Shell ovate, dark-brown, shining, smooth, bluntly keeled dor-sally, slightly convex at the lateral slopes. Anterior valve a little broader than the posterior one, with a few concentric growth-lines (in the one specimen with 4 concentric regularly distant sulci), otherwise smooth and showing only microscopic radiating striae and impressed dots which are densest towards the margin. Intermediate valves beaked, with elevated lateral areas, totally smooth except for lines of growth (and occasional impressed dots towards the margins, and furrows: 4 concentric ones in one specimen), microscopically striated longitudinally. Posterior valve with a pre-median mucro; its central area separated, by means of straight lines, from the posterior elevated area, which has a straight or slightly concave slope. All valves porous, the pores appearing as microscopical regular dots all over the surface. Eyes in a small number, occupying a median ray on the lateral areas, most obvious in their upper parts. Girdle narrow, rusty brown, lighter at the margin, with close elongate scales and fringed with short acicular ones. Interior of the valves crimson; anterior valve with 16 slits, posterior one with 12, median valves with 4 or 5; teeth solid, propped outside, eaves porous, sutura! plates united, sinus shallow, broad. — Gill cordon extending from the anterior corners of the foot to near the foot end.

Compared with C. steinenii from South Georgia, which has a similar colour, the present species shows much smaller and denser girdle-scales, as well as completely smooth, not granulose, and distinctly microscopically striated valves; in the latter respect it differs also from C. platessa, in which a tendency to granulation of the lateral and central areas appears. The number of slits of the intermediate valves (in C. steinenii 2, in C. platessa 3) is another point of distinction.

Locality: Campbell Island