Mizuhopecten yessoensis pseudoyessoensis (Akiyama & Miyajima, 1960)
AKIYAMA, M. & K. MIYAJIMA. 1960. A New Species of Patinopecten and its associated fauna from the Nojima Formation, Kanagawa Prefecture, Japan. Science Reports of the Tokyo Kyoiku Daigaku [Section C - Geology, Mineralogy and Geography], 7 (61): 99-103, pI. 5, text-figs. 1-2. [p. 99, pl. 5, figs. 1-5]
1960 Patinopecten pseudoyessoensis Akiyama & Miyajima, 1960
M. Akiyama & K. Miyajima, plate 5.
|
«Description. Shell moderate in size and thickness, height nearly equal to length, inequivalved, equilateral except for auricles; both valves somewhat convex. Right valve with 18 flat-topped, rather low, radial ribs which are bipartite or nearly tripartite at both margins of the disc; interspaces narrower than ribs themselves; concentric striations are inconspicuous even at the ventral margin; both auricles are small; anterior auricle ornamented with weak radial threads and fine concentric lines, furnished with deep byssal area; posterior one truncated behind, with a few weak, radial threads and concentric lines; hinge with distinct cardinal crura and resilial pit provided with lateral ridges on both margins. Left valve with 20 elevated, round-topped, narrower, radial ribs which become obsolete or disappear near the ventral and lateral margins, and furnished with 4 periodic distinct growth lines and network sculptures only on the dorsal above 2nd periodic line; both small auricles similar to each other, sculptured with a few, weak, radial threads and fine, concentric striations; hinge corresponding to the right valve; smooth inner surface impressed by a rather small muscular impression and a mantle line.
Remarks. This new species is characterized by the right valve provided with 17-20 flat-topped, radial ribs which usually split into 2 parts by a shallow furrow near both lateral margins. The left valve is characterized by the convex shell with 16-21 round-topped, radial ribs which disappear near the ventral margin and by the surface network sculpture on the upper part of the disc.
KINOSHITA (1949) reported the number of radial ribs of Patinopecten yessoensis (JAY) variable between 15 and 32 in the right valves and between 13 and 31 in the left ones and stated the most frequent value is 22 in the right and 20 in the left, while P. pseudoyessoensis AKIYAMA and MIYAJIMA, n. sp. has slightly less number of radial ribs as shown in text-fig. 1. Text-fig. 1. Frequency curve of the number of ribs in Patinopecten pseudoyessoensis AKIYAMA and MIYAJIMA, n. sp.
The abscissa: number of ribs, the spindle: number of specimens. The dotted line: right valve, the full line: left valve. The present new species is much allied to Patinopecten yessoensis living in the sea around Northern Japan, but the former can be distinguished therefrom by the disc with smaller auricles, the right valve with bipartite ribs near lateral margins and more convex left valve with smooth surface at the ventral margin.
Locality and horizon. The cliff near Oyato, Totsuka-ku, Yokohama City, Kanagawa Prefecture; Lat. 35°20'24'' N., Long. 139°33'40'' E. Nojima formation, Pliocene. Repository. The Geological and Mineralogical Institute, Tokyo University of Education, Reg. Nos. 5371 (holotype), 5372-5378 (paratype).» MASAHIKO AKIYAMA & KEIJI MIYAJIMA, 1960
|
«Remarks: – The present one is very closely related to yessoensis (s. s.), but it can be distinguished by the right valve having radial ribs which tend to divide into two or three riblets near the dorsal margins, the left valve with radial ribs which tend to become obsolete or disappear ventrally and the smaller auricles. As the general morphological features are quite similar to each other, the present one is placed in subspecific ranking of yessoensis and may be an allopatric form of yessoensis.
Type locality, Geological formation and Age: – Cliff near Oyato, Totsuka Ward, Yokohama City, Kanagawa Prefecture (Lat. 35°20'24"N., Long. 139°33'40"E.). Nojima formation. Early Pliocene. Distribution: – Nojima and Koshiba formations, Kanagawa Prefecture: both Early Pliocene in age.» 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. 215]
|
1926 [erratum: 1920]. Pecten (Patinopecten) yessoensis, YOKOYAMA: Jour. Coll. Sci., Imp. univ. Tokyo, Vol. 39, Art. 6, pl. 13, figs. 14-15.
1960. Patinopecten pseudoyessoensis AKIYAMA and MIYAJIMA: Sci., Rep. Tokyo Kyoiku Daigaku, Sec. C, No. 61, pp. 99-103, pl. 5, figs. 1-5. Remarks. This species characterized by the right valve provided with 17-20 flat-topped, radial ribs which usually split into two parts by a shallow furrow near both lateral margins. The left valve is characterized by the convex shell with 16-21 round-topped radials which disappear near the ventral margin and by the surface network sculpture on the upper half of the disc. It may be stated that the present species is a descendant of P. paraplebejus from the surface sculpture of valves.
Comparisons. The present species is much allied to P. paraplebejus and P. planicostulatus, but can be distinguished from paraplebejus by the right valve with bipartite radials and smaller auricles, and from planicostulatus by the right valve with folded byssal area and with more conspicuous radials. This species also resembles P. yessoensis living in the sea around Nothern Japan, but differs from yessoensis in the disc with smaller auricles, the right valve with bipartite radials near submargins and more convex left valve with srnooth surface at the ventral margin. Type locality. The cliff near Oyato, Totsuka-ku, Yokohama City, Kanagawa Prefecture. Nojima formation. Reg. no. 5371 (holotype), 5372-5328 (paratype). Distribution. Nojima formation and Koshiba formation in Kanagawa Prefecture. 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. 106]
|
Pecten yessoensis Jay; M. Yokoyama, 1920, Fossils from the Miura Peninsula and its inmediate North, plate 13, figures 14, 15.
|