Cryptopecten nux sematensis (Oyama in Taki & Oyama, 1954)
TAKI, I. & K. OYAMA. 1954. Matajiro Yokoyama's the Pliocene and later faunas from the Kwanto region in Japan. Paleontological Society of Japan, Special Papers, 2: 1-68, pls. 1-49. [pl. 35, figs. 1, 2]
1954 Aequipecten (Cryptopecten) sematensis Oyama in Taki & Oyama, 1954
«C. sematensis is so closely related with C. nux (REEVE, 1853), a Recent species of Southeastern Pacific, that they can hardly be distinguished from each other at species level. However, they reveal a slight but clear difference, that is, the latter has inflated and equivalve shell. Under this circumstance, C. sematensis is treated here as a subspecies of C. nux. Also the present one can be distinguished from Cryptopecten vesiculosus (DUNKER, 1877), a common Recent species of Southern Japan, by its smaller shell, inflated left valve and larger number of radial ribs.»
SATO, Y., K. MASUDA & T. SHUTO. 1986. Pelecypod fauna of the Shimajiri Group in Miyako-jima, Okinawa Prefecture, Japan. Memoirs of the Faculty of Sience, Kyushu University, [Serie D - Geology], 26 (1): 1-49, pls. 1-5. [p. 25]
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Cryptopecten nux sematensis (Oyama, 1973); Y. Sato, K. Masuda & T. Shuto, 1986, Pelecypod fauna of the Shimajiri Group in Miyako-jima, Okinawa Prefecture, Japan, plate 2, figures 5-8.
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«Cryptopecten nux sematensis (Oyama)
Pl. 3, Figs. 3, 4 1922. Pecten tissoti [sic] Bernardi: Yokoyama, Jour. Coll. Sci. Imp. Univs. Tokyo, vol. 44, art. 1, p. 182, pl. 15, figs. 1, 2. [non Pecten tissotii Bernardi, 1858]
1954. Aequipecten (Cryptopecten) sematensis Oyama in Taki and Oyama, Palaeont. Soc. Japan, Spec. Papers, no.2, pl. 35, figs. 1, 2. 1973. Aequipecten (Cryptopecten) sematensis Oyama: Oyama, Palaeont. Soc. Japan, Spec. Papers, no. 17, p. 85, pl. 34, figs. 9a, b, 10a, b. 1976. Aequipecten vesiculosus (Dunker): Masuda and Noda, Check List and Bibliography of the Tertiary and Quaternary Mollusca of Japan, p. 8 (only). [non Pecten vesiculosus Dunker, 1877] Type.—The holotype is a nearly complete right valve (UMUT CM21561) which was described by Yokoyama (1922, pl. 15, fig. 1) as "Pecten tissoti" and designated by Oyama in Taki and Oyama (1954) as the holotype of Aequipecten (Cryptopecten) sematensis. lt occurred from "the Upper Nlusashino at Shito", which is certainly identical with the upper part of the Yabu Formation at Ochishimoshinden of Semata, Toki Town, Chiba Prefecture (35°31.5'N, I40°13.7'E), where the sample Szz (N) was newly obtained. 15.2 mm long, 17.1 mm high, 6.2 mm thick. Material.— In addition to the holotype, twelve specimens of the sample Srz (N) are concerned with the following description and discussions. Diagnosis.— Subspecies of C. nux, characterized by relatively tall outline, very strongly inflated right and left valves, and short and broad byssal wing without any prominent spiny scales. Description.— Shell small, rarely exceecling 18 mm in length ánd height, nearly acline throughout growth, right-convex except for very young stages; height isometric or slightly negatively allometric to length, commonly exceeding length; thickness decidedly positively allometric to length (and height) in each valve; form ratio T/L as large as 0.36-0.41 in adult right valve and 0.26-0.29 in adult left valve; apical angle of disk about 100-105 degrees; test relatively thick; byssal wing broad, not much elongated, marked with several (commonly five) weak radial ribs without development of any prominent spiny scales; disk sculpture and other morphological characters essentially similar to those of C. nux nux. Remarks.— When Oyama in Taki and Oyama (1954) made a taxonomic revision on Yokoyama's illustrated molluscs from the Quaternary of Kanto region, a new name, Aequipecten (Cryptopecten) sematensis, was introduced for two tumid specimens of a pectinid which had been referred to Pecten tissoti [error of tissotii] by Yokoyama (1922). Two different views are possible as to the validity of Oyama's proposal of this taxonomic name. One interpretation, as pointed out by Hayami (1973) and Masuda and Noda (1976), is that A. (C.) sematensis Oyama, 1954, is a nomen nudum, because the proposal was not accompanied by any indication of diagnostic characters. Under this interpretation the present description would constitute the first valid proposal. The other view, as personally communicated by Dr. K. Oyama (October 16, 1982), is that the