Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)
(Pl. 35g): Nukumaru Brown Sand, Nukumaru Beach, Wanganui, Nukumaruan (GNS, ex Marshall and Murdoch Collection)
Beu & Maxwell (1990): Chapter 14; p. 282; pl. 35 g.
Synonymy: Cyclina dispar Hutton 1873b, p. 22; Dosinia subrosea Gray (in part) of Suter 1914, p. 50; Lucinida levifoliata Marshall and Murdoch 1919, p. 257; Lucinida dispar; Callucina (Pteromyrtea) dispar
Type species of Pteromyrtea Finlay, 1926
Description: Small for family (18-35 mm high), almost circular, thin and fragile, weakly inflated, weakly sculptured. Some specimens almost smooth, except for weak growth ridges; others sculptured with low, very thin, irregularly placed commarginal lamellae. Lunule small, deeply impressed, deeply cut into hinge plate, narrow, bounded by a raised ridge; protrusion of a minute, shallowly impressed, anterior lateral tooth and socket causes extension of hinge plate in front of lunule, to form a second rounded, wing-like "pseudolunule", 3 times as long as true lunule, demarcated from disc by a weakly defined, angling ridge from umbo to antero- dorsal margin. Hinge very narrow; with a long, straight, narrow nymph occupying lower half of width of posterior hinge plate, margined above by a wide, rather shallow ligamental groove, completely inside valve margin; remnants of 2 very weak, obscure cardinal teeth and sockets and of very small anterior and posterior lateral teeth are present in some specimens but obliterated from most large specimens. Anterior adductor scar very tall and narrow; posterior one shorter, oval; pallial line without sinus; interior irregularly radially wrinkled.
Comparison: The abrupt extinction of Pteromyrtea dispar is one of many that make the end of the Nukumaruan Stage readily recognised. P. dispar is easily identified by its almost circular outline, its thinness and fragility, its hinge obliteration in large specimens and, in particular, its winglike anterior "pseudolunule". Well preserved specimens have many obvious, fine commarginal lamellae. Earlier species of Pteromyrtea are smaller and rarely have obliterated hinges.
Distribution: Opoitian-Nukumaruan; Hautapu Limestone (i.e., a limestone bed within Matemateonga Formation), Hautapu Falls, upper Rangitikei River (Opoitian), type of Cyclina dispar ; "Moa beds", i.e., Tewkesbury Formation, Nukumaru Beach, Wanganui (Nukumaruan), type of Lucinida levifoliata (the type localities, ages and synonymy were discussed by Boreham 1965, p. 56-57). Uncommon in Opoitian rocks, but a very common shell in Waipipian to Nukumaruan soft-bottom, near-shore sandstone and shellbeds throughout New Zealand, and particularly in shallow facies of Nukumaruan age in Wanganui basin and Hawke's Bay.
Cite this publication as: "A.G. Beu and J.I. Raine (2009). Revised
descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990). GNS
Science miscellaneous series no. 27."
© GNS Science, 2009
(Included with a PDF facsimile file copy of New Zealand Geological Survey Paleontological Bulletin 58 in CD version from: Publications Officer, GNS Science, P.O. Box 30368 Lower Hutt, New Zealand)