Entolium orbiculare (J. Sowerby, 1817)
SOWERBY, J. & J. DE C. SOWERBY. 1812-1846. The Mineral Conchology of Great Britain; or coloured figures and descriptions of those remains of testaceous animals or shells. 7 vols., pls. 1-648 (and 2 bis). London [vol. 2, p. 193, pl. 186]
1817 Pecten orbicularis J. Sowerby, 1817
1822 Pecten laminosus Mantell, 1822
1837 Pecten concentricus Koch & Dunker, 1837
1843 Pecten nummularis Fischer de Waldheim, 1843
1850 Pecten neptuni Orbigny, 1850
1850 Pecten calypso Orbigny, 1850
1850 Pecten darius Orbigny, 1850
1869 Pecten sublaminosus Favre, 1869
1871 Amusium sulcatellum Stoliczka, 1871
1874 Pecten polylasmites Gemmellaro & Di Blasi, 1874
1874 Pecten (Amusium) balticus Dames, 1874
1883 Pecten orbicularis var. magnus Keeping, 1883
1893 Pecten (Entolium) theodosianus Retowski, 1893
1893 Pecten (Entolium) erraticus Fiebelkorn, 1893
1896 Pecten (Syncyclonema) losseriensis Vogel, 1896
1896 Pecten orbicularis var. lohmanni Wollemann, 1896
1900 Pecten germanicus Wolleman, 1900
1908 Pecten (Entolium) gothicus Krause, 1908
1923 Syncyclonema masticonenese Lissajous, 1923
1960 Entolium rossicum Glasunova, 1960
1822 Pecten laminosus Mantell, 1822
1837 Pecten concentricus Koch & Dunker, 1837
1843 Pecten nummularis Fischer de Waldheim, 1843
1850 Pecten neptuni Orbigny, 1850
1850 Pecten calypso Orbigny, 1850
1850 Pecten darius Orbigny, 1850
1869 Pecten sublaminosus Favre, 1869
1871 Amusium sulcatellum Stoliczka, 1871
1874 Pecten polylasmites Gemmellaro & Di Blasi, 1874
1874 Pecten (Amusium) balticus Dames, 1874
1883 Pecten orbicularis var. magnus Keeping, 1883
1893 Pecten (Entolium) theodosianus Retowski, 1893
1893 Pecten (Entolium) erraticus Fiebelkorn, 1893
1896 Pecten (Syncyclonema) losseriensis Vogel, 1896
1896 Pecten orbicularis var. lohmanni Wollemann, 1896
1900 Pecten germanicus Wolleman, 1900
1908 Pecten (Entolium) gothicus Krause, 1908
1923 Syncyclonema masticonenese Lissajous, 1923
1960 Entolium rossicum Glasunova, 1960
J. Sowerby & J. de C. Sowerby, 1812-1846, plate 186.
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«SPEC. CHAR. Orbicular, much depressed, concentrically striated; striae elevated, sharp; one valve smooth; ears nearly equal, broadest at the base.
A THIN tender shell; the striae are many, a line distant from each other; the length and breadth are equal; the ears rather large.
One of the tender products of the green sand of the Devizes canal, preserved by Mrs. Gent. It appears to be unfrequent, as I have seen but one individual.» JAMES SOWERBY, 1817
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«Description. Very brittle, thin, almost flat shell; suborbicular in shape, and subequivalve, with relatively small, well-developed, subequal auricles; apical margins usually straight, but can be curved. Right valve is smooth and more convex than left valve, and has an obsolete byssal sinus. Left valve bears a variable number of concentric ridges separated by shallow furrows; its auricles are equal, triangular and obtusely angled, though still rounded, and project dorsally more than those of the right valve.
Remarks. Distinguished from the smooth E. membranaceum by the concentric ridges on its left valve, and in having more dorsally elevated auricles and a slightly narrower apical angle. Occurrence. Berriasian-Turonian; found throughout southern England and as far north as Lincolnshire and Norfolk; one of the commonest and most widely distributed of pectinids.» 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 orbiculare (J. Sowerby); R. J. Cleevely & N. J. Morris, 2002, Introduction to molluscs and bivalves, plate 21, figure 3; plate 26, figure 16.
