The original figure of this species was rather diagrammatic and requires supplementing. Figs. 1 and 2 are from the type-specimen, Fig. 2 being given to illustrate the rectangular aperture which the species has a tendency to develop, though quite irregularly; this rectangular outline appears to be due to the amalgamation of the two prominent tubercles into a straight-edged cap which hides the arched part of the upper lip and may be oœcial in function. The typical form of the species is definitely established in the Marsupites zone (= Marsupites band of many authors), though it only becomes abundant in the succeeding zone.