Diestothyris frontalis (Middendorff, 1849)

Terebratula frontalis Middendorff, 1849: 518–519; 1851: 241, pl. 18, figs. 9–14.

Terebratella frontalis Davidson, 1887: 86, pl. 15, figs. 1–8.

Diestothyris frontalis Thomson, 1916: 504.

Description . Strong ashy-gray or yellowish-gray shells up to 20 mm in length, round or oval-round in plan. Anterior commissure is convex to pedal valve (sulcate). Posterior part of pedal valve is inflated, foramen is large and open. Deltidial plates are narrow and rudimental. Brachial middle septum is low posteriorly and steeply lifts at 1/3 of the length of valve; descending branches link the septum to cardinal margin through crura; ascending branches form a ring at the top of septum. Gonads are thick and bilobate.

Distribution . Pacific boreal species known from the Aleutian Islands to Peter the Great Bay and the Tsugaru Strait along the coast of Asia, and to the British Columbia (Canada) along American coast.

Ecological data. The species is distributed along the vertical scale from littoral zone to the depth of 435 m. In Primorye D. frontalis is usual to the depths of 50 m where this species reaches density about 100 sp./m². It is the most numerous in the Sea of Okhotsk, where D. frontalis reaches density 900 sp./m² at the depth of 121 m off the mouth of Shelikhov Bay. At the interdial zone O.G. Kussakin found this species in splits and caverns of the stones and rocks at the western shores of Kamchatka near Ust-Tagil (Babushkin Cape).

References:

The World Register of Marine Species