Range.—Silurian to Lias.
Principal Genera.—Koninckina, Koninckella, Coelospira, Davidsonia.
Family. Spiriferidae
Shell biconvex. Brachial apparatus consisting essentially of two descending calcareous lamellae which by spiral enrolment form a pair of laterally-directed cones (Fig. [330]).
Range.—Chiefly Palaeozoic, but a few forms pass up into the Lias.
Principal Genera.—Spirifera, Cyrtia, Uncites, Athyris, Merista.
Family. Atrypidae
Brachial apparatus consists of two descending calcareous lamellae which bend outwards at the extremity of the crura and are coiled into two spiral cones, the apices of which either converge towards each other (Glassia) or towards the dorsal valve (Atrypa, Fig. [332]), or diverge towards the dorsal valve (Dayia); shell structure impunctate.
Range.—Ordovician to Trias.
Principal Genera.—Atrypa, Dayia, Glassia.