Nipponopecten akihoensis (Saga in Matsumoto, 1930)
MATSUMOTO, H. 1930. On the Marine Faunae of Three Fossil Zones of the Upper Miocene of Natori District, Province of Rikuzen. Science Reports of the Tohoku University [2nd. Series - Geology], 13 (3): 96-109, pls. 39-40. [p. 106, pI. 40, figs. 7, 8]
1930 Pecten (Pseudamusium) [sic] akihoensis Saga in Matsumoto, 1930
1954 Patinopecten nakajimai Masuda, 1954
1956 Placopecten wakuyaensis Masuda, 1956
1954 Patinopecten nakajimai Masuda, 1954
1956 Placopecten wakuyaensis Masuda, 1956
H. Matsumoto, 1930, plate 40.
|
«SAGA'S MS. on the present species reads as follows.
"Locality: Akio Natori District, Prov. of Rikuzen. "Shell thin, inequivalve, equilateral, flattened. Left valve more convex than the right; submargins straight, pointed at the top forming an angle of ca. 110º, the anterior and the posterior area reduced to a line. In the right valve, the auricles are subequal in size, though the anterior one is furnished with wide and shallow byssal notch; external surface smooth, but with fine concentric striae; inner surface smooth, devoid of radial lirae. In the left valve, the anterior auricle is slightly larger than the posterior, without any notch; external surface ornamented with a great number of fine, uniqual ribs, as well as with very fine concentric striae, wich in the interstitial grooves grow into fine imbrications — a sort of Camptonectes-striation. "Dimension (in mm.)
"The present species resembles Pecten tokunagai YOKOYAMA from the Pliocene of Koshiba, being, distinguished, however, from it by the more numerous ribs, by the general outline and by the shape of the auricles."
Fairly frequent. Age: Upper Miocene.» HIKOSHICHIRÔ MATSUMOTO, 1930
|
«Discussion.— The species from the Ichibu Formation has a rather small-sized, compressed lenticular shell with a left valve sculpture consisting of ca. 35–40 fine radial ribs and interstitial shagreen microsculpture, and a nearly smooth right valve. The hinge has a small, trigonal resilifer and weak dorsal teeth in the left valve. The denticles at the distal ends of the auricular crura and internal rib carinae are absent. Based on these characters, the species from the Ichibu Formation is safely referred to Nipponopecten akihoensis (Saga in Matsumoto, 1930), originally described from the middle Miocene Moniwa Formation in Miyagi Prefecture, Northeast Japan.
Matsumoto (1930) has long been accepted as the author of this species. However, the authorship should be attributed to the late Mr. Ichirô Saga (?1888–1954), an ex-student of Tôhoku Imperial University (under ICZN Art. 50.1.1), because the original species name “Pecten (Pseudamusium) akihoënsis” and its description in Matsumoto (1930) were quoted from Saga’s unpublished manuscript. Nipponopecten akihoensis shows broad morphological variations especially in the umbonal angle and radial sculpture. Patinopecten nakajimai Masuda, 1954, and P. wakuyaensis Masuda, 1956b, are considered to fall within its intraspecific variation as pointed out by Sato (1991). Nipponopecten akihoensis has sometimes been confused with “Placopecten” protomollitus (Nomura, 1935), originally described from the lower–middle Miocene Tanosawa Formation in the Nishi-Tsugaru area, Aomori Prefecture, Northeast Japan. Some authors referred the latter species to the genus Nipponopecten Masuda, 1962 (e.g. Ozawa et al., 1996; Kurihara, 2010). However, “P.” protomollitus should not ever be referred to the genus Nipponopecten because its right valve has a blunt Vshaped hinge line, the anterior auricle has a shallow, angulated byssal notch, and the left valve is entirely lacking a shagreen microsculpture. In addition, the shell is smaller and thinner, and radial ribs are more regular than those of N. akihoensis. Although Matsubara (2011) tentatively referred a Japanese Miocene “Placopecten” species to the genus Serripecten Marwick, 1928 (type species: Pecten hutchinsoni Hutton, 1873, by original designation), further study is needed to clarify its precise taxonomical status. Miyagipecten saromensis Hasimoto and Kanno, 1958, from the middle Miocene Chirai Formation in Hokkaidô, Northeast Japan, is distinguished from N. akihoensis by having fewer (ca. 20: Hasimoto and Kanno, 1958) rounded radial ribs on the left valve, which tend to become obsolete ventrally. Distribution.— Middle Miocene: Kaigarabashi Formation in Hokkaidô (Kanno, 1962 and Sawada, 1962, as Placopecten wakuyaensis; Sato, 1991); Shiritaka Formation in Akita Prefecture (Masuda and Yoshida, 1991, as Placopecten protomollitus (Nomura)]; Moniwa and Oido formations in Miyagi Prefecture (Matsumoto, 1930; Nomura, 1940a; Masuda, 1952, 1954; Sato, 1991); Nanao Calcareous Sandstone Member of Akaura Formation in Ishikawa Prefecture (Yokoyama, 1929, as Pecten sp.; Otuka, 1935b, as Chlamys sp.); Matsué and Ichibu formations in Shimane Prefecture (Takayasu, 1980, as Placopecten sp.; Sato, 1991; this study).» MATSUBARA, T., I. NORO, Y. MATSUURA & T. IRIZUKI. 2014. Miocene Mollusca from the Ichibu Formation on Nishinoshima Island, Oki Islands, Southwest Japan. Paleontological Research, 18 (1): 6-32, figs. 1-11. [p. 20, 21]
|
Nipponopecten akihoensis (Saga in Matsumoto, 1930); T. Matsubara, I. Noro, Y. Matsuura & T. Irizuki, 2014, Miocene Mollusca from the Ichibu Formation on Nishinoshima Island, Oki Islands, Southwest Japan, figures 7.5, 7.7a–7.7c.
