Dhondtichlamys britannicus (H. Woods, 1902)
WOODS, H. 1899-1903. A monograph of the Cretaceus Lamellibranchia of England. Vol. I: 1-232, pls. 1-42. Paleontological Society Monograph. The Paleontographical Society. [p. 167, pl. 31, figs. 1a, 1b, 2a, 2b]
1902 Pecten (Chlamys) britannicus H. Woods, 1902
H. Woods, 1899-1903, plate 31.
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«Description.—Shell thick, ovate, flattened, with even margins, slightly inequilateral, antero-dorsal margin a little longer than the postero-dorsal, apical angle about 90º.
Left valve with sixteen or more (sometimes probably thirty) strong, rounded, radial ribs, separated by deep grooves, which are frequently as broad as or broader than the ribs. The ribs merge into the smooth margin of the shell; they do not bifurcate, and only rarely is a new rib introduced between two others. Both ribs and grooves are marked by concentric, linear ridges, which are more distinct in the grooves than on the ribs, and are placed close together at regular intervals; the ridges imbricate upwards. Anterior ear moderately large, with the outer angle nearly rectangular, and one or two radial ribs. Posterior ear not seen. Right valve not seen. Measurements: Length . . . . . . . 26.0 mm.
Height . . . . . . . 28.5 " Affinities.— This species resembles P. Espaillaci, d'Orbigny,¹ from the Senonian of Dordogne and Charente-Inférieure, but it differs from the French form in possessing fewer ribs, with a stouter shell and thick margin (instead of a sharp and corrugated edge); also in the concentric ridges imbricating upwards, instead of downwards. A specimen of P. Espaillaci from the Dordonian has been sent me by M. A. de Grossouvre, and its concentric ridges are much better developed on the ribs, and in places are more scale-like, than in our species.
P. decemcostatus, Goldfuss,² possesses fewer ribs and is apparently without the fine concentric ridges. P. fissicosta, Etheridge (p. 163), presents some resemblance to this species, but is easily separated by the divided and usually fewer ribs; also by the corrugated margin and the much less distinct concentric ridges, and by the occurrence of fine radial striae A specimen from Lewes, figured by Mantell,³ may perhaps be an example of this species. Remarks.— Only three specimens have been seen, all coming from nearly the same horizon. The number of ribs on those from Haling and Gravesend is fewer than on the specimen from Stratford, but in other characters they agree. Types.— In Dr. Blackmore's collection, and in Mr. Gr. E. Dibley's collection. Distribution.-- M. cor-anguinum zone of Haling pit, South Croydon, of Gravesend, and of Stratford, near Salisbury.» ¹ ' Pal. Franç. Terr. Crét.,' vol. iii (1847), p. 614. pl. ccccxxxix, figs. 1-4 ; d'Orbigny, ' Prodr. de Pal.,' vol ii (1850), p. 251 ; F. J. Pictet and Campiche, " Foss. Terr. Crét. Ste. Croix " ( ' Mater Pal. Suisse," ser. 5, 1870), p. 215.
² ' Petref. Germ.,' vol. ii (1833), p. 53, pl. xcii, fig. 2; Geinitz, ' Das Elbthageb. Sachsen.' pt. ii (1872), p. 35, pl. X, figs. 8, 9. ³ ' Foss. S. Downs ' (1822), p. 203, pl. xxv, fig. 6 HENRY WOODS, 1902
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«Description. Orbicular species ornamented by broad, simple, rounded, rod-like ribs separated by intercostal areas that are the same width or wider than the ribs. Very fine concentric lines present throughout shell, sometimes become stronger and form bars. Ribs infrequently developed intercostally. Concentric ridges present, but more characteristically, the ribs merge to form a smooth, thickened rim to the shell. Anterior auricle of left valve ornamented by radial ribs, and its growth lines are perpendicular to the hinge-line. Anterior auricle of right valve has a small byssal sinus and its growth lines follow this. Both valves have identical ornament, but the primary ribs of the right valve tend to divide near the margins.
Remarks. Distinguished by its broad, simple, rounded ribs that commonly do not divide. Other comparable species appear to be restricted to the Upper Cretaceous of Europe. In M. septemplicatus (Nilsson) (Campanian-Maastrichtian), which also shows a thick marginal rim, there are only a few very wide ribs. Occurrence. Santonian, M. coranguinum Zone; Surrey, Kent, Hampshire and Wiltshire; scarce.» 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. 126]
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Microchlamys brittanicus (Woods); R. J. Cleevely & N. J. Morris, 2002, Introduction to molluscs and bivalves, plate 20, figure 15.
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