4. Genus Otapiria nov.

Genotype: Pseudomonotis marshalli Trechmann, Middle Jurassic. New Zealand.

Shell of moderate size; regularly obliquely ovate, little inflated, inequivalve, right valve almost flat. Posterior wing well developed, anterior of shell generally produced, but having a short dorsal margin and a narrowly convex outline and so no defined wing. Sculpture of numerous, close, radial riblets. The right valve has, immediately in front of the umbo, a small ear, the finely striated upper surface of which is a continuation of the ligamental area. This surface tapers anteriorly, and its raised margins unite to form a thickened, rounded, upper border to the ear, which below this is concave, and is overlapped considerably by the disc with its straight, erect, dorsal margin. Hinge-area narrow, edentulous. Ligament set in a strongly and regularly grooved triangular ligament-pit, the base of which is about half the length of the posterior dorsal margin. Left valve with the hinge-margin sinused upwards and inwards immediately in front of the umbo, but further forward the margin projects outwards and bears several deep grooves.

Otapiria is nearest to Entomonotis in general characters and may be the Jurassic descendant of that stock. Otapiria has much finer sculpture, an exceptionally ovate shape, and the hinge of the left valve has a sinus instead of a projection immediaetly in front of the ligament and beak. The ligament-pit is more deeply entrenched than that of Entomonotis, thus resembling Echinotis, but the well incised parallel grooves increase the affinity with Entomonotis.

The anterior ear of the right valve of Otapiria shows important differences from that of Entomonotis, being set more in the plane of the disc, the upper edge of which is consequently not bent inwards.

Otapiria marshalli (Trechmann). Figs. 10, 12, 28–32, 34, 35.

• 1886. Pholadomya sp. Hector, Cat. Ind. and Col. Exhibit., append. p. 69, f. 34, No. 4.

• 1923. Pseudomonotis marshalli Trech., Q.J.G.S., vol. 79 (3), p. 270, pl. 15, fs. 6–9.

• Localities: Slopes of Flag Hill, Hokonui Hills, Trechmann, type.

• G.S.343 “Upper ‘Plagiostoma’ beds,” west face, Flag Hill.

• G.S.344 “Upper ‘Plagiostoma’ beds,” north face, Flag Hill.

• G.S.345 “Upper ‘Plagiostoma’ beds,” west face, Bastion Hill.

• G.S.348 “Higher part Lower ‘Plagiostoma’ beds,” Otapiri Ck.

• G.S.349 “Lower part Lower ‘Plagiostoma’ beds,” Otapiri Ck.

All of these localities are in the Hokonui Hills, Southland (see McKay, 1878, Rep. Geol. Expl. during 1877–8, vol. 11, pp. 83, 84). The Plagiostoma of Cox, and McKay's “Plagiostoma beds” probably referred to Otapiria.