Parvamussium paradoxum (Maxwell, 1969)
MAXWELL, P. A. 1969. Middle Tertiary Mollusca from North Otago and South Canterbury, New Zealand, with a review of New Zealand species of Venericardia (Carditidae, Pelecypoda). Transactions of the Royal Society of New Zealand, Geology, 6 (13): 155-185, pls. 1-3. [p. 170, pl. 2, fig. 14]
1969 Ctenamusium paradoxum Maxwell, 1969
P. A. Maxwell, 1969, plate 2.
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«Shell small, thin, subcircular, almost equilateral, byssal notch small. Right valve little inflated, anterior ear larger, both ears with thin low concentric lamellae, which on anterior ear cross six radial ribs decreasing in strength dorsally. Dorsal margin horizontal, lightly serrated by concentric lamellae. Sculpture on exterior of disc of very weak, narrow, flat-topped concentric ridges, c. 18/mm on mature shell. Interior with 15-21 weak radial ribs, normally visible only near edge of disc as low tubercles. Hinge plate with a minute, central triangular resilifer. A single tubercle (auricular crus) on each ear near junction with disc. A small indistinct ovate adductor muscle scar high up near base of posterior ear. Left valve similar, but with coarser, more widely spaced and hence more prominent concentric sculpture, typically about 10/mm but on some individuals almost as closely spaced as in the right valve. Radial sculpture completely absent, even on ears. Anterior ear the larger, depressed below level, of disc; posterior ear continuous with disc. Internal ribbing and auricular crura as in right valve. Adductor muscle scar triangular, larger than in right valve, and situated lower down, just behind centre of disc.
DIMENSIONS OF TYPE: Length 4.7 mm, height 4.9 mm, inflation (one valve) 0.7 mm.
TYPE (TM3896) and 7 damaged paratypes from GS1822, left bank Waitaki R., opposite Wharekuri, collected by Dr J. Marwick.
FORMATION: Wharekuri Greensands.
AGE: Duntroonian.
LOCALITIES: GS1821; G59514, “borrow-pit” for Aviemore Dam, hillside above Lake Waitaki (Duntroonian) (numerous paratypes); G59805, MacDonald’s farm, Waikaka Stream, Southland (Duntroonian).
KNOWN RANGE: Duntroonian.
REMARKS: Ctenamusium paradoxum n.sp. is readily distinguished from other New Zealand species by the lack of external radial sculpture on the left valve and by the abnormally large number of internal ribs (only 9-10 in C. vafer Marwick, Miocene, New Zealand). Both C. vafer and the genotype have 2 or 3 elongate crura on each ear, whereas paradoxum and an undescribed species from McCullough’s Bridge (Kaiatan, Upper Eocene) have a single tubercle on each ear.
Pecten intuscostatus Yokoyama (Recent, Japan), the type of Polynememussium Habe 1951, also has numerous internal ribs (29 in specimen figured by Habe, 1951: 68) but the left valve is strongly costate, and no information on details of auricular crura is available. Grau (1959: 15) has synonymised both Ctenemusium and Polynememussium with Pervamussium Sacco 1897 (type, Pecten (Pleuronectes) duodecim-lamellatus Bronn, Upper Miocene, Italy), but since he does not discuss the auricular crura of any of these groups, the validity of his action cannot be judged.» PHILLIP ALAN MAXWELL, 1969
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