Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)
(Pl. 43i): GS4013, R22/f6353, Tainui Shellbed, Castlecliff, Wanganui, Castlecliffian (GNS), a posterior valve
Beu & Maxwell (1990): Chapter 16; p. 332; pl. 43 i.
Synonymy: Chiton canaliculatus Quoy and Gaimard 1835, p. 394; Chiton insculptus A. Adams 1854, p. 91; Lepidopleurus canaliculatus; Anthochiton canaliculatus
Description: Valves small (median valves to about 11 mm wide, 3.5 mm long), much wider than long, highly arched, with prominently angled dorsum; sutural laminae short and wide, insertion plates thick and very short, square-edged, very finely and complexly denticulate. Anterior valve semicircular, with 8 slits in anterior edge, and about 30-40 moderately prominent wide, low closely spaced, weakly nodulous radial costae. Median valves with 1 slit in each side, almost smooth but obscurely defined jugal area, pleural areas dissected by 10-15 wide, shallow grooves (producing the same number of smooth-topped, low, narrow longitudinal costae), lateral areas with 3 or 4 prominent, low, wide, weakly nodulous radial costae (as on anterior valve). Posterior valve semicircular, with 10-12 slits- around posterior edge, much lower than anterior valve of same specimen; mucro slightly in front of centre, narrow pleural areas sculptured as on valves 2-7; jugal area longitudinally costate on most specimens, i.e., not as well defined and smooth as on valves 2-7; posterior half of valve bears 24-30 low, close, weakly nodulous radial costae, as on anterior valve and lateral areas of median valves.
Comparison: Rhyssoplax canaliculata is much more steeply arched and with pleural areas much more subdivided into costae than on the intertidal to shallow subtidal species R. aerea (Reeve, 1847) (= clavata Suter, 1907; = oliveri Mestayer, 1921; =suteri Iredale, 1910). Also, in modern specimens, R. aerea is usually glaucous green but varies to yellow or speckled brown, and some specimens have pink dorsal areas, whereas R. canaliculata is consistently brownish pink, with a dark grey stripe on each side of the pink jugum on median valves. Fossils that could be identified as R. aerea rather than R. canaliculata have not been recognised with certainty, although a few valves from Castlecliff possibly belong there. The only other chiton with which this could be confused is Lorica haurakiensis (below) but R. canaliculata differs from L. haurakiensis in its much smaller size, lower dorsal (jugal) angle, lower and wider pleural costae, and very much lower and wider lateral radial costae lacking the small, sharp nodules of Lorica; the anterior valve is deeply concave and coarsely costate and the posterior valve has a posterior mucro in Lorica.
Distribution: Nukumaruan-Recent; Recent, New Zealand (types of both Chiton canaliculatus and C. insculptus). R. canaliculata is abundant today throughout New Zealand, dredged with Notoplax rubiginosa (but not quite as common as N. rubiginosa) on hard substrates (usually on shells) in about 5-100 m. It is an uncommon fossil, however; specimens have been seen from Hawke's Bay blue-grey siltstone (Nukumaruan), from several shellbeds at Castlecliff, Wanganui (Castlecliffian), from Te Piki, Cape Runaway (Haweran, oxygen isotope stage 7), from Ohope Beach and Matata, Bay of Plenty (Castlecliffian; NMNZ), and from the shellbed on Hauriri Terrace, Waverley Beach, west of Wanganui (oxygen isotope stage 5a, 80 000 years BP).
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)