Mizuhopecten tokyoensis hokurikuensis (Akiyama, 1962)
AKIYAMA, M. 1962. Studies on the phylogeny of Patinopecten in Japan. Science Reports of the Tokyo Kyoiku Daigaku [Section C - Geology, Mineralogy and Geography], 8 (74): 63-122, pls. 1-8, text-figs. 1-3. [p. 94, pl. 7, figs. 1-3, 5]
1956 Patinopecten hokurikuensis Ogose, 1956 (MS)
1962 Patinopecten tokyoensis hokurikuensis Akiyama, 1962
1962 Patinopecten tokyoensis hokurikuensis Akiyama, 1962
M. Akiyama, 1962, plate 7.
|
«Shell rather small, medium in thickness, suborbicular in outline, inquivalved, equilateral except for auricles, apical angle 105 degrees.
Right valve moderatly convex, with 7 round-topped radials which become lower towards the ventral margin; each radial on both submargins bipartite or tripartite but the rest not divided; interspaces as wide as radials near the umbo but become narrower towards the ventral margin. Left valve rather flat, with 7 low, roof-shaped radials which become inconspicuous towards the ventral margin. Remarks. This subspecies is characterized by the right valve with 7 radials which are not divided and the left valve with 7 low, roof-shaped radials. Described specirnen. Omma, Sakiura-mura, Ishikawa-gun, Ishikawa Prefecture. Himi group. This specimen is one of the YOKOYAMA's collection and is deposited in the Geological Institute, Faculty of Science, Tokyo University. Comparisons. The present subspecies is allied to P. kimurai kimurai and P. murayarnai murayamai, but can be distinguished therefrom by less numerous and less elevated radials. P. tokyoensis tokyoensis and P. tokyoensis sematensis are much allied to the present species, but the latter can be distinguished by the right valve with radials not divided into riblets. Distribution. Himi group in Ishikawa Prefecture, Naganuma formation in Kanagawa Prefecture, and Higashi-Higasa and Umegase formations in Chiba Prefecture.» MASAHIKO AKIYAMA, 1962
|
«Patinopecten tokyoensis kimurai (Yokoyama), Otuka, 1935c, p. 887, pI. 57, fig. 212.
Patinopecten hokurikuensis Ogose (MS), 1956, p. 110. (fide Akiyama, 1962) Patinopecten taiwanus (Nomura), Shuto, 1960, p. 125, pI. 12, figs. 2, 10, 11, pI. 14, figs. 7, 15 (?). Patinopecten tokyoensis (Tokunaga), Aoki, 1960, pI. 34, figs. 10-11; Takayasu, 1961, pI. 1, figs. 23-24. Patinopecten (Patinopecten) tokyoensis hokurikuensis Akiyama, 1962, p. 94, pI. 7, figs. 1, 3, 5. Patinopecten (Patinopecten) tokyoensis hokurikuensis Masuda, 1962, p. 211, pI. 25, fig. 6, pI. 27, figs. 6-7. Patinopecten tokyoensis hokurikuensis Akiyama, Kaseno and lVlatsuura, 1965, pI. 9, figs. 1-2. Mizuhopecten tokyoensis hokurikuensis (Akiyama), Hayasaka, 1973, p. 102, pI. 1, figs. 1a-b. Description:
Shell large in adult, moderately thick, nearly equilateral, inequivalved and orbicular with large ears in outline. Shell nearly equal in height and length. Anterior dorsal margin slightly concaved and descending into well rounded dorsal end as like as posterior. Ventral margin well concaved. Right valve rather well inflated and sculptured with fine concentric growth line and 6 or 7 radial ribs; ribs are roundly topped on the umbonal area and gradually become to flat and wide to ventral margin. And surface sculptured also with some riblets, it is rather distinct on the both extremities. Left valve flat and sculptured with 6 or 7 narrowly elavated radial ribs and numerous fine concentric growth lines. The main radial ribs, three, in number occupying central part of disk are distinct. Hinge line long and nearly straight. Ears trianguler in outline and nearly equal in size with vvide and shallovv byssal notch. Six radial threads on the anterior ear are sculptured. Cardinal crura is simple and shallow resirifer pit is setted. Inner surface is folded, corresponing to surface sculpture. Remarks: Mizuhopecten tokyoensis (Tokunaga, 1906) has been divided into three subspecies as tokyoensis s.s., tokyoensis sematensis (Akiyama, 1962) and