Radulonectites Hayami, 1957
HAYAMI, I. 1957. Radulonectites, a new pectinid genus, from the Liassic Kuruma group in central Japan. Transactions and Proceedings of the Palaeontological Society of Japan [N. S.], 27: 89-93, pI. 16. [p.89]
«Genus Radulonectites HAYAMI, new genus
Type species:-- Radulonectites japonicus HAYAMI, n. sp., Lias, Central Japan.
Diagnosis:— Shell highly inequivalve, inequilateral in right valve, acline or slightly opisthocline in mature, more or less higher than long; left valve more convex than right; antero-dorsal margin of shell-body profoundly sinuated in right valve, but the sinuation is shallow in left; byssal notch very deep; auricles well defined in each valve; right anterior one protruded forwards as in Chlamys, while others are subtrigonal and obtusely truncated at the extremities; almost straight radial grooves appear at first in middle stage, never bifurcate and gradually strengthened; umbonal region almost smooth except for faint concentric lines of growth.
Remarks:— Radulonectites is characterized by the Pleuronectites-like outline and radial grooves in adult stage.
In the smooth umbonal area it is sometimes similar to Camptonectes MEEK (1864), flourished in the jurassic and Cretaceous, but flabellate "Camptonectes-striations", curving outwards and often punctated, are absent in this genus. Its radial grooves are profound, nearly straight and widely spaced, if compared with the "Camptonectes-striations". As to the outline, it differs from Camptonectes (s. s.) or non-striated "Camptonectes" in the more inequilateral shellbody and deep byssal notch. Length is almost equal to or even larger than height in that genus, but the former never exceeds the latter in this genus. In Pecten (Camptonectes) lens LEANZA (non SOWERBY) (1942) radial striations are almost straight and distributed on the antero-ventral side. In outline it is so similar to the type species that it is referable to Radulonectites. In this type species the intervals among radial grooves look like Chlamys radial costae. but a close examination soon proves that the smooth surface is carved by radial grooves which appear in middle stage and develop in mature stage. Incidentally. radial ribs, if present, diverge from the very umbones, and shells are almost equilateral exclusive of auricles in many species of Chlamys. In outline the type species is fairly similar to Pleuronectites laevigatus (SCHLOTHEIM) (PHILIPPI, 1900; DIENER, 1925; SCHMIDT, 1928) from the Lower Muschelkalk in Germany, which has also a highly inequivalve shell. profound byssal notch and sigmoidal antero-dorsal margin of right valve. In that species, however. the surface is quite smooth except for fine concentric lines of growth. Pleuronectites laterstriatus (PHILIPPI) (1899; SCHMIDT, 1928). also from the Lower Muschelkalk, has a left valve of similar outline and fine radial striae in the antero-ventral side. That species was referred to Streblopteria MCCOY by PHILIPPI (1899) and is regarded as an abnormal species of Pleuronectites by AMANO (1955). Streblopteria MCCOY (1851) was thoroughly discussed by NEWELL (1937) as a late Palaeozoic genus or the Aviculopectinidae. Judging from the well defined auricles, deep byssal notch and literally partial ornaments, laterestriatus is referable to neither Streblopteria nor Pleuronectites but to this genus. Streblochondria NEWELL (1937) is characterized by the lattice ornamentation in the umbonal area, nearly equivalve and poorly defined posterior auricles, although it looks sometimes close to this genus in outline. Unfortunately, neither hinge structure nor musculature is known in the type species. Therefore, it is indeterminable directly from internal characters whether this genus belongs actually to the Pectinidae or to some other more primitive family. Ctenolia are often met with in Pleuronectites, and a clear inner resilifer is illustrated in P. laevigatus by SCHMIDT (1928). Because Pleuronectites seems the most intimate relative to this genus among the above-mentioned Palaeozoic or Mesozoic pectinids, this may belong to the Pectinidae. List of species:--
1. Radulonectites japonicus HAYAMI. Liassic, Central Japan. 2. Pecten (Pleuronectites) laterestriatus PHILIPPI (1899; SCHMIDT, 1928), Lower Muschelkalk. Germany. 3. Pecten (Camptonectes) lens LEANZA (1942), Liassic. Piedra Pintada. The following species are similar to this genus, though the generic references cannot be warranted.
4. "Chlamys" kakanuia MARWICK (1956), Hokonui System. New Zealand.
5. Chlamys sp. DIETRICH (1933, pl. 8, fig. 130). Tendaguruschichten. S. E. Africa. 6. Chlamys sp. DECHASEAUX (1936, pl. 4, fig. 6), Bajocian, Paris Basin.» ITARU HAYAMI, 1957
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Radulonectites japonicus Hayami, new species; I. Hayami, 1957, Radulonectites, a new pectinid genus, from the Liassic Kuruma group in central Japan, plate 15, figures 1-7.
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