|
«Remarks:— Since the holotype of this species cannot be found, the description given above was based upon a neotype specimen. Besides the neotype there are several topotype specimens which enabled the writer to study the variation in rib formation and also the relation between this species to related forms by direct comparison with the respective type materials.
In the left valve the primary riblets show no bi- or trifurcation, but remain roundly elevated and about equal to or slightly narrower than the broadly round-bottom interspaces. The primaries vary in number from 35 to 40 and may or may not have intercalary threads, but when present between the primaries, they may become nearly as strong at the margin, but are usually much weaker and occur only ocassionally. Slight imbrication of the primaries is seen only in specimens which have their surface well preserved. The other features such as convexity of the valves, presence of fine concentric growth lines, shallow byssal notch and subequal auricles remain invariable. Although H. MATSUMOTO (1930, p. 106) described the right valve to possess fine concentric striae, the specimens of the right valve at hand are usually sculptured with numerous, very fine, faint radial threads which are additional to the fine concentric growth lines. The radial threads on the right valve are generally much weaker than those of the left. The radial threads seem to be totally lacking in several young specimens. The left valve of this species resembles Pecten protomollitus and P. arakawai, both described by S. NOMURA (1935, p. 40, pl. 4, figs. 1, 2; p. 41, pl. 6, fig. 3) from the Miocene Tanosawa formation of the Nishi-Tsugaru District in Aomori Prefecture. In the former species there are generally three smaller threads in the interspaces between the larger ones and in the latter the primary riblets are squarish in profile and bifurcated, thus in the type of radial sculpture, those two species can be distinguished from the present one. Placopecten akihoensis resembles Placopecten setanaensis KUBOTA (1950, pp. 183-184, pl. 7, figs. 1-4) from his Upper Miocene Setana formation in southwestern Hokkaido in having suborbicular, compressed, thin shell provided with numerous fine radial threads, but is distinguished therefrom by the larger auricles compared with the height of the shell, lack of internal sculpture, smaller apical angle (110º) and slightly imbricated radials, broader interspaces between the radials in the left valve, and by the left valve having much stronger sculpture than the right valve. Placopecten clintonia rappanhannockensis MANSFIELD (1936, p. 186, pl. 23, figs. 1, 2, 3) can be distinguished from the present species by the surface of the valves being similarly culptured with many nearly flat radials, dichotomous striation, much closer arranged radials and the right anterior with a very shallow byssal notch.» MASUDA, K. 1952. On some Miocene Pectinidae from the environs of Sendai. Transactions and Proceedings of the Palaeontological Society of Japan [N. S.], 8: 249-254, pl. 24. [p. 250, 251]
|
Placopecten akihoensis (Matsumoto); K. Masuda, 1952, On some Miocene Pectinidae from the environs of Sendai, plate 24, figures 1a, 1b, 2, 3.
|
«Nom. Jap., Akyu-tukihigari-modoki n. n.
Pecten (Pseudamusium) akihoënsis (SAGA MS.) MATSUMOTO, Sci. Rep. Tôhoku Imp. Univ. Ser. 2, Geol., Vol. 13, No.3, p. 106, pl. 40, fig-so 7, 8, 1930.
Pecten (Pseudamusium) akihoënsis MATSUMOTO. NOMURA and HATAI, Saitô Hô-on Kai Mus., Res. Bull., No. 10, p. 121, pl. 16, fig-. 12, 1936. Localities: Matumori (very frequent), Reg. No. 2635. Sabusawa (rare), Reg. No. 12669.»
NOMURA, S. & K. HATAI. 1937. A List of the Miocene Mollusca and Brachiopoda Collected from the Region Lying North of the Nanakita River in the Vicinity of Sendai, Rikuzen Province, Japan. Saito Ho-on Kai Museum of Natural History Research Bulletin, 13: 121-145, pls. 17-21. [p. 131, pl. 20, figs. 2, 3]
|
Pecten (Pseudamusium) akihoënsis Matsumoto; S. Nomura & K. Hatai, 1937, A List of the Miocene Mollusca and Brachiopoda Collected from the Region Lying North of the Nanakita River in the Vicinity of Sendai, Rikuzen Province, Japan, plate 20, figures 2, 3.
|