Neithea nipponica 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. 296, pl. 40, figs. 1-6; pl. 52, figs. 1, 2]
1965 Neithea (Neithea) nipponica Hayami, 1965
1965 Neithea (s. l.) aketoensis Hayami, 1965
1965 Neithea (s. l.) aketoensis Hayami, 1965
I, Hayami, 1965, plates 40, 52.
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«1934. Pecten (Neithea) sp., NAGAO, Jour. Fac. Sci. Hokkaido Imp. Univ., Ser. 4, VoI. 2, No. 3, p. 209, pI. 29, figs. 8, 9.
Material.— The holotype is a right valve (GK. H 6263) collected from the Aketo formation at loc. Hn. 6201, northeast of Raga, Tanohata village, Shimohei County, Iwate Prefecture (HANAI colI.). Paratypes (GK. H. 6264-GK. H 6266, GK. H 6364, GK. H 6365, GK. H 6632, GK. H 6633) from the type area (HANAI, OBATA and HAYAMI colI.).
Description.— Shell medium-sized, pIano-convex, polygonally ovate in outline, subequilateral, slightly higher than long; test moderately thick. Right valve strongly inflated, with incurved and prominent umbo; antero- and postero-dorsal margins long, broadly concave, forming an apical angle of about 80 degrees; ventral margin distinctly concave between primary ribs; auricle subequal in size, comparatively small, although anterior auricle is more acutely angulated and has a shallow byssal notch; hinge-line moderate in length, provided with numerous fine internal crenules; six primary ribs nearly straight in perpendicular view, not very strong but clearly distinguishable from secondary ribs; each interval between primary ribs slightly concave, provided with four regular secondary riblets which are subequal to their interspaces in breadth; both lateral areas and auriclesmarked with a number of still finer riblets; 'Concentric lines comparatively weak. Left valve nearly flat in early stage and weakly concave after mid-growth; apical angle exclusive of auricles approximately 100 degrees; auricles subequal in size, the posterior one right-angled or obtuse, the anterior one acute and demarcated by a deep angular groove; radial ribs about 30 or slightly more in number, slightly narrower than interspaces, increasing gradually in number by irregular bifurcation which occurs on the weak radial ribs corresponding to primary ribs of right valve; both lateral areas marked with several weaker radial riblets; auricles apparently smooth except for growth-lines.
Observations and comparisons.— The holotype shows nearly complete outline and ornament of right valve, although the test is partly exfoliated. The paratypes of left valves reveal the interior and exterior, and their specific identity with the holotype can be readily presumed from their association with it in the same fossil bed and from the correspondence in ornamentation. The striated lateral areas are observable in every right valve. The Neithea sp. described by NAGAO (1934) from the Miyako group is probably conspecific with these specimens in view of the similar ornamentation.
This species is clearly distinguishable from Neithea (Neithea) ficalhoi (CHOFFAT), which is sometimes associated with it, by the weaker convexity of the right valve, the larger number of secondary riblets (invariably two in N. ficalhoi), the finer primary and secondary ribs, the distinctly striated lateral areas and the larger apical angle of left valve. The height is scarcely in excess of 35 mm. in N. ficalhoi, whereas the adult specimens of the present species are as tall as 40 mm. The ornamentation of both valves of the present species is similar to that of Neithea quinquecostata (SOWERBY, 1814), a well-known species from the Aptian-Turonian of Western Europe, Africa and India. In comparison with WOODS' figures (1902) and with the Upper Greensand specimens of typical N. quinquecostata, however, the umbo of the right valve is slightly narrower and the umbonal area of the left valve is less concave. The shell convexity is distinctly weaker in N. niponica than in N. quinquecostata. Occurrence.— Lower (?) and upper Miyakoan (?) Aptian and Albian). Grey sandy shale of the Aketo formation at loc. Rn. 6201, and grey sandy shale of the upper part of the "Orbitolina sandstone" at loc. Rn. 6203 and Rn. 0671, northeast of Raga, Tanohata village, Shimohei County, Iwate Prefecture. NAGAO's specimens of Neithea sp. are said to have come from the Hiraiga formation at Riraiga in the same village, although I have been unable to find this species in the lower part of the Miyako group.»
ITARU HAYAMI, 1965
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«N. aketoensis Hayami (1965) was based on a single specimen from the upper Aptian of the Hiraiga Formation on the Pacific coast of the Northeast Honshu (Loc. 10 in Fig.1). We re-examined the type specimen, and concluded that the rib morphology and its distribution pattern both on the inner mould and external shell surface resembled those of N. nipponica Hayami (1965).Therefore, N. aketoensis should be considered as a junior synonym of N. nipponica. Taking into account the discussion above we excluded N. kochiensis and N. aketoensis from further consideration in this paper. Detailed taxonomic revision of these two species will be provided elsewhere.»
