Class Rhynchonellata – articulate brachiopods

Valves are connected not only with muscles but also with hinge (valves can be separated when the hinge part of the shell is destroyed), which is composed of two symmetric bulbs (teeth) at the sides of posterior (cardinal) margin in pedal valve and of corresponding hinge-sockets in brachial valve. Gut is blind and has no anus.

Among 10 orders of rhynchonellate brachiopods (Protorthida, Orthida, Pentamerida, Rhynchonellida, Atrypida, Athyridida, Spiriferida, Spiriferinida, Thecideida, Terebratulida) only three ones are represented in the recent fauna (Rhynchonellida, Thecideida, Terebratulida), and only two ones (Rhynchonellida and Terebratulida) occur in the Sea of Japan.

Key to orders of the class Rhynchonellata

1(2). Brachial-supports (crura) look like short plates, which are going off hinge (cardinal) margin and are not jointed to transverse band or to middle septum. Shell has no pores. Lophophore consists of two conical spiral lobes (spiroloph)Rhynchonellida

2(1). Brachial-supports (crura) join each other with transverse band (to form free loop) or join to complicated brachial skeleton which is formed at the top of the middle septum elevating up the valve bottom. Shell is porous. Lophophores are from the most simple round (trocholoph) to threelobate (plectoloph) or multilobate (ptycholoph) Terebratulida