Neithea syriaca amanoi 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. [pl. 41, figs. 8-10; pl. 42, figs. 1-4; text fig. 4]
1965 Neithea (Neithea) amanoi Hayami, 1965
I. Hayami, 1965, plates 41, 42.
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«1957. Pecten (Neithea) cf. morrisi (PICTET and RENEVIER), AMANO, Kumamoto Jour. Sci., Ser. B, Sec. 1, VoI. 2, No. 2, p. 88, pI. 1, figs. 17,18, 20, 25,27-29.
Material.— The holotype is a right internal mould (GK. H 6267) collected
from the Hagino formation at loc. Hy. 6011, Hagino, Mirafu village, Kami County, Kochi Prefecture (OGAWA coll.). Paratypes (GK. H 6268-GK. H 6274) from the type locality (OGAWA coIl.). Description.— Shell comparatively small, subovate, planoconvex, subequilateral in the main part. Right valve higher than long, comparatively narrow, moderately inflated; umbo prominent, incurved; antero- and postero-dorsal margins broadly concave when viewed perpendicularly to valve margin; valve margin with weak serrations corresponding to radial ribs of two orders; apical angle about 65-70 degrees; primary ribs six in number, comparatively narrow, not much raised, but clearly distinguishable from secondaries, nearly straight or only slightly curved outwards; each interval of primary ribs not much concave, ornamented with three (sometimes four) secondary riblets which are slightly broader than their interspaces, almost uniform in breadth and prominence; lateral areas quite smooth; auricles well inflated, remarkably unequal; anterior auricle about twice as large as posterior one; hinge-line comparatively long, provided with numerous fine crenulations internally: Left valve flat, suborbicular, slightly higher than long; antero- and postero-dorsal margins straight, forming an apical angle of about 90 degrees; anterior auricle acutely triangular, about twice as large as posterior one; surface weakly plicated; radial ribs about 24 in number, nearly as wide as their interspaces, almost uniform in breadth and prominence; auricles apparently smooth except for growth-lines; ventral margin weakly serrated but never digitated.
Observations and comparisons.— All the available specimens are represented only by internal and/or external moulds. No bivalved specimen was obtained, but the right and left valves can be assigned with certainty to the same species, because of the corresponding ornamentation and coexistence in the same fossiliferous bed.
As noted by AMANO (1957a), the present species is characterized by the comparatively small apical angle of the right valve, the narrow primary ribs, the nearly smooth lateral areas, the unequal auricles and the three (sometimes four) persistent secondary riblets in each interspace. These characters indicate that this species belongs to the subgroup of Neithea morrisi of the group of N. quinquecostata of the present classification. AMANO compared this species with Neithea morrisi (PICTET and RENEVIER, 1858) from the Aptian of Western Europe, citing the references of WOODS (1902) and NAGAO (1934). In fact, Neithea morrisi as recorded by WOODS (1902) from the Lower Greensand of England has a similarly tall outline and smooth lateral areas, but the present species has narrower primary ribs, larger anterior auricle and, generally, a smaller number of secondary riblets. The species described as Neithea morrisi by NAGAO (1934) from the Miyako group is certainly related neither to N. morrisi nor to the present species. and will be described in this paper as N. ficalhoi. Neithea morrisi has been considered by some authors to be conspecific or closely related to Neithea syriaca (CONRAD, 1852) from the Aptian of Lebanon (VOKES, 1946; CASEY, 1961). The present species resembles N. syriaca in the outline and ornamentation of both valves. The shape of auricles is, however, somewhat different and more unequal in the present species. The secondary riblets of N. syriaca are constantly four in number like N. morrisi. Neithea (Neithea) nipponica sp. nov. from the Miyako group has a larger size, a larger apical angle, a broader outline, less unequal auricles, distinctly striated lateral areas and typically four secondary riblets in each interspace between primaries. Neithea (Neithea) matsumotoi sp. nov. from the Yatsushiro formation has a broader outline, broader and more roof-shaped primary ribs, more numerous and delicate secondary riblets, a slightly larger apical angle, distinctly striated lateral areas and