Pecten natoriensis Matsumoto, 1930
MATSUMOTO, H. 1930. On the Marine Faunae of Three Fossil Zones of the Upper Miocene of Natori District, Province of Rikuzen. The Science Reports of the Tōhoku University [2nd. Series -Geology], 13 (3): 96-109, pls. 39, 40. [p. 104, pl. 40, figs. 10, 11]
«Pecten natoriensis, sp. nov.
PI. XL (11), Figs. 10 & 11. Shell medium-sized, rather thick-valved, swollen, inequivalve, the right valve being more convex than the left in younger stage, but becoming just the reverse in the adult, equilateral in the typical form except for the ears, subcircular, shorter than to as long as wide in the typical form; ribs prominent, rounded, few and stout at first, numbering six on the right valve and five on the left in the primordial condition, but soon becoming numerous ventrally by means of both bifurcation and intercalation; on the right valve, the primary bifurcation always precedes the primary intercalation and the secondary bifurcation the secondary intercalation, while on the left valve, the primary, and often also the secondary, intercalations precede the primary bifurcation. The right valve is regularly convex, with the anterior ear longer and larger than the posterior; byssalnotch shallow; ribs broader than the interstices; the multiplication of ribs ventrally is caused essentially by their bifurcation and only auxiliarily by intercalation; the bifurcation goes up to the tertiary order, and the intercalation rarely to the secondary order, the secondary intercalation occurring in the valleys arisen as a result of the primary bifurcation. The left valve in the adult is convex, abruptly bent at a certain distance from the beak, so as to form a flat platform proximally; the five primordial ribs form a boss at this bending; the anterior ear is more or less longer and larger than the posterior, provided with a shallow byssal notch; primordial ribs narrower than the primordial interstices; multiplication of ribs ventrally caused essentially by intercalation and only auxiliarily by, bifurcation; the bifurcation goes up only very rarely to the secondary order, with the exception of the foremost and the hindmost primordial rib; all the intercalaries occur only in the four primordial interstices; the primary intercalaries soon acquire almost the same strength as the primordial ribs and divide ventrally just as the same; the secondary intercalaries, which are one or two in number on either side of the primary, are weaker. The outer surface of both valves is very finely concentrically striated; the ears of the same are radiately ribbed; the inner surface is folded, corresponding to the ribs and interstices of the outer.
One right valve at hand measures 57 mm. in length, 61 mm. in width and 11 mm. in convexity; one left valve 64.5 mm. in length, 66 mm. in width and 20 mm. in convexity; and another left valve 55 mm. in length, 57.5 mm. in width and 13.5 mm. in convexity. Abundant. Natural position: The present sepecies appears to stand near P. cosibensis YOKOYAMA so as to be nearly ancestral to it. Age: Upper Miocene.» HIKOSHICHIRÔ MATSUMOTO, 1930
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H. Matsumoto, 1930, plate 40.
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