Tosapecten suzukii nabaensis Nakazawa, 1952
NAKAZAWA, K. 1952. A study on the pelecypod-fauna of the Upper Triassic Nabae group in the northern part of Kyoto Prefecture, Japan. Part 1. Pectinids and limids. Memoirs of the College of Sience, University of Kyoto [Series B -Geology and Mineralogy], 20 (2): 95-106, pls. 7-10. [p. 9, pl. 8, figs. 1, 2, 5]
1952 Tosapecten nabaensis Nakazawa, 1952
1952 Tosapecten suzukii forma distincticostatus Nakazawa, 1952
1952 Tosapecten suzukii forma distincticostatus Nakazawa, 1952
K. Nakazawa, 1952, plate 8.
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«Description: – All specimens are remarkably deformed, but their general outline or aspect closely resembles Tosapecten suzukii (Kobayashi) (Kobayashi, 1931. Kobayashi and Ichikawa, 1949).
Shell conspicuously inequivalve, nearly equilateral, suborbicular, postero- and anterodorsal margins slightly arcuate, ventral margin subcircular. Right valve moderately convex; anterior ear larger and about one and a third width of posterior, convex along anterodorsal margin; byssal sinus distinct, deep, particulary in the young stage; posterior ear trigonal, a little rolled up with linear posterior margin. Surface sculptured with 13 to 15 ribs narrower than their interspaces, gradually weakended towards anterior and posterior margins; at posterior side corresponding to submargin of left valve, 2 or 3 secondary ribs are inserted, along anterior margin 1 or 2 striae are observed. Left valve almost flat or slightly convex; anterior ear larger than pesterior one, flat, trigonal wich almost straight anterior margin, curving forward along disk, but not sinuous; posterior ear slightly concave, nearly right-angled triangular; submargins at both sides of disk distinct, anterior one remarkably inflated with a disctinct furrow and a riblet inside, posterior one not so conspicuous but wider. Ornament consists of round-topped primary and secondary ribs, 6 to 8 secondary ribs alternating with 8 to 9 primaries; at posterior half, secondary ribs strengthened almost as strong as primaries and closely set with them; furthermore, a few tertiary striae are often observed here. Surface covered with feeble dense growth-lines, distinct on both ears. Ligament pit triangular, small. Dimensions :
Comparison:– This species shows a close affinity to Tosapecten suzukii (Kobayashi) from Upper Triassic formation of Sakawa in Shikoku, but differs from the latter in obscure submargin of right valve and less number of ribs of both valves, i. e, nabaensis possesses 8 to 9 ribs on right valve except along submargins, while 11 to 12 are present in suzukii. In the ornamentatlon of left valve the present species is similar to T. suzukii var. paucicostatus Kob. & Ichi. and T. suzukii var. inflatus Kob. & Ichi., but differs from paucicostatus in more numerous ribs, and from inflatus in less convex shell and development of secondary ribs. (Kob. & Ichi., 1949).
Occurrence: – Middle and upper part of N3 bed, Nishimitsumatsu and Nabae. (Loc. No. N-201, 202, 207, 212 to 216. Reg. No. JM. 10004-8 except 10007) forma distincticostatus
Pl. VIII, Figs.6-8. is represented by attached valves, i. e., strongly convex right valve and nearly flat left one, and distinguished from nabaensis by uniformly developed and regularly arranged radial ribs and strong secondary ribs of the left valve.
The right valve resembles closely that of nabaensis. The medial ridge of the right valve and both of the strongly inflated submargins of the left valve may be caused by deformation. Occurrence: – Upper part of N3 bed, Nishimitsumatsu. (Loc. No. N-201, Reg. No. JM 10007. a, b, c).» KEIJI NAKAZAWA, 1952
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