IBA, Y. & S. SANO. 2008. Paleobiogeography of the pectinid bivalve Neithea, and its pattern of step-wise demise in the Albian Northwest Pacific. Palaeogeography, Palaeoclimatology, Palaeoecology, 267: 138-146, figs. 1-6. [p. 139]
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«Remarks.— This species commonly occurs in the uppermost part of the Hiraiga Formation and the Aketo Formation of the Miyako Group. The age is certainly lower Albian as indicated by associated ammonites. This appears to have suddenly
substituted the pre-existing species, N. (N.) ficalhoi, but the two species are rarely but strictly associated with each other at a few localities. There are no individual showing intermediate morphology, and no direct phyletic relationship can be expected between the two species. Rather it seems more reasonable to consider that N. nipponica migrated from somewhere to this area and usurped within a short time the niche which had been occupied by N. ficalhoi. N. nipponica clearly differs from other Japanese species of Neithea in the combination of the following characters: medium size, rather weak contrast of prominence between the primary and secondary ribs, almost invariably four secondary ribs on each interspace in the right valve, indistinctness of six pairs of ribs in the left valve, and fine radial riblets on the lateral areas of the right valve. In many of these characters N. nipponica resembles some specimens of Neithea quinquecostata (SOWERBY, 1814). DHONDT (1973) regarded the morphological differences between the two species stated by HAYAMI (1965) as insignificant and interpreted N. nipponica as a junior synonym of N. quinquecostata. As the result of further comparative studies with the specimens of N. quinquecostata preserved at the British Museum (Nat. Hist.) and some other institutions in western Europe, however, one of us (I. H.) maintains the taxonomic distinction between the two species. In comparison with the Upper Greensand and Chalk specimens of N. quinquecostata including the lectotype (BMNH no. 43324), the Japanese specimens have much narrower umbonal area, more slender primary ribs and more rounded anterior and posterior extremities of right valve, less polygonally angulated ventral margin of two valves, smaller size and probably thinner test. These difference cannot be attributed to the difference of ontogenetical stages between the materials, though once assumed by DHONDT. At the same time, however, it must be noted that the morphological differences are not so significant between the Japanese specimens of N. nipponica and those of "N. quinquecostata" from the Lower Greensand and Gault of England. Our specimens are likewise similar to the illustrated specimens of N. quinquecostata from the upper Albian of Crimea (DRUSCHIZA and KUDRJAVTSEVA, 1960), which also show more rounded ventral margin and weaker contrast between the radial ribs of two orders. It is thus suggested that somewhat significant morphological change occurred in the stock of N. quinquecostata, and that the name of N. (N.) nipponica is possibly applicable also for the fossil populations from the Lower Cretaceous of Europe. We think the nomenclatural translation of N. (N.) nipponica to subspecies is not necessary, even if it proves to be phyletically related to N. quinquecostata.» HAYAMI, I. & M. NODA. 1977. Notes on the morphology of Neithea (Cretaceous pectinids) with taxonomic revision of japanese species. Transactions and Proceedings of the Palaeontological Society of Japan [N. S.], 105: 27-54, pls. 5, 6. [p. 47, 48]
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«I cannot distinguish N. (N.) nipponica HAYAMI, 1965 from N. quinquecostata on the basis of the description and figures. According to HAYAMI (p. 297) the differences are the following:
" — umbo of the right valve is slightly narrower; — umbonal area of the left valve is less concave; — shell convexity is distinctly weaker in N. niponica (sic) than in N. quinquecostata." The umbo of the right valve is by definition subject to variation; if it appears somewhat narrower on N. nipponica this could, however, also be explained by the fact that the Japanese specimens are rather smaller and, on Pectinidae, the umbo is wider on larger (older) valves than on smaller (younger) ones. Umbonal area of the left valve: N. quinquecostata is characterized by a completely flattened left valve, so it seems pointless to talk of "less concave" since it is never concave. Shell convexity: plate 40 , figure 1 represents the holotype of N. nipponica; it has to be admitted that this specimen is not very convex; the figures of the other specimens (pl. 40, figs. 5 and 6, pl. 52, figs. 1 and 2) are normaly convex individuals; the weaker convexity of the holotype could be explained by its much larger size: larger (older) specimens usually grow less convex than smaller (younger) ones.» DHONDT, A. V. 1973. Systematic revision of the subfamily Neitheinae (Pectinidae, Bivavia, Mollusca) of the European Cretaceus. Mémoires du Institute Royal des Sciences Naturelles de Belgique, 176: 1-101, pls. 1-5. [p. 34]
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«It is possible that what I. HAYAMI, 1965 (pp. 309-310, pl. 43, fig. 6) describes as N. (s.l.) aketoensis sp. nov. from the Albian of Japan is in fact a N. aequicostata. The figured specimen, which is the only one that has been found so far, is incomplete and does not allow a definite conclusion to be drawn.»
DHONDT, A. V. 1973. Systematic revision of the subfamily Neitheinae (Pectinidae, Bivavia, Mollusca) of the European Cretaceus. Mémoires du Institute Royal des Sciences Naturelles de Belgique, 176: 1-101, pls. 1-5. [p. 13]
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Neithea (s. l.) aketoensis sp. nov.; I. Hayami, 1965, Lower Cretaceous marine pelecypods of Japan, part I, plate 43, figure 6.
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