Genus Parvithracia (Bivalvia: Thraciidae) with descriptions of a new subgenus and two new species from the northwestern Pacific

A new subgenus, Pseudoasthenothaerus, of the genus Parvithracia Finlay, 1927, and two new species, Parvithracia (Pseudoasthenothaerus) lukini and R (Pseudoasthenothaerus) sirenkoi, are described from the Pacific seas of Russia. Previously representatives of the genus Parvithracia, with the type species Parvithracia suteri Finlay, 1927, were found only off New Zealand. Study of thraciid genera showed that Japanese species assigned to the genus Asthenothaerus Carpenter, 1864, should be placed in the genus Parvithracia. For Asthenothaerus sematana (Yokoyarna, 1922) and A. isaotakii Okutani, 1964, new combinations are suggested: Parvithracia (Pseudoasthenothaerus) sematana (Yokoyama, 1922) and P (Pseudoasthenothaerus) isaotakii (Okutani, 1964). The main morphological characteristics of the new subgenus are the presence of conspicuous lateral teeth in both valves and a subumbonal plate for the attachment of the lithodesma that is supported by pillars. Expanded descriptions of R suteri, R (Pseudoasthenothaerus) sematana, and P. (Pseudoasthenothaerus) isaotakii are given.