p. 4

Irechenalia concentrica, Hall, Pal. of N.Y. vo 2 p. 171 pl. 40 E fig. 5

Frond circular, slightly cup-form in the young state, flattened at maturity and often variously contorted from irregular growth or accident, and thickened at intervals; surface concentrically marked by fine striaand prominent concentric folds as ridges; celluiferous side less strongly striated, cells arranged in concentric lines, almost coincident with the striae; apertures of cells narrow, opening upon the summit of an elongated pustule; con-celluliferous side of the frond showing oval stigmata produced by the bases of cells on the opposite sides.