The genus Adontorhina (Bivalvia: Thyasiridae) from the northern Pacific Ocean: Description of two new species, an expanded description of Adontorhina cyclia S. S. Berry, 1947, and remarks on other species

We describe two new deep-sea species of bivalves of the genus Adontorhina that were found at a depth of 788–2532 m off the Kuril Islands and the southeastern coast of the Kamchatka Peninsula (Adontorhina inflata sp. nov.), as well as at a depth of 3210–3432 m in the Sea of Okhotsk and on the oceanic slope of the Kuril Islands (Adontorhina diaphana sp. nov.). Herein, we provide an expanded description of the amphi-Pacific Adontorhina cyclia, which is supplemented with new data on shell morphology, body anatomy, and distribution, based on the analysis of samples collected by 12 expeditions between 1949 and 2015 in various regions of the northwestern Pacific from the coast of Japan to the Bering Sea. The main feature distinguishing the new species from all other Adontorhina species is a unique sculpture of the prodissoconch consisting of thin folds and wrinkles of varying length, shape, and number. In addition, A. inflata sp. nov. differs from other Adontorhina species by having an obliquely ovate, strongly inflated shell with a poorly defined escutcheon and indistinct auricle; and A. diaphana sp. nov. is distinguished by having a subcircular shell with a long and strongly prominent auricle. A thorough study of the prodissoconch morphology of A. cyclia from the western and eastern Pacific, as well as Adontorhina sphaericosa and Adontorhina zelayi, showed that the sculpture of the prodissoconch in all species studied is unique and can be used as an important additional character for the identification and differentiation of Adontorhina species.