Parvamussium cf. maorium Dell, 1956

(Fig. 5 E1)

Material. One specimen from Moonlight North (Y16/f0694),?adult, TM9241, UOA (borrowed from GNS).

Description. Incomplete internal mould; auricles absent; anterior half ca. 70% intact, posterior half ca. 50% intact of which posteroventral region almost totally absent, if shell assumed to be typical; only segments of margin that are (possibly) intact are short (ca. 1 mm) anterodorsal portion and even shorter (ca. 0.4 mm) anteroventral part; at least nine strong, wide internal radial ribs emanate from beak region, widely and more or less evenly spaced, anteriormost six with gentle curvature away from posterior margin, posteriormost three straight for their short continuation before they are cut off by erosion, two anteroventral ribs are preserved in entirety to show rounded terminations at ca. 90% of distance from beak to margin; small umbonal notch present.

Remarks. The specimen is a partial internal mould that does not yield sufficient information for confident specific placement. What remains of it is comparable to the Mangapanian (late Pliocene)–Recent Parvamussium maorium Dell, 1956 (Fig. 3 B2) from several localities throughout New Zealand. The radial ribs of the fossil specimen are of similar size, spacing, and position relative to the umbo compared to the modern species. The rounded terminations of the ribs, at 90% of the beak–margin distance, in the fossil specimen also are typical of P. maorium, and the umbonal notch is of a similar size in both species. The portions of intact margin in the fossil species also are of a similar shape and position relative to the umbo and ribs to that of P. maorium, and the fossil specimen is of a similar size to adult P. maorium.