shallow, without definite boundary. Ventral margin generally concave. Each valve has a narrow anterior ear separated from lunule by a groove; ear of left valve of variable size, formed by thickening of margin; ear of right valve somewhat larger, groove deeper, and with small notch byssal in margin. Surface smooth except for some prominent concentric growth-stages towards margin. Hinge edentulous. Ligamental area of moderate size, obliquely triangular to trapezoid, with a broad often ill-defined central pit. Muscular impressions not visible in any of the specimens. Valve-margins thin, smooth.

Type: Aucella marshalli Trechmann. (Plate 71, figs. 1–6.)

Trechmann (1923, p. 269) recognized the unusual characters of this shell, and thought it probably represented a new genus, perhaps related to his Hokonuia. Additional material collected from Kawhia by Henderson and Grange throws further light on these peculiarities, and confirms Trechmann's opinion that a new generic division is required. The anterior ear of the right valve is but little stronger than that of the left, and so is not nearly so specialized as that of Hokonuia or Aucella. The size of the left ear is, however, more variable than that of the right; occasionally it is scarcely developed.

There is little or no difference in the inflation of the valves, apparent inequalities of one or the other being due to accidents of pressure during fossilization. The shape is extremely variable, and, as there is generally considerable deformation due to rock-changes, the original outline is difficult to make out. Most of the specimens have beaks about the anterior fourth, a strongly concave escutcheon, and a concave ventral margin; some, however, have almost median beaks, a shallow escutcheon, and straight ventral margin. From its occurrence uniformly low in the Jurassic strata this shell is most useful as a zone-determiner.

Localities.—1191, 1195, 10 chains south along coast from Ururoa Point, Te Maika Peninsula, Kawhia Harbour; Awakino Valley (Henderson and Ongley, 1923, p. 20); north of Sandy Bay, Nugget Point (Trechmann, 1923, p. 269).