Chlamys miyatokoensis (Nomura & Hatai, 1937)
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. 127, pl. 19, figs. 2-4; pl. 20, fig. 1]
1937 Pecten (Chlamys) miyatokoensis Nomura & Hatai, 1937
S. Nomura & K. Hatai, 1937,
plates 18, 19. |
«Nom. Jap., Miyatoko-itayagai n. n.
Shell rather small in size, averaging about 55 mm or slightly less in length; outline subcircular, somewhat higher than long, equilateral except· for ears; both valves equal and slightly convex; test rather thin; sides of disc apparently non-serrated with a slightly concave curve above. Right valve with many (about twenty), narrow, more or less flat-topped imbricating radial ribs, which increase their number anteriorly by means of division and intercalation; main ribs generally divided into two or four equally smaller ones with one to three almost equal-sized intercalaries along ventral border; hinge-line slightly shorter than one-half of disc-length; anterior ear produced in front, sculptured with five or six imbricating riblets and a few finner ones; byssal notch quite prominent, triangular; posterior ear shorter than the anterior (about one-half), nearly rectangularly truncated behind, sculptured as in right valve, but the radiating riblets are finner and more numerous. Left valve similar to the right in convexity and sculpture, except for the shape of ears. Interior surface smooth with no distinct grooves, marginally crenulated, corresponding to external sculpture. Length 48, height 51, depth ca. 20 mm (intact valves), length of ears ca. 20 mm (type) .
Some paratype specimens give the following dimensions (in mm);
This species resembles P. kaneharai YOKOYAMA mentioned above in general features, but the shell is decidedly smaller, and the ribs are more equally divided into finner ones in proportion to size. YOKOYAMA figured two fragmental specimens (figs. 5, 6) showing the external sculpture of P. kaneharai from Siobara, these are closely similar to the present species, but the imbricating nature of the ribs seems apparently less distinct in those specimens. When the shell size is left out of consideration, the sculpture is similar to the specimen figured also by YOKOYAMA on specimens from the Tanagura Miocene as P. kaneharai 1), but compared side by side, the Tanagura specimen has more distinct main ribs and deeper interspaces than our's.
P. miyatokoensis is also related to P. hericius, P. hindsii and P. islandicus from the cold waters of the North Pacific and Bering Sea; from the first one this shell differs chiefly in possessing smaller ribs in the left valve, from the second by having more pronounced imbrications on the surface of the right valve and from the third by the shell being smaller with more imbricated ribs. P. erythrocomatus DALL has a large shell with a less number of ribs. Localities: Ôtutumi (abundant), Reg. No. 4518. Sabusawa (rare), Reg. No. 12676.» 1) M. YOKOYAMA, Jour. Fac. Sci. Imp. Univ. Tokyo, Sec. 2, Vol. 3, Pt. 4. p. 203, pI. 13, 1931.
SHICHIHEI NOMURA & KOTORA HATAI, 1937
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«Comparison: Chlamys (s.s) kaneharai (Yok.) is the nearest, from which Chlamys (s.s) miyatokoensis is differentiated by thin spinules of radial ribs, considerably smaller in their height, smaller subdivisions (only into 3-4 radial ribs) radial ribs, presence of 2-3 delicate radial filaments in the intercostal intervals, less developed ctenolium und smaller byssal sinus.
Another nearby species is considered to be Chlamys (s.s) hastatus, from which Chlamys (s.s) miyatokoensis is differentiated by shell which is distinctly stretched in height, radial ribs are smaller in height, without a clearly visible middle keel, which carries high curved snubbed spines and a small number of intercalated riblets in the intercostal intervals. Age: Middle Miocene
Location: Japan, prefektura Miyagi, Miyatoko river; Kamchatka, western coast, mouth of Voaympolkj River (promontory Ivirvilkin) (outcrop 6, bed 9, sample 87).
Distribution: Otsutsum Formation, Ytsuno, Ginzan, Ogana Japan; Etolon suite west of Kamchatka.»
SINELNIKOVA, V. N. 1975. Mio-Pliocene Pectinidae of Kamchatka [Pektinidy mio-pliotsena Kamchatki]. Transactions of the Geological Institute, Academy of Sciences of the USSR, 229: 1-140, 25 pls. (in Russian with English title and contents; Translated for the U. S. Department of the Interior, Geological Survey, and the National Science Foundation, Washington, D. C., by the Al-Ahram Center for Scientific Translations, 1977). [p. 57]
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Chlamys (Chlamys) miyatokoensis Nomura et Hatai; V. N. Sinelnikova, 1975, Mio-Pliocene Pectinidae of Kamchatka, plate 18, figures 7, 9; plate 24, figures 4, 7, 9.
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«Holotype: – SM, Reg. No. 4518.
