1892. Geol. and Pal. Queensl. and New Guinea, p. 451.
Genotype (original designation): Avicula barklyi Moore, Aptian, Australia.
Figured: Moore, Quart. Journ. Geol. Soc., vol. 26, p. 245, pl. 11, figs. 1. 2.

Maccoyella magnata n.sp. Plate 61, figs. 1, 2.

Left valve large, moderately inflated, thick towards the beaks, inequilateral; outline subcircular, posterior wing apparently not produced nor defined. Umbo overhanging the area. Anterior margin descending steeply, having a wide byssal sinus, the upper margin of which is slightly reflected (forming scobinate lamellae). Owing to erosion of the surface, the number of radial ribs is uncertain, but there are probably about 22 subequal ribs with weaker ones in the wide flat interspaces. On analogy with the Australian species, these would be ranked as about 11 primary ribs, and 11 secondaries of approximately equal strength, each interspace containing a small tertiary rib about half as strong. A feeble quaternary rib is to be seen on some of the interspaces. Hinge having a broad, finely striate, ligamental area extending from the top of the scobinate lamellae backwards across the top of the cardinal socket to an obliquely triangular, slightly concave resilifer. Cardinal socket sharply raised along a definite line posteriorly, and having, anteroventrally, a thick, high, rounded margin.

Holotype (unique) in New Zealand Geological Survey collection.

Length, 122 mm. (estimated); height, 120 mm.; inflation, 25 mm.

Locality: Geol. Surv. 1989. Moanui Survey District, on track 50 chains north of Koranga River, and 5 miles 10 chains at 315° from Maungatapere Trig.

Age: Taitai Series (probably Upper Aptian).

This species appears to be the largest yet described. The posterior dorsal margin has been broken off, but sufficient of the posterior margin remains to show its position fairly closely.

The radials tend to be weakly moniliform and the interspaces weakly concentrically ridged. This, however, is a development of weathering on the peculiar “lenticular” internal structure of the shell remarked on by Etheridge (1902), and showing plainly over much of the surface.

Maccoyella magnata has sculpture at almost the same stage as that of M. reflecta (Moore) as figured by Etheridge (1902, pl. 2, fig. 1). It differs widely from this species, however, in shape, having no flat posterior wing and a considerably smaller apical angle, viz., about 120° instead of 180°. It thus approximates M. barklyi in outline, but its sculpture is much more advanced.

The stratigraphic significance of Maccoyella in New Zealand is discussed below.