taxonomic name has been available since 1954, because the citation [synonym list first appeared in Oyama (1973)] of Yokoyama's description of "Pecten tissoti [sic]" constitutes and valid indication as required by ICZN articles 13 (a) (ii) and 16 (a) (i). I am now in favor of the latter interpretation, applying Oyama's trivial name, sematensis, for this taxon. My examination of one of Yokoyama's specimens (UMUT CM21561) (the other specimen CM21562, which was erroneously designated as the holotype of A. (C.) sematensis by Oyama (1973), is missing from the University Museum, the University of Tokyo), and also of a newly collected sample Srz (N), showed that the structure of radial ribs and many other characteristics are quite similar to those of C. nux from other fossil localities and the present seas. The convexity of the two valves is, however, unusually strong, and the thinkness (T) is far more positively allometric to length (L) and height (H). The difference in the shell convexity and growth ratio (a) between this subspecies and C. nux nux is clearly recognizable from the profiles of the two valves (Pl. 3, Fig. 1c versus Fig. 3c; Fig. 2c versus Fig. 4c) as well as the discrepant slopes of reduced major axes in the double logarithmic diagrams of relative growth (see Figs. 15 and 16). Another criterion for the subspecific distinction seems to exist in the shape and sculpture of the byssal wing. In the present subspecies the byssal wing is not so narrowly elongated nor so strongly ornamented as in the nominate subspecies. Distribution.— This subspecies has been known almost exclusively from the Yabu Formation at Ochishimoshinden of Semata, Chiba Prefecture. I have found only one left valve of this subspecies in the old collection of the Tohoku University (IGPS no. 13369), a specimen which was collected by Prof. Yabe from "Narita Beds at Takakura, Shito Village" and registered as Pecten (Chlamys) vesiculosus. Late Pleistocene (ca. 0.29 Ma).» HAYAMI, I. 1984. Natural history and evolution of Cryptopecten (A Cenozoic-Recent Pectinid Genus). The University Museum, The University of Tokyo, Bulletin, 24: 1-149, pls. 1-13. [p. 103, 104]
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Cryptopecten nux sematensis (Oyama); I. Hayami, 1984, Natural history and evolution of Cryptopecten, plate 3, figures 3, 4.
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«1922. Pecten tissoti BERNARDI: YOKOYAMA, Foss. Kazusa Shimosa, p. 182, pI. XV, figs. 1, 2. — 1954. Aequipecten (Cryptopecten) sematensis OYAMA in TAKI & OYAMA, Plioc. & later Faunas, p. 35, pI. 35, figs. 1, 2, new name for Pecten tissoti BERNARDI sensu YOKOYAMA.
Fossil occurrence.—Shitô. Ecology.— Unknown.» OYAMA,
K. 1973. Revision of Matajiro Yokoyama's type mollusca from the
Tertiary and Quaternary of the Kanto area. Paleontological Society of
Japan, Special Papers, 17: 1-148, pls. 1-57 [p. 85]
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Aequipecten (Cryptopecten) sematensis Oyama; K. Oyama, 1973, Revision of Matajiro Yokoyama's type mollusca, plate 34, figures 9, 10.
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«302. Pecten tissoti, Bernardi.
PI. XV. Figs. 1, 2. Pecten tissoti. Pilsbry, Cat., p. 144. Bernardi in Jour. d. Conch., VII, 1859, p. 91, pl. I, fig. 2. Kobelt in Syst. Conch. Cab., VII, part 2, Spondylus and Pecten, p. 143, pl. XL, fig. 5.
A small neat Pecten circular in form and tolerably swollen, with the anterior ear larger than the posterior. About twenty radiating ribs accompanied by a smaller one on both sides ornament the surface. Crossing these ribs, there are fine concentric lamellae which are most distinct in interspaces where there are one or two radiating striae. Inner margin crenate. Only three valves of which two are right. The larger of the right valves measures 16.8 millim. in height, 16 millim. in breadth and 6.2 millim. in depth. Fossil occurrence.— Shito. Living.— Japan (according to Dunker).» YOKOYAMA, M. 1922. Fossils from the upper Musashino of Kazusa and Shimosa. Journal of the College of Science, Imperial University of Tokyo, 44 (1): 1-200, pls. 1-17. [p. 182]
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Pecten tissoti, Bernardi; M. Yokoyama, 1922, Fossils from the upper Musashino of Kazusa and Shimosa, plate 15, figures 1, 2. [non Pecten tissotii Bernardi, 1858: Aequipecten (Cryptopecten) sematensis Oyama in Taki & Oyama, 1954; fide Sato, Masuda & Shuto 1986, p. 24]
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