Sectipecten diffluxus (Hutton, 1873)
HUTTON, F. W. 1873. Catalogue of the Tertiary Mollusca and Echinodermata of New Zealand, in the collection of the Colonial Museum. Colonial Museum and Geological Survey Deptartment, xvi, 48 p. G. Didsbury, Gobernment Printer. Wellington. [p. 31]
1873 Pecten diffluxa Hutton, 1873
Sectipecten diffluxus (Hutton); A. U. E. Boreham, 1965, A revision of F. W. Hutton's pelecypod species described in the Catalogue of Tertiary Mollusca and Echinodermata, plate 7, figure 2, holotype, a right valve (TM 2753).
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«110. P. diffluxa, sp. nov. Sub-orbicular; with twenty-five radiating ribs; ribs broad, flat, simple near the umbo, but grooved near the margin, and occasionally divided into two; covered with sharp scales; grooves round, narrower than the ribs, finely transversely striated, without any central raised line; ears sub-equal, radiately ribbed and transversely striated. Height, 3; length, 3.
Localities.— Weka Pass (M).» FREDERICK WOLLASTON HUTTON, 1873
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«Holotype (TM 2753), an incomplete right valve. Figured topotype (TM 2754), a complete individual with separate valves, from the "Bryozoan Bed", Weka Pass (GS 6374).
TYPE LOCALITY. "Weka Pass (Middle Beds)", North Canterbury, possibly the horizon of the Hinnites Shellbed (fide Professor R. S. Allan). Age: Waiauan (Middle Miocene). DISTRIBUTION. Sectipecten diffluxus is limited to the Waiauan of North Canterbury. Additional specimens have been collected from the Hinnites Shellbed, also from the overlying Bryozoan Bed and a somewhat higher horizon in the Glenmark Limestone Formation, Weka Pass (Wilson, in press). Only one specimen, a complete individual, has been collected outside the Weka Pass area: from Cascade Stream, Pahau River, North Canterbury, from beds of Waiauan age (Mason, 1949, p. 424). REDESCRIPTION (based on the holotype right valve and 6 topotypes from Weka Pass, including one welI-preserved complete individual, here figured). Shell moderate in size, equilateral, both valves inflated, right rather more than left. Ears large, unequal, with deep byssal notch parlIicularly in early growth stages, becoming shallower later. Sculpture discrepant on opposite valves: ribs of right valve square-cut in profile, flat or slightly concave on top, slightly wider than their interspaces; those of the left valve high rounded and rather sharpcrested in early stages, becoming low rounded later, narrower than their interspaces. One or two right valve ribs tend to divide, either partially or completely, by the development of a central groove at varying stages in growth, a tendency not seen on the left valve. Initial number of ribs on both valves varies from 24 to 30 in 7 specimens (holotype 24). Fine close regular concentric lamellae cover the whole of disc on left valve, and interstices and sides of ribs on right valve, but are usually worn off the rib tops. Both valves folded into 5 shallow plications, more strongly marked proximally, dying out distally. Ears sculptured by 4 or 5 fine serrated riblets, often nearly obsolete on all but the right anterior ear.
Most specimens, including the holotype, have subequal, regularly-spaced ribs, but one topotype left valve has slightly unequal ribs that are somewhat grouped, with stronger ribs forming the crests of the radial plications and weaker ones between. This tendency for irregular rib development is more marked on the specimen from Cascade Stream (GS 3789), which is otherwise similar to Weka Pass shells. The Cascade Stream specimen has 24 ribs.
DIMENSIONS. Lectotype: height, 76 mm; length, 83 mm (est.); inflation (one valve), 17 mm. The largest specimen collected (from GS 7199) measures 87 mm x 96 mm.»
BOREHAM, A. U. E. 1961. The New Zealand Tertiary Genus Sectipecten Marwick (Mollusca). Transactions of the Royal Society of New Zealand, 88 (4): 655-668, pls. 45-49. [p. 659, 660]
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Sectipecten diffluxus (Hutton); A. U. E. Boreham, 1961, The New Zealand Tertiary Genus Sectipecten Marwick, plate 45, figures 1-3.
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