Pseudamussium princeps (J. de C. Sowerby, 1826)
SOWERBY, J. & SOWERBY J. DE C. 1812-1846. The Mineral Conchology of Great Britain. 7 vols. London [vol. 6, p. 80, pl. 542, fig. 2]
1826 Pecten princeps J. de C. Sowerby, 1826
J. de C. Sowerby, 1826, plate 542.
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«SPEC. CHAR. Orbicular, compressed; decorated with 90 rounded nearly equal radii surmounted with erect concave scales; ears large, unequal, squamose; valves equal.
NEARLY orbicular, but rather wider than long: the scales upon the ribs are large and distant, chiefly upon the left valve; but as there are rudiments upon the right also, that probably had them when the shell was entire. A handsome rare Crag fossil, found at Ramsholt. It ornaments the collection of the Rev. G. R. Leathes.» JAMES DE CARLE SOWERBY, 1826
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«Discussion — Juvenile specimens have been collected from the Petaloconchus level, a portion of the Kattendijk Formation until now sampled extensively in situ at Kallo only. This level is characterised by dense colonies of the bivalve Petaloconchus glomeratus (Linné, 1758) and has yielded highly interesting molluscan faunas, amongst which numerous representatives of the Emarginulidae and Polyplacophora are of particular note. The Petaloconchus colonies probably provided sufficiently hard substrates for those molluscs to settle. Juveniles of P. princeps have a well-developed ctenolium, which suggests that they were probably attached to the substrate by their byssus. The active ctenolium is lost in adults, and these were probably free living; adult shells are absent from the Petaloconchus level, but do occur scattered in the overlying beds, where juveniles are absent. The Petaloconchus level sand is well sorted, contains large amounts of glauconite, and in the uppermost part yields a number fossils probably preserved in life position, e.g. crustacean burrows of the Ophiomorpha type, which suggests deposition in calm waters. However, P. princeps is also known from the Late Miocene 'Heterocetus Sand' (Diest Formation), which is a coarse sand unit, with gravel and large blocks of sandstone, suggesting deposition in a turbulent setting.
In P. princeps, there usually is a clear difference between the ornament of juvenile, subadult and adult specimens; juveniles have far more numerous radial ribs than subadults, which also are subequal. In adults, the distinction between primary and secondary ribs disappears and the number of radials increases. In some specimens, the onset of subadult ornament is gradual, in others it is sudden. The development of spines on auricle ribs is highly variable, the ornament of the remainder of shell however showing little variation, mainly in strength of striae on juvenile portion of shell and in size of area covered in striae instead of secondary radial ornament. Pseudamussium princeps differs markedly from P. edegemense in lacking hollow radial ribs, and in the relative size of the posterior auricles (i.e. in excess of 50% of length of anterior auricles in the former, and barely a third in the latter species). Pseudamussium princeps also grows to a much larger size than P. edegemense. Wood (1851) considered Pecten sublaevigatus Nyst, 1843 to be a synonym of Pseudamussium princeps. However, Nyst's species is much smaller than P. princeps, has an utterly different adult ornament, and occurs in the Wemmel Member (Late Eocene) at Laeken, near Brussels.» MARQUET, R. & H. H. DIJKSTRA. 1999. Neogene species of Pseudamusium (Mollusca, Bivalvia, Pectinidae) from Belgium. Contributions to Tertiary and Quaternary Geology, 36 (1-4): 45-57. [p. 48]
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Pseudamussium princeps (J. de C. Sowerby, 1826; R. Marquet & H. H. Dijkstra, 1999, Neogene species of Pseudamusium from Belgium, plate 1, figures 1, 2 (above); plate 2, figure 3 (below).
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