Pectinella? miyakoensis (Nagao, 1934)
NAGAO, T. 1934. Cretaceous Mollusca from the Miyako district, Honshû, Japan. Journal of the Faculty of Science, Hokkaido University [Series 4, Geology and Mineralogy], 2 (3): 177-277, pls. 23-39. [p. 209, pl. 31, figs. 11, 12]
1934 Pecten (Camptonectes) miyakoensis Nagao, 1934
T. Nagao, 1934, plate 31
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«Shell small, compressed, suborbicular in outline, higher than long. Right valve regularly convex with the dorsal half triangular and the ventral half semi-eircular or elliptical. Antero-dorsal margin slightly concave and the postero-dorsal nearly straight; anterior and posterior margins convex. Umbo small and pointed, with the aptical angle about 80º. Hinge line rather short. Ears small, inequal and distinctly separated from the valve by sulci; of them the posterior one very narrow, smaller than the anterior, with the outer angle obtuse, the anterior one broad, provided with a convex outer margin forming a right angle with the dorsal margin and distinetly notched below.
In the Ieft valve, the anterior ear broad, with its outer margin straight, forming an acute angle with the dorsal margin; posterior ear narrow with its outer margin very obtuse. Surfaee almost smooth except for fine, faintly raised concentric lamellae and very fine, almost invisible radial striae; interspaces between the striae nearly equal to or slightly broader than the striae themselves. Test very thin.
Localities and geological horizons: The Hiraiga Sandstone; Haipe, Hiraiga and Hideshima. The Akito Sandstone; Akito. The Orbitolina Sandstone; Raga.
A number of specimens were examined. Most of the specimens are very small, but the largest example measures 20 mm. in height.
The present form is not unlike P. (C.) placitus WHITE (1) from the Upper Cretaceous of Brazil but has a slightly smaller apical angle and a narrower and higher anterior Ieft ear than the latter. By its smaller apical angle it is also distinguished from P. Iaevis HILLS (2) from the Cenomanian of Germany and P. simplicius CONRAD (3) from the Upper Cretaceous of North America.» (1) C. A. WHITE: Contribution to the Pa!eontology of Brazil. Archivos do Mus. Nacional do Rio de Janeiro, Vol. VII, 1888, p. 40, Pl. IV, figs. 8, 9.
(2) H. B.GEINITZ: Das Elbth. Plaeontagraphica, Vol.XX, pt.1, 1872, p.192, Pl. 43, figs. 12, 13. (3) R. P. WHITFIELD: U. S. Geol. Surv. Monogr., Vol. IX, 1885, p. 51, PI. VII, figs. 11, 12. S. WELLER: A Report Cret. Paleont. New Jersey. Geol. Surv, N. J., Vol. IV, 1907, p. 480, Pl. LI, fig. 6. J. S. GARDNER: Maryland Geol. Surv., Upper Cret., p. 596, Pl. XXXIV, figs. 8, 9. L. W. STEPHENSON: The Cret. Form. North Carolina, Vol. V, pt. 1. Invert. Foss. Upper Cret. Form., 1923, p. 199, Pl. 55, figs. 6-11. TAKUMI NAGAO, 1934
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«A similar and possibly related species is Pecten miyakoensis Nagao from the Aptian - ?Albian Hiraiga Formation of southwest Japan, assigned by Hayami (1965a: 318-320, pI. 45, figs 2-11, pI. 46, fig. 5) to the genus Pectinella Verrill. Speden (1967: 24, 25) compared P. miyakoensis with the type species of both Syncylonema and Pectinella, noting differences from both in the crural and auricular features and sculpture and from Syncyclonema also in the cardinal area. The relationship of P. miyakoensis to the present and other small, smoothshelled Cretaceous pectens requires further study, as noted by Speden»
KENDRICK, G. W. & A. V. VARTAK. 2007. Middle Cretaceous (Cenomanian) bivalves from the Karai Formation, Uttattur Group, of the Cauvery Basin, south India. Records of the Australian Museum, Supplement 72: 1-101, figs. 1-20. [p. 49]
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«Hayami (1965, p. 318) reclassified a Japanese Lower Cretaceous species, Pecten miyakoensis Nagao, under Pectinella. Study of casts of P. miyakoensis (NZGS-WM 8473) shows that externally the species resembles both S. halli and P. sigsbeei. But the strongly projecting dorsal extremities of the auricles of the right valve of P. miyakoensis are not matched by either species. P. miyakoensis more closely resembles S. halli in the strength and form of the radial and concentric ornament and lack of a pseudopunctate effect. As recognized by Hayami the ornament of P. miyakoensis tends to be discrepant with stronger concentrics on the right valve and stronger radials on the left valve. Also the ornament of P. miyakoensis is markedly finer, and the radial ornament more restricted to the dorsal flanks of the disc.
