I.—Notes on new or imperfectly known Chalk Polyzoa

Extract

Zoarium always unilaminate, nearly always adherent.

Zoœcia broad, length ·65–·8 mm., breadth ·45–·55 mm., aperture primarily semicircular but tending to quadrangular by thickening of the upper lip, and with a semicircular collar, the two ends of which are expanded into large perforated spine bases: the arched front wall springs from the inside of the side walls and is pierced along its edge by a row of large pores, and at a short distance inside this row by a row of smaller pores in fairly close correspondence with the outer row; the centre of the front wall is pierced by a very variable number of fine pores, which sometimes give clear indications of a third row corresponding to the two outer ones; the zoœcia always have common side walls of considerable width; in immature stages (Fig. 1) the front walls rise high above the side walls, and small beak-shaped avicularia are scattered irregularly between them; in a further stage of development (Fig. 2) the side walls are greatly thickened and raised above the front walls, absorbing the avicularia, and the perforated spine bases at the corners of the aperture grow inwards until they meet, and are fused into a stout bar across the aperture, so qualifying the species for admission to the Steginoporidæ; in the stage of maximum development (Fig. 3) the bar develops a stout denticle in the middle of the lower edge.