Mizuhopecten yokoyamae Masuda, 1962
MASUDA, K. 1962. Tertiary Pectinidae of Japan. Science Reports of the Tohoku University [2nd. Series - Geology], 33 (2): 117-238. [p. 215, pl. 25, figs. 2, 3; pl. 26, fig. 7; pl. 27, fig. 12]
1962 Patinopecten yessoensis yokoyamae Masuda, 1962
K. Masuda, 1962, plates 25, 26, 27.
|
«Description: — Shell moderately thick, orbicular, equilateral except for auricles, right valve more convex; valves radiately ribbed, apical angle about 105º. Right valve moderately inflated, with 25 to 30, elevated, more or less squarish, flatly round-topped radial ribs, fine concentric growth lines and obtuse network; radial ribs nearly equal to or a little narrower than their interspaces, rarely divide into two riblets by a shallow longitudinal furrow ventrally; interspaces between radial ribs deep, round-bottomed, with fine, lamellated concentric lines; auricles moderate in size, subequal to each other; anterior with wide and shallow byssal notch, a few, faint, fine radial threads and concentric lines; posterior one similar to anterior in sculpture except for obtuse network; hinge with rather distinct, simple cardinal crura and wide and deep resilial pit with rather distinct lateral ridges. Left valve slightly inflated, with rather distinct, fine radial ribs, fine concentric growth lines, and rather distinct network; radial ribs narrower than their interspaces, more or less sharp near beak but tend to become more or less rounded ventralIy; auricles subsequal to each other in size, sculptured with several, fine radial threads, concentric lines and obtuse network. Interior surface more or less folded corresponding to external sculpture, but rather distinctly folded near ventral margin.
Dimensions (in mm.): --
Comparison and Affinity: — The present subspecies is distinguishable from yessoensis (s. s.) by its larger number of elevated, squarish radial ribs which are nearly equal to or a little narrower than their interspaces. However, it is rather difficult to distinguish the left valve of the present one from yessoensis (s. s.), though the radial ribs of the present one are somewhat more elevated and distinct than those of yessoensis (s. s.). Also this one is related to Patinopecten poculum (Yokoyama), but Yokoyama's species differs from yokoyamae by the greatly inflated right valve, larger auricles, larger number of and more elevated, flat-topped radial ribs of the right valve compared with those of yokoyamae and by the left valve having nearly flat or more or less concaved shell, distinctly elevated squarish radial ribs.
Remarks: — Yokoyama's Pecten kurosawaensis from the Onma formation in Ishikawa Prefecture differs from the Pecten kurosawaensis from the Sasaoka formation in Akita Prefecture. However, it is considered that the kurosawaensis from the Sasaoka formation at Kurosawa is a synonym of Patinopecten poculum (Yokoyama). Type locality, Geological formation and Age: — Left cliff of the Chikagawa River, about 200 m. from the sea shore at Chikagawa, Mutsu City, Aomori Prefecture (Lat. 41°11'01" N., Long. 141°16'36''E.). Hamada formation. Early Pliocene. Distribution: — Hamada and Narusawa formations, Aomori Prefecture; Onma formation, Ishikawa Prefecture; Nishiyama and Haizume formations, Niigata Prefecture; Sasaoka formation, Akita Prefecture; Setana formation, Hokkaido: all Early Pliocene in age. Occurrence: — Common in the fine- to medium-grained sandstone of the Hamada formation; common in the medium- to fine-grained sandstone of the Onma formation.» KÔICHIRÔ MASUDA, 1962
|
«The present species was stablished by Masuda in 1962 upon the characteristic form of ribs om the right valve wich are squarish, 25 to 30 flately-topped. This subspecies is distinguished from Mizuhopecten yessoensis yessoensis (Jay) in having larger number of radial ribs. The present subspecies is also different from Mizuhopecten poculum poculum (Yokoyama) by its rather flat right valve, less number of radial ribs and smaller auricles.
According to Masuda (1962) the original specimens of kurosawaensis of Yokoyama (1926) from Akita City is conspecific with poculum of Yokoyama. The present writer agreees with Masuda's view. Yokoyama (1927) and Kaseno and Matsuura (1965) recorded kurosawaensis from the Omma Formation, but judging from the shell character, their species could be synonymous with the present subspecies yokoyamae. Remarks: The present subspecies has been reported from the following formations; Setana and Tomikawa formations in Hokkaido, Hamada and Narusawa formations in Aomori Prefecture, Sasaoka Formation in Akita Prefecture, Nishiyama and Haizume formations in Niigata Prefecture, and Omma Formation in Ishikawa Prefecture.» OGASAWARA, K. 1977. Paleontological Analysis of Omma Fauna from Tokoyama-Ishikawa Area, Hokuriku Province, Japan. Science Reports of the Tohoku University [2nd. Series -Geology], 47 (2): 43-56. [p. 96, 97]
|
Mizuhopecten yessoensis yokoyamae (Masuda, 1962); K. Ogasawara, 1977, Paleontological Analysis of Omma Fauna from Tokoyama-Ishikawa Area, plate 8, figures 1, 2; plate 9, figures 3, 5.
|