P. miyakoensis, like S. halli, lacks auricular crura. Contrary to Hayami's (1965) description, a right valve of P. miyakoensis has cardinal crura. (These are faintly visible on pl. 45, fig. 6, of Hayami, 1965, Gk H6622). On either side of the sunken resilifer, flat longitudinally striaed surfaces pass laterally into narrow crura which extend halfway to the extremities of the auricles. Small rounded protuberances extend ventrally from the triangular surfaces on either side of the resilifer. The form and arrangement of the crura and the strongly projecting auricles of the right valve distinguish P. miyakoensis from both Syncyclonema and Pectinella, and the lack of tooth-like processes on either side of the resilifer distinguishes it from Syncyclonema. Future studies on small, relatively smooth pectinids may warrant placement of P. miyakoensis in a separate supra-specific taxon. Until adequate studies are completed, it is best to treat it as a Pecten (sensu lato). Except for the large byssal auricle and notch P. miyakoensis superficially resembles species of Entolium by its possession of strongly projecting dorsal margins and short cardinal crura.» SPEDEN, I. G. 1967. Revision of Syncyclonema (Upper Cretaceous) and comparison with other small pectinid bivalves and Entolium. Postilla, 110: 1-36, pls. 1-7. [p. 24, 25]
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«Observations and comparisons.— More than 10 specimens exhibiting external features are available for study. In addition, numerous individuals are adhered to several slabs of the present collection. The protruded part of byssal auricle is very thin and seems liable to be damaged before fossilization. The ratio of length/height is somewhat variable among NAGAO's and the present specimens, but it is generally larger in the right valve than in the left. Although both valves in juxtaposition are not available at present, the anterior and posterior perhipheral parts of the right valve were probably not covered by the left valve. It is presumably also from the anterior and posterior peripheral areas of the left valve, which are slightly but distinctly concave. The faint radial capillae are seemingly more distinct in the left valve than in the right, but the converse applies to the concentric lines. The apical angle of two valves is almost constant.
NAGAO (1934) assigned the present species to Camptonectes MEEK, 1864 (type-species: Pecten auritus SCHLOTHEIM, 1820), a well-defined Mesozoic pectinid genus of the Chlamydinae. It is, however, obviously different from Camptonectes and other genera of the Chlamydinae in view of the absence of a fasciole area at the base of the byssal auricle, the absence of Camptonectes striations, its dorsal margin extending well above hinge axis, its translucent shell and the presence of weakly concave areas on the anterior and posterior parts of the left valve. In the general outline and smooth test this species apparently resembles some species of Entolium MEEK, 1865, especially the group of Entolium hehli in DECHASEAUX's (1936a) classification, which includes several Lower Juraassic species having very unequal auricles. The byssal auricle of this species is protruded upwards and is fairly similar to that of Entolium. In Middle Jurassic and later species of Entolium, however, auricles are generally subequal or only slightly unequal. Crural ridges of Entolium-type are almost invisible in the present species. In the Cretaceous there are several small pectinids which are comparable with the present species in many characters, but no suitable generic name has been given to them. Pecten conradi WHITFIELD and Pecten simplicius CONRAD from the Upper Cretaceous of U.S.A. may be congeneric with the present species, but the auricular protrusion in such species is less striking and the apical angle is much larger than in the present species. Pecten nilssoni GOLDFUSS, 1836 (WOODS, 1903) from the Upper Cretaceous of western Europe resembles the present species in the shape of the byssal auricle and some other characters. However, the apical angle of the Japanese species is much smaller than in GOLDFUSS' figure and the antero-dorsal margin is not so deeply concave as in the specimens figured by WOODS. This differs from Pecten (Syncyclonema) inconspicuus CRAGIN, 1895, from the Albian of the Comanche series of Texas (ADKINS, 1920; STANTON, 1947) by the more unequal auricles, the more profound byssal notch and the longer antero- and postero-dorsal margins. Occurrence.— Lower and (?) upper Miyakoan (Aptian to ? Albian). Calcareous sandstone of the Hiraiga formation at loco Hn. 0016, 0017, 0018, southern coast of Hiraiga, Tanohata village, Shimohei County, Iwate Prefecture. Besides, NAGAO (1934) reported this species from the "Orbitolina sandstone" at Raga and the Aketo formation at Aketo, the same village.»
HAYAMI, I. 1965. Lower Cretaceous marine pelecypods of Japan, part I. Memoirs of the Faculty of Sience, Kyushu University, [Serie D - Geology], 15 (2): 221-349, pls. 27-52. [p. 319, 320]
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Pectinella miyakoensis (Nagao); I. Hayami, 1965, Lower Cretaceous marine pelecypods of Japan, part I, plate 45, figures 2-11; plate 52, figure 5.
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