Shell small, ovate-oblong, thin, fragile, compressed, especially below the middle, very inequilateral; umbones broad, flattened, in the foremost part of the upper side. Front margin abruptly descending, but little convex, strongly bent into the almost straight under side, hind margin produced-rounded, upper margin long, a little curved. Sculpture: 14 strong radiating costae, strongest in the left valve, of which 9 directed to the posterior margin somewhat flexed upwards with their ends, elevated, rather compressed (most so the uppermost ones), and 5 lower costae in the median part; front half of the shell without radiating sculpture; besides irregular concentric lines, in the posterior and upper parts raised as lamellae crossing the costae and lending them a subsquamose ap¬pearance. Colour white, dorsally brownish. Interior white, brownish behind and above; mantle line with a very small sinus below the posterior adductor. Anterior adductor scar small, ovate, not distinctly separated above from the retractor scar, posterior one broader, rounded ovate, Hinge-plate narrow. Hinge: left valve with a single median cardinal; shell margin depressed on both sides of it; right valve with 2 prominent stout teeth embracing the left one, and fitting into the sockets of the left valve margin; no cardinals in either valve. Ligament external, long, thin, with a strong resilium. Dimensions: L. 4, h. 2,6, crass. 2.2 mm. — Locality: Auck¬land Island: 41 (Masked Island), 1 empty, somewhat injured shell.

Externally this species is strikingly similar to a Cardita calyculata; but the hinge shows quite different characters and proves that it is referable to the genus Kidderia. It is similar to K. bicolor in the teeth and the shape of ligament and protoconch; only a slight difference is present between their left valves which is, however, easily explainable in the obliteration of the foremost tooth (4a) of the new species; even in K. bicolor it is not constant. A further difference is that in K. bicolor the retractor muscle scars are well separated from the adductorial ones. Also in K. bicolor a tendency to forming a small sinus of the mantle line is observable. The participating of the shell margin in forming the left cardinal sockets is no doubt a secondary apparition.