Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)
(Pl. 9g): GS14431, J42/f6573A, right bank, Kakanui River near mouth, Whaingaroan (GNS)
Beu & Maxwell (1990): Chapter 9; p. 131; pl. 9 g.
Description: Small for family (height 17-24 mm), left valve weakly inflated, right valve slightly more inflated. Some specimens bent in towards commissure distally, producing a humped profile. Ears unequal, particularly in right valve; byssal notch well developed, moderately wide, ctenolium present. Radial sculpture in both valves commencing near beaks as 11 or 12 narrow, well spaced costae that quickly become broad plicae (i.e. involve whole thickness of shell), those on left valve developing 2 (or rarely 3) longitudinal grooves so that distally they consist of 3 (or 4) narrow, subequal, scaly costae and are typically of reduced relief. Secondary costae, also scaly, developing by intercalation at 6-11 mm from beak, and tertiary costellae at a later stage, either by intercalation or by branching off sides of plicae. Plicae on right valve developing 1 (or less commonly 2) deep longitudinal grooves, the resulting costae remaining rounded and smooth apart from weak growth lines. Secondary and tertiary radials similar in character to plicae. Ears with 4 or 5 narrow radial costae. Both valves with antimarginal microsculpture on disc and ears, in most specimens much better developed on left than on right valve, and in places (particularly on lateral margins) bearing microsculpture resembling shagreen where intersected by fine growth ridges.
Comparison: Although this distinctive little pecten superficially resembles some species of Mesopeplum, it differs in having distinct antimarginal (and locally shagreen) microsculpture and a relatively broad byssal notch. Mesopeplum lacks antimarginal and shagreen microsculpture but has numerous fine, closely spaced commarginal ridges extending over the whole disc and ears, and has a narrow byssal notch (becoming obsolete in the adult of the Australian Recent type species, M. caroli Iredale, 1929, = fenestratum Hedley, 1901). The Kakanui species was referred to Notochlamys, proposed by Cotton in 1930 for Australian Mesopeplum-like shells with shagreen sculpture, by Beu & Maxwell (1990, p. 131), but further examination has shown that it is a member of an unnamed New Zealand genus with numerous, prominent, narrow, almost smooth radial plicae and much fine interstitial sculpture, bearing antimarginal and shagreen sculpture in some radial interspaces. "Serripecten" venosus is another member of the genus, and several others unnamed ones occur in Oligocene and early Miocene rocks of North Otago-South Canterbury. The unnamed genus is referred to as "Notochlamys" for now.
Distribution: Whaingaroan, McDonald Limestone, Kakanui River, near mouth (uncommon); Deborah Volcanic Formation, Everett's Quarry, Kakanui (moderately common); tuffs below limestone, All Day Bay, Kakanui (rare).
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)