The family Myophorellidae was proposed by Cooper (1991) to
include species with generally conspicuously nodate and oblique
flank ribs, although with sub-commarginal flank costae in early
growth stages (Fig. 1). The genus Myophorella Bayle includes a wide
range of species with these general characters. Kobayashi and
Tamura (1955) ordered its variability subdividing the genus into
three subgenera: Myophorella s.s., Promyophorella Kobayashi and
Tamura, and Haidaia Crickmay. Myophorella s.s. was characterized
by having a typical triangular shell with strong tubercles on the
flank (Fig. 1(G)). Promyophorella, on the other hand, had a more
varied shell outline (often crescentic), ornamented by narrower
costae on the flank, with small tubercles aligned on the top
(Fig. 1(E)). As suggested by its name, this subgenus was regarded as
the ancestor of the other two by Kobayashi and Tamura (1955). The
genera Steinmanella Crickmay and Quadratotrigonia Dietrich were
considered by Kobayashi and Amano (1955) as two independent
offshoots of the Myophorellinae.