Genus Nucula Lamarck, 1799.
Type: Area nucleus Linné.

Nueula tersior n. sp. (Figs. 4, 6.)

Shell small, fragile, ovate; beaks prominent, about posterior sixth.Posterior end regularly convex; anterior end narrowly rounded, dorsal margin descending rapidly in moderate curve. Lunule not defined apart from increased curvature of disc. Escutcheon slightly bulging the outline, its outer margindefined by weak depression. Sculpture of low, bevelled, concentric ridges becoming obsolete over central part of disc which is smooth and shining; traces of fine radial threads appear in some of the interspaces. Hinge with about eleven anterior and five posterior teeth. Resilifer but little projecting. Valve-margins finely, regularly crenulated.

Holotype in collection of N.Z. Geological Survey.

Height, 2.5 mm.; length, 2.6 mm.

Suter identified this shell with N. hartvigiana Pfr. to which it is probably ancestral. The differences in the fossil, however, easily warrant specific recognition, e.g., much smaller size, more regularly rounded posterior end, fewer hinge teeth, broader beaks.

Nueula vestigial n. sp. (Figs. 1, 3.)

Shell small, ovate; beaks high, about posterior fourth. Lunule not well defined; escutcheon broad, bounded by low ridge. Sculpture of fine radial riblets, crossed by irregular, interlocking ridges in the spaces between which are the radials. Towards margin fairly regular, sharp, concentric ridges occur. Hinge strong, with about 11 anterior and 7 posterior teeth. Internal margin finely, regularly crenulated.