Nippononectes eleganus Tashiro, 1982
TASHIRO, M. 1982. A new pectinid genus Nippononectes from the Cretaceous of Japan. Memoirs of the Faculty of Science, Kochi University [Series E - Geology], 3: 1-6, pIs. 1, 2. [p. 4, pl. 1, figs. 1-9; pl. 2, figs. 12-14]
1982 Nippononectes eleganus Tashiro, 1982
M. Tashiro, 1982, plates 1, 2.
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«Material:— Holotype, KSG 2986, external mould of right valve, from Doiban of Odochi, Monobe Area, Kochi Prefecture; paratypes, KSG 2987-KSG 2991, external moulds of left valves; paratypes, KSG 2993 and KSG 2994, internal moulds of left valves; a paratype, KSG 2992, external mould of right valve; another paratype, KSG 2995, internal mould of right valve; the paratypes, from the same locality of the holotype.
Description:— Shell very small, roundly ovate, somewhat taller than length, moderately inflated; umbo small, located at nearly midpoint or slightly anterior than the mid-point of the valve length: apical angle about 65º; anterior and posterior margins nearly straight, slightly longer with about three fourths length of the valve, in posterior than in anterior; ventral
margin semicircular; hinge line straight and horizontal with about two thirds of the valve length; anterior segment of the line elongated with about twice to posterior one; both lateral margins of ears subverticaly truncated; byssal notch deep with 5 or more ctenolia; disk raindrop-like in outline, ornamented with two sorts of the ribs; one of them radiated, narrower than their interspaces, generally spinose on the marginal part of the later stage, roundtopped, with about 40 in left valve, 80 or so in right valve, stronger and broader in left valve than in right one; the other sort of ribs are divaricated striae, finely crowded on the interspaces of the radial ribs; growth line distinct, generally developed on the marginal part, finely laminated; ears ornamented within the same way as the disk, larger in anterior than in posterior; two pair of cardinal crurae distinct; a pair of the crurae small, appear on the upper half of either sides of trigonal resilifer; the ether pair of cardinal crurae weak, below the hinge line, parallel to cardinal axis; inner surface smooth except for an optical adductor scar which is indistinctly impressed and located at a little posterior than the central point of the inner surface; inner margin smooth. Observation:— The secondary radial ribs are very weak and rarely appear on the anterior and posterior lateral parts near the margins in the left valve. The divaricated striae are very fine and are about 16 in the distance of 1 mm on the ventral part of the mature specimens.
Comparison:— This resembles Nippononectes monobensis (op. cit.) in its small valve, but differs in its small apical and larger ears than those of N. monobensis. This species are discriminated from Nippononectes tamurai s. l. Tashiro (op. cit.) in its smaller valve, less developed secondary radial ribs and stronger and broader primary radial ribs than those of the Senonian species.
Occurrence:— Sandstone of the lower part of the Hibihara Formation of the Monobegawa Group, at Doiban of Odochi, Monobe Area, Kochi Prefecture; Lower Aptian.»
MASAYUKI TASHIRO, 1982
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