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«Material: Two incomplete specimens from Orosei, lowermost Hauterivian (n. 286, 365).
Dimensions (mm): n. 365: H = 8.5; L = 8.2; n, 286: H = 12; L = 13. The specimens are relatively small for the species.
Discussion: As can be seen in the synonymy given above, E. orbiculare has been restudied recently by several authors. The Sardinian specimens bring no further new information.
Distribution: Recent authors (JOHNSON, 1984 and KELLY, 1984) have extended the stratigraphical range of E. orbiculare downwards into the Oxfordian. The youngest E. orbiculare specimens were recorded by DHONDT (1971) from the Actinocamax plenus Zone, Late Cenomanian. Thus E. orbiculare could be an example of an unusually long ranging species. On the other hand this long range could only demonstrate that E. orbiculare has so few characteristics that it is difficult to differentiate it from other species in the same genus. Oxfordian-latest Cenomanian: north Temperate Realm (including Northern Canada, Alaska, Northern Siberia, Greenland), northern margin of the Tethys, SE Africa. JOHNSON (1984) indicated a llboreal" dis r'bution for the Late Jurassic (Text-fig. 12).»
DHONDT, A. V. & I. DIENI. 1988. Early Cretaceous bivalves of eastern Sardinia. Memorie di Scienze Geologiche, 40: 1-97, pls. 1-13, text-figs. 1-25. [p. 27]
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Entolium orbiculare (Sowerby); A. V. Dhondt & I. Dieni, 1988, Early Cretaceous bivalves of eastern Sardinia, plate 6, figure 9.
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«E. orbiculare (SOWERBY) differs from E. membranaceum (NILSSON) in having macroscopical concentric ridges on its left valves and more dorsally elevated auricles. The apical angle is sligtly narrower in E. orbiculare (84°-118°; E. membranaceum 94°-122°). The U.P.D./W. index variation is practically the same: 0.936-1.211 for E. membranaceum and 0.958-1.196 for E. orbiculare. The smoothness of both valves in E. membranaceum is the only easily usable distinctive characteristic.
E. orbiculare and E. membranaceum are undoubtedly closely related. Chronologically they follow each other : when E. orbiculare disappears in the late Cenomanian it is replaced by E. membranaceum. It could be that the first evolved into the second: indeed the specimens from Ventnor (Isle of Wight) by having a partly smooth left valve seem to lose the E. orbiculare-characteristics. The same happens in Entolium noetlingi from the Middle Cenomanian Dniester Deposits as described by SOBETZKI, 1960. This species, according to the author, is supposed to be very similar to E. membranaceum. He keeps both species separated because they have different measurements; however. those given by him for E. noetlingi fall within the variability of both E. orbiculare and E. membranaceum. Also the statement that E. noetlingi has smooth valves is denied by the figure (pI. 8, fig. 2) which shows an only partly smooth valve. The ridges on it are few but they are present. It is thus very likely that E. noetlingi is a late variety of E. orbiculare. From the Carpathian Senonian of Premysl (Poland) T. WISNIOWSKI described in 1919 Pecten (Syncyclonema) woodrowwilsoni. This species is a typical Entolium: it has the subequal auricles and the auricular crurae. and the smooth shell-surface with ridges. It is different from E. orbiculare because the auricles are much more dorsally projected. almost as in Pernopecten WINCHELL, 1865 (*). From E. membranaceum it differs by the auricle shape and by the ridges on the shell surface.» (*) See E. PHILIPPI, 1900, p. 79: « ...man hat wohl das gelegentliche Auftreten dieser spitzen Ohren bei mesozoischen Arten als Riickschlagerscheinung aufzufassen ».
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. 21, 22]
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Entolium orbiculare (Sowerby, 1817); A. V. Dhondt, 1971, Systematic revision of Entolium, Propeamussium (Amusiidae) and Syncyclonema (Pectinidae, Bivalvia, Mollusca) of the European Boreal Cretaceous, plate 1, figures 1a, 1b.
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