M. tokyoensis hokurikuensis (Akiyama, 1962). According to Akiyama (1962) and Masuda (1962), their subspecies are distinguished from each other in the convexity of shell, the shape of radial ribs, presence or absence of riblets on the surface of right valve. The writer agrees with the classifications of Akiyama (1962) and Masuda (1962) concerning subspecies. Foutunatelly, many well preserved specimens were collected from several exposures of the Omma Formation, and examined from the view point of ontogenetic study. The result of observation on some specimens collected are shown in the follovving lines. Right valve: (shell length in mm) 3 mm: Shell flat, and few threads on the ears become distinct. 5.6 mm: Radial ribs on the shell surface indistinct. 7.0 mm: Three roof-like elevated radial ribs become distinct. 9.5 mm: Four radial ribs with few riblets are seen. Left valve 5.6 mm: Five or six radial threads can be observed on the shell surface. Generally, the ratio of height to length of disk is constant, and apical angle is variable from 90° to 110°. The largest one attains more than 170 mm in length, 150 mm in height and about 100 mm in hinge length among the specinlens from the Omma Formation. The present subspecies is one of representatives of the Omma fauna from its geological and geographical distributions. Locality: Abundant specimens were collected fronl Loc. nos. KK-03, KK-21, KO-15, 22.» OGASAWARA, K. 1977. Paleontological Analysis of Omma Fauna from Tokoyama-Ishikawa Area, Hokuriku Province, Japan. The Science Reports of the Tōhoku University [2nd. Series -Geology], 47 (2): 43-56, pls. 3-22 [p. 98, 99]
|
Mizuhopecten tokyoensis hokurikuensis (Akiyama, 1962); K. Ogasawara, 1977, Paleontological Analysis of Omma Fauna from Tokoyama-Ishikawa Area, plate 8, figures 5, 5a, 5b; plate 9, figure 4; plate 10, figures 1a, 1b.
|
«Comparison and Affinity: – The present subspecies can be distinguished from tokyoensis (s. s.) by its more inflated right valve, prominent, round-topped radial ribs which are a little smaller number and usually remain undivided in central part of disc in the right valve, and by the left valve having nearly flat or rarely somewhat concaved shell in younger stage and the distinctly elevated, roof-like shaped radial ribs with no intercalary threads. However, sometimes it is difficult to distinguish the present one from tokyoensis (s. s.) in the case of the left valve and in the case of the younger stage of the right valve.
The present subspecies was first named by Ogose (1947), but it was not published. The present one is closely related to tokyoensis (s. s.) and its occurrence is restricted to the Early Pliocene, while the occurrence of tokyoensis (s. s.) is restricted to the Late Pliocene to Pleistocene. Moreover, the morphological characters of the present one are considered to be more primitive than those of the tokyoensis. Therefore, this one may be an ancestor of tokyoensis (s. s.). Recently Shuto (1960) illustrated Patinopecten taiwanus (Nomura) from the Pliocene Takanabe formation in Miyazaki Prefecture, southeast Kyushu. However, taiwanus consisting of only one immature right valve (IGPS, coIl. cat. no. 45007) from the Byoritsu formation in Formosa can be distinguished from the Takanabe specimens by having much compressed valve, smaller proportion of hinge-length and shell height and posterior auricle truncated behind at obtuse angle. Therefore, the writer considers that Shuto's specimens differ from Nomura's and may be a synonym of the present subspecies. It is noticed that Shuto's specimens are usually smaller in size than those of the northern regions.» MASUDA,
K. 1962. Tertiary Pectinidae of Japan. Science Reports of the Tohoku
University [2nd. Series - Geology], 33 (2): 117-238, pls. 18-27. [p. 212]
|
Patinopecten (s. s.) tokyoensis hokurikuensis Masuda, n. subsp.; K. Masuda, 1962, Tertiary Pectinidae of Japan, plate 25, figure 6.
|