Biplicate

North American biplicate species are not considered in the present paper because of poor knowledge of their internal morphology.

Platystrophiid with uniplicate or biplicate origination of costae in ventral sulcus; dorsal interior with well-developed notothyrial platform; brachiophores with massive basal parts; fulcral plates absent; anterior and posterior pairs of adductor scars closely spaced in the central part of the dorsal valve; median septum well developed, bisecting the adductor scars medially; adductor muscle impressions smooth.

Origin of costae in sulcus and on fold, biplicate type of Cumings (1903).

Origin of costae in sulcus and on fold of biplicate type of Cumings (1903).

Platystrophiid genus with costae in ventral sulcus of biplicate type; cardinalia with sessile septalium; fixlcral plates present; posterior adductor scars separated by gap; small median septum variously developed only between anterior adductor scars; muscle scars smooth; posterior adductor scars bisected by variously developed ridges.