Remarks: – This species resembles Chlamys kaneharai (Yokoyama) in general features. However, it is distinguishable from kaneharai by its decidedly smaller shell, finely scaled and low radial ribs which are equally divided into three or rarely four riblets, two or three, finely scaled intercalary threads in the interspaces, narrow byssal area, less distinct ctenolium and cardinal crura. This is also related to Chlamys hericius (Gould) (Arnold, 1906, p. 110, pI. 43, figs. 3, 3a), a Recent Northern Pacific species and as fossil in the Upper Miocene to Pleistocene formations of California, but it differs therefrom in having a larger shell, divided radial ribs in which the middle part is the most prominent, and several intercalary threads in the interspaces between the radial ribs. This species was thrown into Chlamys islandica hindsi (Carpenter) by Kuroda and Habe (1952). However, from the above mentioned characters it is evident that this is a species distinct from hindsi. Type locality, Geological formation and Age: – Southern cliff of the lake at the junction of two tributaries of the upper course of the Miyatoko-River, Otsutsumi, Taiwa-machi, Kurokawa-gun, Miyagi Prefecture (Lat. 38°22'44"N., Long. 140°49'08''E.). Otsutsumi formation. Middle Miocene. Distribution: – Otsutsumi and Utsuno formations, Miyagi Prefecture; Ginzan formation, Yamagata Prefecture; Ogane formation, Tochigi Prefecture: all Middle Miocene in age. Occurrence: – Abundant in the granule conglomerate to conglomeratic tuffaceous very coarse-grained sandstone of the Otsutsumi formation; few in the tuffaceous coarsegrained sandstone of the Utsuno formation; few in the conglomeratic coarse-grained sandstone of the Ginzan formation.» MASUDA, K. 1962. Tertiary Pectinidae of Japan. The Science Reports of the Tōhoku University [2nd. Series - Geology], 33 (2): 117-238, pls. 18-27. [p. 178]
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«Remarks:— This species is characterized by its rather small, orbicular and subequivalve shell numerous, densely scaled, low radial ribs which are divided into three subequal parts by longitudinal furrows, two or three finely scaled intercalary threads, larger anterior auricle sculptured with imbricated and distinct radial threads and concentric lines; furnished with deep byssal notch and more or less narrow byssal area, much smaller posterior auricle sculptured with imbricated fine radial threads, indistinct ctenolium, fine marginal serration, rather wide hinge plate ornamented by fine striae parallel to the hinge in full adult specimens, and left valve with sculpture almost similar to the right valve.
The subspecies matumori described by S. NOMURA and K. HATAI (1936, p. 129, pl. 20, fig. 11) from the lowermost part of the Nanakita formation at Matsumori is said to differ from miyatokoensis by the fewer radial ribs (about ten) and slightly more distinct main ribs. According to the writer's examination of the type specimen, the following features must be added: the majority of the radial ribs consist of a set of two main radial ribs, and the interspace between the main radials of a pair is somewhat deep and narrow; interspaces between the pair are much narrower than the pair itself and more or less deep; the number of nmin radial ribs are almost similar to miyatokoensis, though S. NOMURA and K. HATAI pointed that the number of radial ribs are fewer than miyatokoensis by counting the paired radial ribs as one. This species resembles Chlamys kaneharai (YOKOYAMA) (YOKOYAMA, 1926, p. 135, pl. 18, fig. 1, pl. 19. figs. 1, 2, 5-7) in general features. The characteristics of the hinge are other differences between kaneharai and the present one as already pointed by S. NOMURA and K. HATAI (1937, p. 128). This species is also related to Chlamys hericius (GOULD) (ARNOLD, 1906, p. 110. pl. 43, figs. 3. 3a) as also pointed by S. NOMURA and K. HATAI, which occurs as Recent in the Northern Pacific and as fossil in the upper Holocene, Pliocene and Pleistocene formations of California, but it differs therefrom in having a larger shell, divided radial ribs in which the middle part is most prominent, and several intercalary threads in the interspaces between the radial ribs. At the type locality of Ôtsutsumi in the northern border of Sendai, many specimens of this species were collected from a granule conglomerate cemented by tuffaceous sand or conglomeratic tuffaceous very coarse-grained sandstone. Here, abundant and intact valves are most common, as already noticed by M. NAKAMURA (1940, pp. 6-7), but many of the specimens are slightly or severely deformed. The associated fauna of this species are not always deformed, therefore, the writer considers that the shells of the present species are probably easily deformed. Miyatokoensis has been known to occur in the Ôtsutsumi and Utsuno formations in Miyagi Prefecture, and from the Ginzan formation at the Ginzan Hot Spring. Obanazawa-machi. Kita-Murayama-gun, Yamagata Prefecture. Though S. NOMURA and K. HATAI reported on occurrence of miyatokoensis matumori from the Nanakita formation at Matsumori, Izumi-mura, Miyagi-gun, Miyagi Prefecture, the writer failed to collect it from the same locality. The geological age of these formations are all Miocene, and the range of this species is considered to be Early Miocene. As noticed in early lines, the present species is closely related to Chlamys hericius (GOULD), and the former is restricted in the Early Miocene formation of the Northeastern Japan, the latter is known as fossil from the upper Miocene Santa Margarita formation, and Pliocene and Pleistocene formations of California and as Recent in the Northern Pacific. According to R. ARNOLD (1906, pp. 110-111,) the fossil shells of hericius are on the average smaller and appear to have slightly more scaly secondary riblets than the living species. Miyatokoensis is on the average small in size, and it may be the ancestral form of Chlamys hericius (GOULD), but further materials are necessary to settle this problem. MASUDA, K. 1956. On the Miocene Pectinidae from the Environs of Sendai; Part 9, On Pecten (Chlamys) miyatokoensis Nomura and Hatai. Transactions and Proceedings of the Palaeontological Society of Japan [N. S.], 24: 247-250, pI. 35. [p. 248, 249]
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Chlamys miyatokoensis (Nomura and Hatai); K. Masuda, 1956, On the Miocene Pectinidae from the Environs of Sendai; Part 9, On Pecten (Chlamys) miyatokoensis Nomura and Hatai, plate 35, figures 1-9.
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