Amusium pleuronectes okinawaensis Masuda, Sato & Shuto, 1986
SATO, Y., K. MASUDA & T. SHUTO. 1986. Pelecypod fauna of the Shimajiri Group in Miyako-jima, Okinawa Prefrecture, Japan. Memoirs of the Faculty of Sience, Kyushu University, [Serie D -Geology], 26 (1): 1-49, pls. 1-5. [p. 18, pl. 2, fig. 11; pl. 3, figs. 1, 2]
1986 Amusium pleuronectes okinawaensis Masuda, Sato & Shuto, 1986
Y. Sato, K. Masuda & T. Shuto, 1986, plates 2, 3.
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«? 1928b. Pecten (Amusium) japonicus (GMELIN). YOKOYAMA, Imp. Geol. Surv. Japan, Rep. no. 101, p. 95, pI. 13, fig. 12.
1933. Pecten (Amusium) pleuronectes (LINNAEUS). NOMURA, Sci. Rep., Tohoku Imp. Univ., 2nd Ser., vol. 16, no. 1, p. 60, pI. 3, fig. 23. ? 1958. Amusium japonicum. MAKIYAMA, Palaeont. Soc. Japan, Spec. Paper, no. 6, pI. 99, fig. 12. 1981. Amusium pleuronectes. MASUDA and OGASAWARA, Mem. Vol. Prof. M. OMORI, pI. 2, fig. 5. Holotype:— GK-L 11827.
Type locality:— Loc. 2. Ôura Formation. Diagnosis:— Shell generally large and thick. Cardinal and auricular crura large and distinct. Auricular crura terminate distally in a conspicuous oblong denticle. Internal radial ribs stout and its total number 20. Description:—Shell medium to large in size, medium in thickness, orbicular in outline, compressed, subequivalve, forming an angle of about 120° at apex. Right valve slightly inflated, smooth, with fine incremental lines; auricles medium in size, nearly equal to each other, a little rounded at anterior and posterior margins, with very shallow byssal notch below anterior auricle and with fine incremental lines; hinge line a little angulated. Left valve slightly inflated, with about 11, very low, faint, fine radial threads near beak and fine incremental lines; radial threads tend to disappear immediately downwards; auricles a little rounded at anterior and posterior margins, with fine incremental lines; hinge line straight. Hinge with conspicuous cardinal crura provided with fine provinculum, wide and shallow resilial pit and distinct auricular crura terminating distally in a conspicuous oblong denticle at each extremity. Interior surface with about 22, distinct internal ribs terminating distally near ventral margin. Comparison:— The present subspecies is closely related with Amusium pleuronectes (LINNAEUS), a Recent species of Southeast Asia (HABE, 1964), and it has hitherto been considered to be a synonym of pleuronectes (NOMURA, 1933). However, okinawaensis is obviously distinguished from pleuronectes by its rather large, moderately thick shell, shallow but distinct byssal notch below anterior auricle, distinct, thick internal ribs which are more prominent at lower half of shell and hinge with conspicuous cardianl crura having rather distinct auricular crura terminating distally in a distinct oblong denticle at each extremity. It is treated here as a subspecies of pleuronectes. Amusium hulshofi (MARTIN, 1909) from the Pliocene of Java, Indonesia, can be distinguished from the present one by its small shell and larger number of internal ribs which terminate at ventral margin. Also this new subspecies is distinguishable from Amusium iaponicum (GMELIN), a Recent species of Southern Japan, by its small, rather thick shell, rounded auricles with very shallow byssal notch below anterior auricle and smaller number of internal ribs. Dimensions (in mm):-- Occurrence:— Loc. 2. (Abundant); Loc. 4. (Abundant).
Geologic distribution:— Byoritsu Formation, Taiwan: ? Canguinsa Formation, Philippines. Geologic range:— Pliocene.» YOSHIO SATO, KÔICHIRÔ MASUDA & TSUGIO SHUTO, 1986
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«Amusium pleuronectes okinawaensis Masuda, Sato and Shuto in Sato, Masuda and Shuto, 1986, p.18, pl. 2, fig. 11; pl. 3, figs. 1 , & 2.
Remarks: Several imperfect specimens. The specimens are characterized by their slightly inflated, medium-sized, rather thin and smooth shell with about 15 pairs of distinct internal ribs which developed from upper part of disc to ventral margin.
The present one differs from Amusium japonicum (Gmelin), a Recent species of southern Japanese Islands, in having smaller number of internal ribs. Also Amusium pleuronectes okinawaensis from the Pliocene Shimajiri Group in Okinawa Prefecture, Japan, can be distinguished from the present one by its large and thick shell, and fewer number of internal ribs. The present one may represent an unnamed species but naming is withe Id until better specimens are collected. Occurrence: Late Miocene Kueichulin Formation (Wuchi section) KS 628, (Pitou section) PT 455, and (Mawutu section) KC 1314.»
MASUDA, K. & C. HUANG. 1990. Miocene Pelecypoda in the western foothills of northern Taiwan (Part II. Sistematyc description). Bulletin of National Museum of Natural Science, 2: 141-189, pls. 1-11. [p. 147]
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Amusium cf. pleuronectes okinawaensis Masuda,
Sato and Shuto, 1986; K. Masuda & C. Huang, 1990, Miocene Pelecypoda in the western foothills of northern Taiwan, plate 4, figures 8-10. |