishable by 1) the serrated crenulation along the inner ventral margin, and 2) a resilifer pit filled only by a fibrous layer. All figures so far identified as L. belcheri from Japan, Korea, and China disagree with the inner shell characters of this syntype. In contrast, the South African species Limopsis chuni Thiele & Jaeckel, 1931 agrees with it in both outer and inner shell characters, and should be synonymized. These observations support the proposal that the type locality of Limopsis belcheri should be off the Cape of Good Hope, as noted in the original description, and not the revised locality proposed by Adams (1863).

The karyotypes of four species and one form of the genus Semisulcospira in Lake Biwa, Japan were examined. The observed diploid chromosome number of each taxon was as follows: Semisulcospira niponica and S. niponica form biwae, 2n=26 (20M+4SM+2ST); S. decipiens, 2n=26 (22M+2SM+2ST); S. takeshimensis, 2n=26 (18M+6SM+2ST); S. shiraishiensis, 2n=28 (18M+8SM+2T).