Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)
(Pl. 4a): GS9957, J40/f6608, South Branch, Waihao River near "Pentland Hills", Mangaorapan? (GNS)
(Pl. 4d): GS9957, J40/f6608, South Branch, Waihao River near "Pentland Hills", Mangaorapan? (GNS)
Beu & Maxwell (1990): Chapter 6; p. 89; pl. 4 a, d
Classification: Arcidae: Arcinae
Description: Very small for family (length 6-7.5 mm), sub-trapezoidal, strongly inequilateral, umbones broad and low, beaks prosogyrous, at about anterior quarter of length. Anterior margin convex, ventral margin almost straight or somewhat sinuous, posterior margin truncate, dorsal margin long and straight. Posterior area very well defined, concave; umbonal ridge strong. Commarginal sculpture of prominent, low, imbricate, frill-like ridges of variable strength and spacing; and fine growth ridges. Radial sculpture on flanks of 15-23 narrow, low costae of variable spacing, crenulating the commarginal ridges or forming semitubular spines at their intersections. Posterior area with costae similar to those on flanks but more closely spaced, some apparently truly radial (i.e. arising from beak) but others diverging from umbonal ridge, all densely spinose. Hinge long and narrow, thinnest and edentulous for a short distance immediately behind beaks, teeth narrow, finely transversely grooved, radiating from a point well below hinge. Cardinal area very narrow, ligament opisthodetic. Adductor muscle scars prominent, anterior scar circular, posterior scar ovate, both slightly but distinctly raised above interior of shell. Anterior and posterior margins internally crenulate.
Comparison: The new species is similar to Acar sociella (Brookes, 1926) (Recent, northern New Zealand) in size and shape but has more prominent commarginal sculpture and better developed spines. This is the oldest known New Zealand member of Acar, a genus that occurs sporadically in shallow-water assemblages up to the present day, and lives now under intertidal boulders on rocky shores, as well as byssally attached to boulders in deep water. Acar is characterised by its relatively small size, coarse commarginal and radial sculpture, very slender opisthodetic ligament, and raised adductor muscle scars. Most modern records are from tropical or subtropical regions.
Distribution: Mangaorapan?, Kauru Formation, South Branch, Waihao River near "Pentland Hills", South Canterbury (not uncommon).
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)