Entolium membranaceum (Nilsson, 1827)
NILSSON, S. 1827. Petrificata suecana formationis cretaceae, descripta et iconibus illustrata a S. Nilsson. Pars prior, vertebrata et mollusca sistens. Londini Gothrum, Ex Officina Berlingiana. viii, 37 p., pls. 1-10 [p. 23, pl. 9, fig. 16]
1825 Pecten fragilis Defrance, 1825
1827 Pecten membranaceus Nilsson, 1827
1842 Pecten rotundus F. von Hagenow, 1842
1842 Pecten latus F. von Hagenow, 1842
1935 Pecten wichmanni Feruglio, 1935
1982 Entolium transcaspicum Sobetski, 1982
1827 Pecten membranaceus Nilsson, 1827
1842 Pecten rotundus F. von Hagenow, 1842
1842 Pecten latus F. von Hagenow, 1842
1935 Pecten wichmanni Feruglio, 1935
1982 Entolium transcaspicum Sobetski, 1982
S. Nilsson, 1827, plate 9.
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«14. Pecten membranaceus.
P. testa ovali-rotunda, depressa, nitida, tenuissima, laevi; auriculis aequalibus, parvis, obtusangulis, basi latioribus, striâ margini parallela notatis. (Tab. nostra IX. f. 16.)
Longit. 25; lat. 21 mm.
— 18 — 15 -- Descript. Tenuissima, planiuscula s. convexo-plana, ovali-rotunda, nilida, oculo nudo laevissima, armato autem tenuissirne & concentrice striata. Auriculae satis parvae, quoad formam & magnitudinem aequales, obtusangulae, inferius latiores, stria longitudinali duplici vel triplici, margini parallela, notatae.
Locus: In calce arenosa & carbonifera ad Köpinge, Kåseberga &c. vulgaris.»
SVEN NILSSON, 1827
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«Description. Completely smooth shell, both inside and out; auricles are almost exactly equal, the byssal sinus is absent but well-developed auricular crurae are present.
Remarks. Distinguished from E. orbiculare by the complete lack of any macrosculpture and its dorsally projecting auricles. The well-developed auricular crurae indicate that the species belongs to Entolium. Its equal auricles and straight apical margins distinguish this species from the similarly smooth Syncyclonema nilssoni (Goldfuss) (Campanian). Occurrence. Turonian-Maastrichtian; unexpectedly rare in the English Chalk; only known from Kent, Lincolnshire and Northern Ireland.» 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. 134]
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Entolium membranaceum (Nilsson); R. J. Cleevely & N. J. Morris, 2002, Introduction to molluscs and bivalves, plate 21, figure 2.
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«Material. A single complete composite mould, from B. 151 at Dolne, one incomplete composite mould fram Drain Croix des Fiancés, one questionable fragment from Drain des Fiancés.
Dimensions. UPD 31.8 mm, W (30) mm.
Remarks. Although generally poorly preserved the complete specimen has well-preserved auricles which permit definite specifie identification.
Distribution. Cenomanian to latest Maastrichtian; also in the Maastrichtian stratotype.»
DHONDT, A. V. & J. W. M. JAGT. 1996. Late Creatceous macrofauna from the Hautes Fagnes area (NE Belgium). Annales de la Societé géologique de Belgique, 119 (1): 39-53, pls. 1, 2. [p. 44]
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Entolium membranaceum (Nilsson, 1827); A. V. Dhondt & J. W. M. Jagt, 1996, Late Creatceous macrofauna from the Hautes Fagnes area (NE Belgium), plate 1, figure 10.
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«Description of the valves:
They are so similar that one description suffices for both.
Outside and inside the valves are smooth; the outside is also shining. On some valves concentric growth lines can be seen; they lie very closely together and can sometimes, when many are close, form concentric ribbons on the disc and have a different colour-shade. The auricles are almost perfectly equal and the byssal sinus is absent. The hinge-margin is sometimes straight but, on other valves, the auricles are projected dorsally. The apical margins are straight. When visible the auricular and cardinal crurae and the umbonal edges are strongly developed. U. P. D. is generally superior to the width, but this is less pronounced on larger than on smaller valves. Discussion
In Cretaceous palaeontological literature two problems appear where E. membranaceum is concerned:
1. differentiation of E. membranaceum from Syncyclonema nilsoni (GOLDFUSS);
2. differentiation of E. membranaceum from E. orbiculare (SOWERBY) (for this problem see above, sub E. orbiculare). The differentiation of E. membranaceum and S. nilsoni has been the origin of great confusion in the systematics of Cretaceous pectinids.
The confusion started with S. NILSSON; he described E. membranaceum correctly, but on plate 9, two figures bear number 16. The lower one is E. membranaceum, the upper one is Pecten corneus NILSSON (non SOWERBY) (= Pecten cretaceus NYST, 1843, p. 299), and this last species could be related to S. nilsoni. In S. NILSSON another unfortunate picture is that for Pecten orbicularis (NILSSON, non SOWERBY) (= S. nilsoni (GOLDFUSS)): it depicts a left valve, which valve is very difficult to distinguish from E. membranaceum. The description of this species by NILSSON is also inaccurate: the auricles are unequal and NILSSON states that they are equal. The confusion caused by these inaccurate and faulty descriptions was ended by D. WOLANSKY: she states clearly that E. membranaceum is characterized by equal auricles and straight apical margins, whereas S. nilsoni has unequal auricles, a clearly developed byssal sinus on the right valve, and curved apical margins. Though both species belong to different families it took palaeontologists a century to differentiate them easily. However, this confusion is understandable because:
— both species have smooth valves without macrosculpture;
— the U. P. D./W ratios vary in a very similar way, even though the ranges of variation do not fully overlap; — the apical angles have similar values. The only good differentiating character is the shape of the auricles, but on most specimens they are broken off and then the shells cannot be determined with certainty. There can sometimes be a slight difference in the U. P. D./W ratio but this only for specimens which are situated at the end of the variation-range. The shell-thickness is slightly different too: E. membranaceum has a thinner and thus more breakable shell than S. nilsoni: this is a character that can only be used if one possesses a large series of specimens from the same locality. The difference in apical margins can be used with much success on larger valves of both species, because on large S. nilsoni-specimens the curving is more accentuated than on smaller specimens.
On well preserved fragments a differentiation is possible by using the microsculpture :
S. nilsoni bears a radial, diverging Camptonectes-like striation which is absent on E. membranaceum. This sculpture, unfortunately, is hardly visible on worn specimens. E. membranaceum is the only species of the genus known to reach the Maastrichtian in Europe. No Entolium is known to have lived after the Maastrichtian.» DHONDT, A. V. 1971. Systematic revision of Entolium, Propeamussium (Amusiidae) and Syncyclonema (Pectinidae, Bivalvia, Mollusca) of the European Boreal Cretaceous. Bulletin de l’Institut Royal des Sciences Naturelles de Belgique, 47 (32): 1-95, pls. 1-4. [p. 33, 34]
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Entolium membranaceum (Nilsson, 1827); A. V. Dhondt, 1971, Systematic revision of Entolium, Propeamussium (Amusiidae) and Syncyclonema (Pectinidae, Bivalvia, Mollusca) of the European Boreal Cretaceous, plate 1, figures 2a, 2b.
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