Lyriochlamys fissicosta (Etheridge, 1881)
ETHERIDGE, R. 1881. Palaeontology (1. General Account of the Palaeontology of the different Divisions of the Chalk. 2. Description of new species). In W. H. Penning and A. J. Jukes-Browne (1881) The Geology of the Neighborhood of Cambridge, Appendix A: 132-148, pls. 1-3. Memoir of the Geological Survey, London, United Kingdom. [p. 141, pl. 2, fig. 1; pl. 3, fig. 1]
1881 Pecten fissicosta Etheridge, 1881
R. Etheridge, 1881, plates 2, 3.
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«Shell elongated, longer than wide, wings or auricles unequal, valves strongly costated, or ribbed, costae broad slightly arched or gently rounded, straight and about 15 in number, lines of growth strongly marked, especially so with age, appearing like two or three shells one within or placed upon the other. The costae, or ribs of the upper half (ligamental portion) or two-thirds of the shell have a line, or depression down the centre, giving them the appearance of being double; this character is not seen in the newer or last formed part of the valve; auricle of right valve deeply notched for bissal sinus, and waved in structure. Hinge line straight, umbo pointed, intercostal or depressed spaces half the width of the raised ribs or costae, and marked by slightly wavy lines crossing at an angle of about 50°, occasionally they are zig-zag between the 3 or 4 of the central ribs, these lines also cross some of the costae at the same angle, on the anterior and posterior sides of the shell, but appear absent on the middle costae, which are more flattened than the side ribs, and delicately lined longitudinally.
The auricles differ greatly, that of the left, or byssal side, being strongly waved, deeply notched, and having corrugated parallel lines of growth. This well-marked shell seems to have been overlooked, probablyowing to its rarity, or the want of good examples from other areas; the Cambridge shells from the Totternhoe Stone of Cherry Hinton and Burwell are, however, admirably preserved and enable good characters to be given. Through the liberality of Professor HUGHES we are enabled to figure both valves of this species (with many other fossils) from the fine collection in the Woodwardian Museum. We possess the left valve of the same shell from the Lower or Grey Chalk of Dover, but badly preserved; it is doubtless the Cherry Hinton and Burwell form. The left valve of this specimen differs in many respects from the right, especially so as regards the costae which are much flatter, and possesses the impressed line or depression along the middle of the rib to the extremity or ventral margin of the shell, as well as in the condition and structure of the wings. Hinge line straight, wings unequal, anterior largest and nearly double the size of the posterior, marked with well-defined inclined costae, the ribs are flatter and less rounded and elevated than in the opposite or right valve; again, the impressed line down the centre of the costae is constant to the completeness of the shell in this valve, this and the flatness of the ribs readily distinguishes one valve from the other. Localities, Burwell, Cherry Hinton, and Orwell (Tottemhoe Stone). Collection, Woodwardian Museum, Cambridge.» ROBERT ETHERIDGE, 1881
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«Description. Medium-sized shell covered with 11-23 broad ribs, which divide occasionally after a strongly developed growth ridge.
Remarks. Distinguished from other Mimachlamys species by its smooth ribs; M. henrici has far more ribs and virtually no intercostal intervals. Occurrence. Lower Cenomanian; southern England and East Anglia; only known from very fine chalky sediments.» 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. 123]
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Mimachlamys fissicostata [sic] (R. Etheridge); R. J. Cleevely & N. J. Morris, 2002, Introduction to molluscs and bivalves, plate 20, figure 16.
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«Where Lyriochlamys differs from Cenozoic and Holocene Chlamys, however, is in the extent of the inner crossed-lamellar aragonitic inner shell layer. In Lyriochlamys fissicosta and the great majority of Mesozoic "Chlamys," this layer extends nearly to the margins of disk and auricles, whereas in Cenozoic and Holocene true Chlamys it is nearly always confined to the area within the pallial line or is absent altogether except very early in ontogeny. This important difference caused Waller (1991) to apply Sobetski's name Lyriochlamys to the whole array of Mesozoic chlamydids bearing costae or plicae that interlock at the shell margin, leading to rib increase by branching on the right valve and by intercalation on the left. (Note that branching is the negative image of intercalation. So long as ribs interlock at the growing margin when they first form, a branching on one valve will be represented by an intercalation on the opposite valve.) Lyriochlamys also has antimarginal microsculpture, which is particularly evident on the disk flanks in Rhaetian, Jurassic, and some Cretaceous taxa. Data thus far accumulated by one of us (Waller) suggest that Lyriochlamys first appears in the Late Triassic (Rhaetian), as exemplified by species such as Pecten acuteauritus Schafhäutl, 1851, P. bavaricus Winkler, 1861, and P. praepollux Hahn, 1910, and extends through the Jurassic and Cretaceous, becoming extinct at the end of the Cretaceous.»
WALLER, T. R. & L. MARINCOVICH JR. 1992. New species of Camptochlamys and Chlamys (Mollusca: Bivalvia: Pectinidae) from nearthe Cretaceous/Tertiary boundary at Ocean Point, North Slope, Alaska. Journal of Paleontology, 66 (2): 215-227, figs. 1-5. [p. 223]
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Chlamys (Lyriochlamys) fissicosta (Etheridge, 1881); V. A. Sobetski, 1977, Late Cretaceous bivalve molluscs of the platform seas of southwestern USSR, plate 3, figures 9-11.
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«Pecten fissicosta ETHERIDGE in PENNING and JUKES-BROWNE, 1881, belongs to the Chlamydinae because of its general shape, the presence of well developed radial ribs, the large auricles and the Camptonectes like-striation near the side margins. The absence of the typical Chlamys-microsculpture make it probable that it belongs to the genus Mimachlamys. The correct name is Mimachlamys fissicosta (ETHERIDGE in PENNING and JUKES-BROWNE, 1881).»
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. 123]
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