Family Tythothyrididae Zezina, 1979

Small (up to 20 mm in length) inflated almost spherical shells with large open foramen, smooth, gray or white with rose margin. Brachial skeleton develops at the back part of free margin of middle septum as bifurcate ascending branches.

The family is represented in the Northern Pacific with 3 genera, which form hypomorphal row from Diestothyris with complete brachioal skeleton to Tythothyris with underdeveloped crura and without descending branches of brachial loop, and to Simplicithyriswithout crura, descending and ascending branches of brachial skeleton.