Genus Maitaia Marwick.

1934.Proc. Fifth Pacific Sc. Congress; 1B, p. 948.

Genotype (by monotypy): Maitaia trechmanni Marwick.

Shell of moderate size, mytiliform, winged posteriorly, equivalve; beaks terminal, slightly raised above the long, straight dorsal margin. Hinge edentulous, margin thickened and flat, and having about three well-defined, longitudinal, ligamental grooves. The shell margins, towards the beaks, thickened and bevelled, and below the beaks, internally, a septal plate was well developed in large shells. Shell substance consisting chiefly of the prismatic layer. Surface with low, rather irregular, concentric waves. Muscle-impressions weak, posterior adductor scar semicircular, of moderate size, placed far back and somewhat nearer the dorsal than the ventral margin.

Maitaia trechmanni Marwick. Figs. 1.3.

Holotype in collection of N.Z. Geol. Survey.

Height, 90 mm. (approx.); length, 105 mm.

Localities: Geol. Surv. loc. 143, Wooded Peak, Dun Mountain, Nelson (type); Geol. Surv. loc. 1456, 1 mile N.E. Clinton.

In most of its characters this shell agrees with Myalina, e.g., the mytiloid shape, longitudinal cartilage grooves, and umbonal septum, the outstanding difference being the prismatic shell. The presence of this septum in the New Zealand fossil has not previously been pointed out, but it shows clearly in Figure 2, and also in Trechmann's figures 1 and 4, though it is not evident from these that the umbos are hollowed out behind the septal plate. In smaller specimens the septum is not wide; however, on the casts some trace of it can generally be seen comparable to the hinge plate of a Mytilus, but, of course, without any teeth.

The Clinton specimens are mostly small, though one measures 65 mm. x 65 mm., and there are intermediates, so that the writer is now of the opinion that all can be included in the same species as the Dun Mountain fossils. Further evidence against close relations with the Australian Aphanaia is provided by the specimen shown in Figure 1. The scar of the posterior adductor appears plainly on this cast, situated nearer the dorsal than the ventral margin and of a semicircular shape.

The difficulty of obtaining undistorted, complete specimens (or even casts) is largely due to the prismatic structure of the shell-substance and its consequent extreme brittleness.

No further evidence as to the age of the Maitai beds has come to light, so that all we can say is that it is upper Palaeozoic.