Amussiopecten praesignis (Yokoyama, 1922)
YOKOYAMA, M. 1922. On a new species of Pecten from the Neogene of Japan. Journal of the Geological Society of Tokyo, 29 (250): 1-2. [p. 1, pl. 5, figs. 1-3]
1922 Pecten praesignis Yokoyama, 1922
M. Yokoyama, 1922, plate 5.
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«Shell large, rather thin, much compressed, orbicular, somewhat broader than high, a little inequivalve, the left valve being slightly flatter than the right, equilateral, radiately ribbed on the surface. The ribs are about sixteen in number, the lateral ones often splitting into two or more riblets, broad, low and flat, straight, longitudinaly striate, flattening toward the margins, the interspaces on the right valve are narrower than the ribs themselves, while on the left just the opposite is the case. Incremental lines more or less distinct, in some parts looking like striations. Ears subequal, the anterior on the right valve with the anterior margin somewhat wavy, and the corner acute, while the posterior ear is obliquely truncate behind; byssal notch hardly inbricated. Right valve 123 millim. broad and 109 millim. hight. Left valve 120 millim. broad and 110 millim. hight. Thickness 18 millim. of wich about 10 millim. belongs to the right valve.
So far as the outside characters of the right valve is concerned, this species has a striking resemblance to Pecten burdigaliensis Lamarck (Hörnes, Foss. Moll. Wien. Beck., Conchifera, p. 418, pl. LXV.) from the Miocene of Promontor near Pest, Hungary. But in the latter the anterior ear of the right valve is rounded at the corner and the byssal notch more distinct. It is much to be regretted that the locality of the japanese fossil is uncertain. But I doubt if it is not from the Musashino Formation of the neigbourhood of Tokyo.» MATAJIRO YOKOYAMA, 1922
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«Remarks:— The present species is characterized by the following features:— Shell large, medium in thickness, compressed, somewhat longer than high, equilateral except for auricles, inequivalve, right valve more convex than left valve; both valves radiately ribbed and forming an angle of about 110° at apex. Right valve moderately inflated, with 15 to 19, low, broad, more or less squarish, roundly flat-topped radial ribs and distinct, lamellated fine concentric growth lines on back of radial ribs and interspaces between radial ribs; radial ribs broader than their interspaces, tend to become gradually obsolete towards ventral and lateral margins, sometimes with faint, fine longitudinal striae near ventral and lateral margins but recognized by reflected light only; radial ribs near submargins much lower than those of central part of disc, tend to divide into two parts towards ventral margin; interspaces between radial ribs sometimes with faint, fine longitudinal striae recognizable by reflected light; hinge line forms an angle of about 160° at beak; auricles medium in size, subequal to each other, with rather distinct, lamellated, fine concentric lines and very faint, fine radial threads; anterior with wide and shallow byssal notch, somewhat folded near submargin; hinge with prominent cardinal crura with fine provinculum, wide and deep resilial pit provided with faint, fine lateral ridge, rather distinct auricular crura which extend along inner margins of auricles and terminate distally in a rounded, more or less oblong denticle at each extremity of hinge. Left valve nearly flat in younger stage but tends to become slightly inflated with growth, with low, round-topped radial ribs and distinct, lamellated, fine concentric growth lines; radial ribs narrower than their interspaces but sometimes nearly equal to their interspaces in breadth at ventral margin; radial ribs near submargins slender, very low, with faint, fine intercalary threads; submargins somewhat more elevated than disc surface in younger stage; auricles subequal, sculptured with rather distinct lamellated concentric lines; hinge line straight; hinge with auricular crura which terminate distally in a rounded denticle at each extremity. Interior surface with prominent paired internal ribs running from upper part of disc to ventral margin.
Comparison:— The present species is closely related with Amussiopecten iitomiensis (OTUKA) described from the Miocene Shizukawa Formation in Yamanashi Prefecture, but iitomiensis can be distinguished from the present one by its rather smaller number of radial ribs which do not usually bifurcate and narrower interspaces compared with those of praesignis. Also Amussiopecten singkirensis (MARTIN, 1903) described from the Pliocene of Java, Indonesia, can be distinguished from the present one by its rounded radial ribs on right valve and roof-shaped radial ribs on left valve. O'HARA and ITO (1980) described the present species from the Miocene Senhata Formation, Chiba Prefecture, but their specimens differ from praesignis in having rounded radial ribs. Dimensions (in mm):-- Occurrence:— Loc. 5 (Abundant).
Geologic distribution:—Dainichi, Hamaishidake and Shirahama Formations, Shizuoka Prefecture; Kurotaki Formation, Chiba Prefecture; Ananai and Koe Formations, Kochi Prefecture; Takanabe Member of Koyu Formation, Miyazaki Prefecture; Nakoshi, Shinzato and Maja Formations, Okinawa Prefecture; Nakazato Formation, Kanagawa Prefecture; Tajima Formation, Kagoshima Prefecture; Off Mishima Island, Yamaguchi Prefecture; Byoritsu Formation, Taiwan. Geologic range:— Pliocene.» 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. 20, 21]
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Amussiopecten praesignis (Yokoyama, 1922); Sato, Y., K. Masuda & T. Shuto, 1986, Pelecypod fauna of the Shimajiri Group in Miyako-jima, Okinawa Prefecture, Japan, plate 2, figures 3a, 3b.
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