Neithea matsumotoi Hayami, 1965
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. 297, pl. 41, figs. 1-7, text fig. 4]
1965 Neithea (Neithea) matsumotoi Hayami, 1965
I. Hayami, 1965, plate 41.
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«1952. Neithea atava (RÖMER), MATSUMOTO and KANMERA, Guide books of the geological excursions. The lower valley of the Kuma, p. 49 (non p. 45), listed.
1954. Neithea atava (RÖMER), MATSUMOTO, Cretaceous System, Japan. Islands, p. 114 (non p. 113), listed. Material.— The holotype is internal and external moulds of a right valve (GK. R 6126) collected from the Yatsushiro formation at loc. Km. 3037, west of Mt. Jôgûsan, Miyaji, Yatsushiro City, Kumamoto Prefecture (KANMERA coll.). Paratypes (GK. R 6125, GK. R 6127-GK. R 6131) from the type area (KANMERA, FUJII and AKATSU coIl.).
Description.— Shell small or medium-sized for genus, pIano-convex, subequilateral, polygonally ovate. Right valve higher than long to a varying extent, well inflated; umbo incurved, protruding prominently above hinge-line, overhanging that of the other valve; anterior and posterior margins slightly concave when viewed perpendicularly to valve margin; ventral margin frilled, corresponding to radial ribs of two orders; apical angle about 75 degrees; primary ribs six in number, roof-shaped in transverse section, very prominent, giving rise to digitations of ventral margin, slightly curved outwards; ribs distinctly plicated; interspaces of primary ribs flattened at bottoms, bearing three-five weak secondary riblets, which are of rather unequal prominence, nearly as wide as their interspaces and sometimes bifurcated at various growth-stages; both lateral areas also striated by secondaries of similar strength; faint concentric growth-lines cross radials; auricles subequal, comparatively small. Left valve nearly flat or even slightly concave, suboctagonal, nearly as high as long; hinge-line of moderate length; ventral margin with angular projections corresponding to intervals between primaries and broadly sinuated at primaries; apical angle exclusive of auricles approximately 90 degrees; primary ribs, which correspond to the intervals between primaries of the other valve, five in number, straight, strongly plicated, much broader than interspaces, marked with three-five secondary riblets; primary grooves bearing also a few radial threads, which are, however, much weaker, densely spaced and not impressed on internal surface; concentrics weak; auricles small, subequal, demarcated clearly from main body by a pair of angular sulci; anterior auricle rectangularly truncated; byssal notch shallow; posterior auricle more obtuse; strong internal ridges extend along antero- and postero-dorsal margins, defining a pair of faintly striated marginal areas; musculature unknown.
Observations and comparisons.— This species is represented by many specimens, seven of which are nearly complete. The test is preserved only in one paratype (GK. H 6127), and all other specimens are composed of internal and / or external moulds. Specimens with both valves in juxtaposition cannot be found, but the two valves certainly belongs to one species in view of their similar frequency and corresponding ornamentation. The variation of outline seems relatively slight, according to my measurements of the ratio of length / height. The secondary riblets, however, are fairly variable in number and prominence.
The present specimens were formerly assigned to Neithea atava (RÖMER, 1839), but they are essentially different from every specimens from Europe hitherto referred to N. atava. Compared with the Lower Greensand specimens of N. atava as described by WOODS (1902), the right valve has narrower ribs, its secondary ornament consists of fine ribs instead of striae of the Neithella type, and the left valve is more weakly plicated and has a much smaller and less projecting anterior wing. The present species is certainly a member of the group of Neithea quinquecostata of the present classification. In the outline of the right valve this species is somewhat similar to Neithea morrisi (PICTET and RENEVIER, 1858) from the Aptian of Western Europe (WOODS, 1902), and allied species to it from Lebanon (VOKES, 1947), but the convexity of the right valve is stronger and the two lateral areas, beyond the outermost primary ribs, are distinctly striated in the present species. In convexity and number the secondary riblets are comparable to those of Neithea quinquecostata (SOWERBY, 1819) from the Aptian-Turonian of western Europe, Africa and India (WOODS, 1902; STOLICZKA, 1871; etc.). However, in the right valve the secondary riblets are more delicate than in that species and the left valve is more distinctly plicated than that of N. quinquecostata. In the left valve of N. quinquecostata radial ornament consists of many ribs which are almost equal in strength, while in the present species it is more complicated, as described above. Such differences also exist between the present species and Neithea (Neithea) nipponica sp. nov. from the Miyako group. Pecten (Neithea) sexcostatus WOODWARD (WOODS, 1902) from the Chalk of England differs from the present species in its weaker outermost primary ribs, more salient umbo and larger size. Neithea attockensis COX, 1935, from the Albian of Attock district of India, is related to the present species, in which, however, the lateral areas are more clearly ornamented and the umbonal region of the right valve is more prominent, and the secondary ribs are generally weaker and more numerous. This species is named after Prof. Tatsuro MATSUMOTO of the Kyushu University, who has given me invaluable advise and encouragement throughout this study. Occurrence.— Upper Miyakoan (Albian). Dark grey sandy shale of the Yatsushiro formation at loc. Km. 3037, west of Mt. Jôgûsan, Miyaji, Yatsushiro City, Kumamoto Prefecture. Grey sandstone of the same formation at the southwest of Bishô, Tôyô village, Yatsushiro County, Kumamoto Prefecture. This species is known also from the Haidateyama formation at loc. U.1005, between Tamarimizu and Ochiai, Nozu town, Ono County, Oita Prefecture (FUJII colI.)»
ITARU HAYAMI, 1965
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«Material.— An incomplete internal mould of right valve (GH.K6807) is concerned with the description below.
Description.— Right valve small, subequilateral, higher than long, moderately inflated (GH.K6807, right in mould, 13.5 mm. long, 15.5 mm. high, 5.0 mm thick); umbo submesial, prominent; antero- and postero-dorsal margins of the flank a little concave; ventral margin frilled in accordance to the radial plications; apical angle approximately 75 degrees; primary ribs six in number, persistent, strongly plicated, roof-shaped in transverse section; interspaces more or less flattened, marked respectively with threefive fine secondary riblets; both lateral areas also ornamented with several riblets of similar strength; anterior auricle probably much larger than the posterior. Left valve unknown. Observations and comparison.— The present specimen is slightly suffered from secondary deformation, and the auricular and ventral parts are incompletely preserved. The test is completely eroded away, but the characteristic ornamentation is clearly impressed on the internal mould. judging from the ornamentation and other characters of right valve, it belongs undoubtedly to the group of Neithea quinquecostata, which was defined recently (HAYAMI, 1965a). Furthermore, it seems to be referable to Neithea (Neithea) matsumotoi HAYAMI, 1965, which was originally proposed on the specimens from the upper Miyakon (=Albian) Yatsushiro formation of Kyushu. Although it is somewhat smaller than the normal adult specimens from the Yatsushiro formation, all the essential characters of right valve, especially the surface ornamentation, are quite identical. Occurrence.— Fine sandy shale of the Doganaro formation at Loc. 2. Kakureyashiki, north of Doganaro, Susaki City, KAWASAWA coll.» HAYAMI, I. & K. KAWASAWA. 1967. Some Lower Cretaceous bivalves from The Shimantogawa group of south Shikoku. Transactions and Proceedings of the Palaeontological Society of Japan [N. S.], 66: 73-82, pl. 9. [p. 76]
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Neithea (Neithea) matsumotoi Hayami; I. Hayami & K. Kawasawa, 1967, Some Lower Cretaceous bivalves from The Shimantogawa group of south Shikoku, plate 9, figure 2.
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