Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)
(Pl. 7j): GS9508, J40/f8803, McCulloch's Bridge, Kaiatan (GNS)
(Pl. 7k): GS9508, J40/f8803, McCulloch's Bridge, Kaiatan (GNS)
Beu & Maxwell (1990): Chapter 8; p. 115; pl. 7 j, k.
Synonymy: Caestocorbula n. sp. Beu & Maxwell, 1990, p. 115, pl. 7j, k; Caestocorbula praeviator Maxwell 1992, p. 78
Description: Size moderate for family (length 10-12 mm), trigonal, subequilateral, strongly inflated, markedly inequivalve, right valve considerably the larger, overlapping along ventral and posterior margins. Right valve with typical corbulid "duplex" shell, inner layer considerably smaller than outer layer, line of concrescence well marked on interior. Umbones very prominent, with large nepioconch (a protruding early growth stage) raised above rest of valve. Anterodorsal margin descending steeply, almost straight, posterodorsal margin less steep, somewhat concave; posterior end narrow, truncate. Umbonal ridge rounded, posterior area slightly concave. Sculpture of prominent, rounded commarginal costae with narrower interspaces, widest somewhat behind middle, narrowing towards either end and weakening on posterior area. Left valve subelliptical, nepioconch large, projecting, sculptured with irregularly spaced commarginal costae, remainder of valve with weak growth ridges and 1 or 2 narrow radial ridges. Some articulated shells retaining a small, subquadrate siphonal plate lodged in space between posterior ends of left and right valves. Hinge of right valve with a peg-like cardinal tooth immediately beneath beak and a sunken triangular resilifer behind. Left valve with a deep triangular socket to take right cardinal and a projecting, triangular chondrophore behind. Adductor muscle scars relatively large, subcircular; pallial line with shallow, arcuate sinus.
Comparison: Caestocorbula praeviator is superficially very similar to the widespread Notocorbula humerosa (Duntroonian-Kapitean) but differs in being more elongate anteroposteriorly, in having a more distinctly rostrate posterior end, in having a larger resilifer in the right valve and, most importantly, in the presence of a siphonal plate. It is on the basis of this last character that this species is referred to Caestocorbula Vincent, 1910 rather than to Notocorbula Iredale, 1930 (Vokes 1944). Caestocorbula is not known to have survived the Eocene, but strong circumstantial evidence suggests that it gave rise to Notocorbula through loss of the siphonal plate (see discussion by Maxwell, 1992, pp. 78-80).
Distribution: Bortonian-Kaiatan, Pahi Greensand, Pahi; Waihao Greensand, McCulloch's Bridge (common).
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)