3. Genus Echinotus nov.

Genotype: Avicula echinata W. Smith, Middle Jurassic, Europe.

Shell rather small, inequivalve, outline subquadrate, left valve well inflated, right valve flat. Both valves have a prominent posterior wing, acutely angled at its junction with the long, straight dorsal and strongly concave posterior margin. Anterior wings weak, right byssal ear well developed, oblique, considerably overlapped by the anterior wing. Sculpture discrepant, right valve with strong, regular, radiate ridges, left valve smooth. Hinge-area edentulous, with a well excavated, triangular ligament-pit. Left hinge having a thickening below the umbo with a gentle sinus anteriorly and bounded by the ligament posteriorly.

Echinotis echinata (W. Smith). Figs. 7, 8, 25–27.

1817. Avicula echinata W. Smith, Strat. Syst. org. Foss., p. 67.

Good figures of the hinge have been published by Pompeckj (1901, pl. 15, fs. 1, 4, 7, 11, 15, 19). These show that the hinge, while built on the same general plan as that of Entomonotis, yet presents important differences. Entomonotis has a wide, scarcely excavated ligament-pit which bears several well incised parallel grooves, but Echinotis has a much narrower, well excavated pit which is not regularly grooved. The left hinges of both Entomonotis and Echinotis bear a prominent thickening immediately anterior to the ligament; but the disposition of this and also of the rest of the hinge differs in the two groups. For instance, in Entomonotis the thickening merges into the ligamental area and is itself finely striated; in Echinotis it is sharply separated from the ligament.

The specimens of E. echinata recorded by Trechmann (1923, p. 271) are the only ones that have been yet found in New Zealand. Since they are rather poor casts, the identity is not beyond doubt, but is accepted by the writer, who, however, wrongly identified as “P. echinata (?)” shells from the Awakino Valley (Henderson and Ongley, 1923, p. 24). The resemblance is only superficial, the smooth right valves have no byssal ear, and the generic relation may be with Placunopsis.