Revised descriptions of New Zealand Cenozoic Mollusca from Beu and Maxwell (1990)
(Pl. 10b): GS9806, F45/f9668, Shell Gully, Chatton, Duntroonian (GNS)
Beu & Maxwell (1990): Chapter 10; p. 141; pl. 10 b.
Synonymy: Loboplax ashbyi Laws 1932, p. 184; Notoplax ashbyi; Acanthochitona (Notoplax) ashbyi, Beu & Maxwell 1990, p. 141
Description: Shell small (holotype, a median valve, 3.5 mm long x 5.5 mm wide; anterior valve 3.3 x 4.7 mm; posterior valve (figured) 2.2 x 3.7 mm), similar to Notoplax mariae (Pl. 44a, d, h) and the large living species Notoplax violacea (Quoy and Gaimard, 1835), but differing in its more steeply arched back (Laws (1932, p. 184) recorded that the holotype has a jugal angle of 90°) and in having very large, flattened granules on the tegmentum. Laws' description and figure of a median valve are of one much like that of N. mariae, with roughly triangular tegmentum and without a ridge between lateral and pleural areas. Anterior valve incomplete in the only known specimen (missing tegmentum), with 5 slits, 5 prominent radial ridges, and a deeply lobate tegmental margin. Posterior valve with 7 deep slits and several shorter ones, a short, wide, oval tegmentum (as in many modern specimens of N. mariae), jugum not well differentiated from latero-pleural areas, and a low mucro at posterior 0.6 of tegmentum length.
Comparison: Notoplax ashbyi evidently lived in a similar environment to that of N. mariae today, i.e. in the intertidal zone, or more commonly, in the shallow subtidal zone, on hard substrates or on boulders and shells in soft substrates. The classification of the Australian and New Zealand chitons of the Acanthochitonidae is unsettled, and different taxonomists have used different genera, Australian workers tending to follow Iredale & Hull's (1931) classification into many very narrow, closely similar genera. In contrast, we are more impressed by the close similarity of many of the proposed groups, and consider at least Notoplax to be a broad genus containing many of the species with a wide girdle, variously spinose or smooth. These species are assigned to Notoplax tentatively here, pending a molecular phylogenetic study of the group.
Distribution: Duntroonian, Chatton Formation, Shell Gully, Chatton (type), uncommon in washings.
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)