Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)
(Pl. 12g): hypotype of Maxwell (1978, fig. 3) (TM 5464, GNS); (Pl. 12h) lectotype (TM2897, GNS), both from junction of Porter and Thomas Rivers, Castle Hill Basin, Duntroonian
Beu & Maxwell (1990): Chapter 10; p. 150; pl. 12 g, h.
Synonymy: Mactra attenuata Hutton 1873b, p. 18 (not of Deshayes in Reeve, 1854); Mactra leda Finlay 1924a, p. 106 (new name for M. attenuata Hutton, preoccupied); Longimactra leda; Oxyperas (Pseudoxyperas) leda, Beu & Maxwell, 1990, p. 150, pl. 12g, h.
Classification: Mactridae: Mactrinae
Description: Rather large for family (length 60-95 mm), elongate-ovate, beaks subcentral, anterior end narrowly rounded, posterior end broader. Posterior weakly differentiated. No sculpture apart from subdued commarginal ridges and grooves. Right hinge with a prominent, lateral socket at each end, bounded below by a prominent, finely transversely grooved tooth and above by a much weaker tooth scarcely differentiated from shell margin; cardinal portion with 2 divergent lamellar teeth anterior to an oblique, triangular resilifer. Left hinge imperfectly known, but having a prominent lateral tooth at each end, and a short, narrow anterior cardinal; median cardinals missing in available shells. Other internal features not known, but pallial sinus presumably moderately deep, tongue-shaped.
Comparison: Oxyperas leda differs from O. elongata (Opoitian-Recent) in its more elongate and more equilateral shell, and in having a more strongly convex ventral margin and longer, more prominently striate lateral teeth. O. exensis (Waitakian, Tengawai River) is less elongate than O. leda, and has a more strongly concave anterodorsal margin and a broader hinge. An early form of O. elongata, named as O. komakoensis (Opoitian-Nukumaruan) has much more prominent commarginal sculpture than O. elongata, but intergrades in sculptural prominence through Nukumaruan-Castlecliffian time (Beu 2006), apparently undergoing anagenetic evolution. Oxyperas has a sporadic record in New Zealand from about Mnagorapan or Porangan (Otaio Gorge) to Recent. The extant species O. elongata occurs in shallow waters down to about 75 m. Beu (2006) regarded Pseudoxyperas as not worth segregating from Oxyperas.
Distribution: Duntroonian; "Broken River (lower beds)" (type) i.e. probably the "Fan coral bed", Thomas Formation, junction of Porter and Thomas Rivers, Castle Hill Basin, the only locality where this species is known definitely to occur.
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)