"Chlamys" elongata (Lamarck, 1819)
LAMARCK, J. B. 1819. Histoire naturelle des animaux sans vertèbres, présentant les caractères généraux et particuliers de ces animaux, leur distribution, leurs classes, leurs familles, leurs genres, et la citation des principales espèces qui s'y rapportent. Tome sixième, Premier partie, 343 p. Chez l'Auteur, au Jardin du Roi. De l'imprimerie de A. Belin. Paris, 1819. [p. 181]
1819 Pecten elongatus Lamarck, 1819
1822 Pecten obliquus J. Sowerby, 1822
1833 Pecten cretosus Goldfuss, 1833
1841 Pecten crispus Roemer, 1841
1841 Pecten comans Roemer, 1841
1847 Pecten raulinianus Orbigny, 1847
1822 Pecten obliquus J. Sowerby, 1822
1833 Pecten cretosus Goldfuss, 1833
1841 Pecten crispus Roemer, 1841
1841 Pecten comans Roemer, 1841
1847 Pecten raulinianus Orbigny, 1847
Pecten elongatus Lamarck; A. d'Orbigny, 1844-1847, Paléontologie francaise, Description des Mollusques Rayonnés fossiles, Terrains crétacés, III Lamellibranches, plate 436, figures 1-4.
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«10. Peigne allongé. Pecten elongatus.
P. testâ longitudinali, ovato-oblongâ; radiis 26 ad 3o tenuibus, inaequalibus subdenticulatis.
Habite... Fossile des environs du Mans, près de Coulaines. Cabinet de M. Menard. Il acquiert au moins 75 millimètres de longueur.» JEAN BAPTISTE LAMARCK, 1819
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«Description. Ornamented by tripartite, often divided ribs that are crossed by concentric striae; these ribs are covered with small scales. The valves are slightly elongated, prosocline and flattened; the auricles are large and very unequal.
Remarks. Differs from other species in having a rather irregular rib distribution and a very high number of small scales on all ribs. Ribs vary with age and their position on the valve. Occurrence. Albian-Cenomanian; widespread throughout southern England.» CLEEVELY, R. J. & N. J. MORRIS. 2002. Introduction to molluscs and bivalves. In A. B. Smith & D. J. Batten (Eds.): Fossils of the Chalk, 99-160. The Paleontological Association. London. [p. 119]
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Chlamys? elongata (Lamarck); R. J. Cleevely & N. J. Morris, 2002, Introduction to molluscs and bivalves, plate 20, figure 9.
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«Variability:
As stated in the description, the development of the ribs varies very greatly, according to the size of the valve; amongst the ribs of one valve there can be a difference in the stage of development, and then some ribs are still tripartite when reaching the pallial margin, whereas others are divided into three ribs and others again have formed two ribs: one side riblet has become an independent rib but the other one is still attached to the original main rib. Exceptionally, ribs do not divide or subdivide and remain undivided even at the pallial margin. The irregularity in rib distribution is more pronounced in older strata than in the Middle Cretaceous ones. There seems to be a trend towards greater regularity which continues in the Upper Cretaceous species C. faujasi (DEFRANCE) (see also H. WOODS, p. 173). Synonymy:
Little can be added to what WOODS wrote, except for Pecten elongatus in D'ORBIGNY: the figure (pI. 436, fig. 1) is unfortunate and gives the impression of belonging to another species. This is probably due to a faulty reconstruction of a poorly preserved specimen which is, however, undoubtedly a Pecten elongatus. Differentiation:
As stated by WOODS (1902, p. 173): «P. faujasi DEFRANCE, from the B. mucronata Chalk of Maestricht, approaches very closely those forms of P. elongatus which have the triple arrangement of the ribs well developed, but appears to differ in having fewer ribs». That both species are closely related seems to be beyond doubt; from the Saxonian Lower Turonian I saw several specimens in the Museum in Dresden which could almost be considered as being the transition between both species: they have a lower ribnumber and only subdivided ribs as in C. faujasi, but they do not have the largish scales typical for C. faujasi in Maastrichtian strata. C? elongata differs from other Chlamys-species in having a rather irregular rib distribution and a very high number of small scales on all ribs. Generic attribution:
A thimble-microsculpture has never been seen on Pecten elongatus LAMARCK, but this is probably due to the generally poor state of preservation of Lower and Middle Cretaceous specimens. Considering the close relation which must have existed between P. elongatus and C. faujasi (DEFRANCE) it is almost certain that the former also belongs to the genus Chlamys. However, as long as it cannot be proved, it seems preferable to give Pecten elongatus LAMARCK the name Chlamys? elongata (LAMARCK).» DHONDT, A. V. 1973. Systematic revision of the Chlamydinae (Pectinidae, Bivalvia, Mollusca) of the European Cretaceous. Part 3: Chlamys and Mimachlamys. Bulletin de l’Institut Royal des Sciences Naturelles de Belgique, 49 (1): 1-134, pls. 1-9. [p. 24, 25]
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Chlamys? elongata (J.-B. Lamarck, 1819); A. V. Dhondt, 1973, Systematic revision of the Chlamydinae (Pectinidae, Bivalvia, Mollusca) of the European Cretaceous. Part 3: Chlamys and Mimachlamys, plate 2, figures 1a-1e.
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