less unequal auricles. In many characters the present species resembles Neithea shawi PERVINQUIÈRE, 1912 (DARTEVILLE and FRENEIX, 1957) and its "varieties" from the Cenomanian of Tunisia, Algeria, Lebanon and the Albian of Gabon. The outline of the main body, the apical angle and the radial ornamentation are especially similar, but the anterior auricle is larger and the umbo may be more salient and narrower than that of N. shawi. It may be also close to Neithea attockensis Cox, 1935, from the Albian of Attock in view of the number of secondary riblets, the smooth lateral areas and the relatively inconspicuous primary ribs, but is distinguishable from the Indian species by its narrower outline, smaller apical angle and weaker shell-convexity. The specific name is dedicated to Prof. Masahisa AMANO of the Kumamoto University, who previously described this species and kindly gave me valuable specimens of Lower Cretaceous pelecypods. Occurrence.— Lower Miyakoan (Aptian). Fine weathered sandstone of the Hagino formation at loc. Hy. 6011, Hagino, Mirafu village, Kami County, Kochi Prefecture.»
ITARU HAYAMI, 1965
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«Remarks.— This subspecies is known only from the type locality and the Choshi Group of east Kwanto. It is characterized by the small size, almost invariably three secondary ribs on each interspace in the right valve, fine radial ribs of nearly one order of prominence in the left valve, almost smooth lateral areas, not strongly inflated right valve and nearly flat left valve without marginal digitations. DHONDT (1973) regarded Pecten (Neithea) cf. morrisi by AMANO (1957), which was described on the basis of the same fossil population, as belonging to Neithea (Neithea) syriaca (CONRAD, 1852). In fact, Janira morrisi PICTET and RENEVIER, 1858, appears to be synonymous with Janira syriaca CONRAD, 1852. According to VOKES (1947), the original specimen of J. syriaca from Abeih of Lebanon was lost, but its specific characters can be recognized in some "neosyntypes" from the type locality. The Japanese specimens actually resemble the topotypes of N. (N.) syriaca and also some European specimens hitherto described under the name of N. morrisi. The small size, relatively narrow and rounded outline and smooth lateral areas of right valve are particularly similar, while the radial ornaments of two valves are somewhat different. In the Lebanon specimens there are commonly four secondary ribs on each interspace in the right valve and 26 to 28 ribs in the left, whereas the Japanese specimens usually possess three secondary ribs in the right valve and totally 22 to 24 ribs in the left. The six pairs of ribs in the left valve, which correspond to the primaries of the right valve, are probably more prominently elevated in the japanese specimens. Although one of us (HAYAMI, 1975) once regarded N. amanoi as a synonym of N. syriaca, the above mentioned slight but clear difference seems to deserve certain taxonomic distinction. At present it is difficult to say with documentation whether the difference represents the distinctness of species or is merely due to geographic variation, but the Japanese specimens are here tentatively treated as a subspecies, considering the japanese and Lebanon populations are almost contemporaneous. Most of European specimens of "N. morrisi" are more similar to the Lebanon specimens than to Japanese ones, and they may be generally referable to the nominate subspecies, N. (N.) syriaca syriaca.
Various diagnostic characters of these Japanese species and subspecies of Neithea are coIlectively shown in Table 4. The taxonomic names here adopted may be considerably different from DHONDT's (1973), but in many cases we agree with her in recognizing the morphological resemblance between European and Japanese specimens. The mode of radial ribbing not only in the right valve but also in the left seems to be important for the taxonomic distinction, and the transverse sections of two valves in main japanese species are diagrammatically shown in Text-fig. 5.»
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. 49, 50]
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Variability of radial ornaments among main Japanese species of Neithea; I. Hayami & M. Noda, 1977, Notes on the morphology of Neithea (Cretaceous pectinids) with taxonomic revision of Japanese species, text-figure 5.
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