Micronectes Ichikawa & Maeda, 1958
ICHIKAWA, K. & Y. MAEDA. 1958. Late Cretaceous pelecypods from the Izumi group. Part 2. Orders Taxodontida, Prionodontida, Dysodontida, Desmodontida and Adapedontida. Journal of the Institute of Polytechnics, Osaka City University, [Series G], 4: 71-122, pls. 3-7. [p. 95]
«Genus Micronectes ICHIKAWA & MAEDA, new genus
Type-species:— Micronectes bellaturus ICHIKAWA & MAEDA, new genus and species.
Diagnosis:-- Shell thin, small, slightly higher than long, moderately inflated, the left valve a little more inflated than the right, a little inaequilateral, slightly prosocline or nearly acline in the immature stages, then a little opisthocline in mature; the antero-dorsal margin of the shell body slightly concave; the postero-dorsal one nearly straight or slightly convex. Auricles comparatively well defined from the shell body, the anterior auricle larger than the posterior. Right anterior auricle subquadrate, with distinct byssal notch below; posterior auricles obtusely truncated posteriorly. Ligament internal; resilifer small, trigonal and nearly vertically directed. Cardinal crura well developed, extending from the umbo, immediately below and parallel to the cardinal axis. Auricular crura and internal ribs absent. Surface of both valves nearly smooth, with regular, broad, mostly flat-topped concentric ribs, separated by very narrow, sharp and regular incised concentric striae and fine radial lines, the latter of which are generally confined to the lateral sides of the shell body and to the auricles where they are commonly more prominent. Besides, exceedingly fine, microscopic, densely set, discontinuous or vermicular, radiating lines on the entire surface of the shell, which are visible under magnification on somewhat eroded specimens. These exceedingly fine lines are fanwise arranged and cross the radial lines, mentioned above, at the subrrargins of the shell body, thus giving the divaricating appearence of the radial elements.
Remarks:— The microscopic discontinuous radiating lines on the subexternal moulds is most certainly a surface expression of the radial crossed-lamellar structure of the upper ostracum. The present genus is monotypic, so that further remarks on this genus will be included in the description and discussion of the type-species.
Comparison:— The present new genus is closely related to Camptonectes (AGASSIZ) MEEK, 1864 (type-species: Pecten lens J. SOWERBY, 1818, Late Jurassic), but differs from the typical members of. Camptonectes in having the well developed cardinal crura and the microscopic discontinuous radiating striations instead of the macroscopic, often punctate Camptonectes-striations. Besides, the representative of the present genus is comparatively small.
In the above-mentioned respects the present genus may be better compared with Eburneopecten CONRAD, 1865 (type-species: Pecten (Eburneopecten) scintillatus CONRAD, 1865 Eocene, Mississippi etc.). Eburneopecten was generally considered to be a synonym of Camptonectes. STEWART (1930, p.122), however, pointed out that Eburneopecten is apparently the first valid name for small pectinids without the long internal rays. HARRIS (in HARRIS & PALMER, 1946, pp.33-35), NORTH (1951, p. 235) etc. recognized Eburneopecten. Recently STENZEL etc. (1957) discussed this genus in detail and pointed out the difference between Eburneopectem and Camptonectes. According to them, Camptonectes has fine but macroscopic striae or riblets with punctate interspaces; microscopic striae and non-punctate interspaces characterize Eburneopecten, which therefore has a smooth and polished surface. Besides, it may be remarked here that the cardinal crura are well developed in Eburneopecten, while it is obsolete (COX, 1952, p.22) in Camptonectes. Species of the former genus are comparatively small (to 36 mm long, according to STENZEL etc., 1957), while some species of the latter genus are quite large, the shell-length being even 132 mm in C. giganteus ARKELL (1930, p. 100). Thus Eburneopecten has much in common with the new genus here concerned. However, Micronectes differs from Eburneopecten in the more inflated and more inaequilateral, that is, a little opisthocline shell form and in the presence of widely spaced, regular incised concentric striae on the surface. The holotype of the type-species of Eburneopecten may have apparently an opisthocline obliquity, so far as the original illustration is concerned (CONRAD, 1865a, pl. 10, fig. 4). But better illustrated topotype specimens in STENZEL etc. (1957, pl. 7, figs. 16-19) do not show so inaequiiateral shell form as the poor original illustration of the holotype may suggest. Some specimens of the type-species of Eburneopecten appear to have the auricular crura (STENZEL etc., 1957, pl.7, flg. 17), whereas the auricular crura are absent in Micronectes. Eburneopecten has microscopic divaricating lines which are generally conflned to the marginal areas and more pronounced on the auricles. Radiating sculpture of Micronectes is composed of two elernents; the one is true (not divaricating) very fine radial lines, restricted to the submargins of the shell body and more pronounced on the auricles; the other is much finer, microscopic, discontinuous fanwise radiating lines, which themselves are sometimes bifurcating and cross the radial lines at the submargins, giving there divaricating appearence of the radiating sculpture. This difference in the radiating elements of the sculpture may not, however, be essential. There are some Triassic and Jurassic srnooth pectinids which are tentatively referred to Camptonectes, but lack macroscopic Carnptomectes-striations (vide lCHIKAWA, 1954, pp. 192-193; HAYAMI, 1957, pp. 122-124). They are, however, not intirnately related to the present genus. The typical representative of the genus Pseudamussium (discussion on the authorship and the type-species of Pseudamussium, see NORTH, 1951) has the Carnptonectes-striations, according to DALL (1898, p. 751), who took P. hybridus GMELIN as the type at least in the same page. Its sculpture is, however, discrepant on the two valves, the left valve being radiaily and the right valve concentrically feebly sculptured (DALL, 1898). Furthermore, it has auricular crura and its shell body is apparently more symmetrical. Thus Micronectes is distinct also from the typicai representative of Pseudamussiurn. Besides, there are a number of genera and/or subgenera of smooth or nearly smooth pectinids, which appear to be closely related to Pseudarnussiurn or Palliolum MONTEROSATO, 1884 (type-species: Pecten testae PHILIPPI). From most of these genera (see VERRILL, 1897; THIELE, 1934, etc.) Micronectes can be distinguished by the comparatively inflated left valve and by the slightly opisthocline, less symmetrical shell form, in addition to its incised concentric sculpture. Judging from the slightly opisthocline shell form with more or less produced antero-ventral part of the shell body, Late Cretaceous Micronectes may be phylogenetically more closely related to the typical members of Carnptonectes than to the more symmetrical camptonectids, the latter of which are not uncommon in the Late Cretaceous. From the former type of Camptonectes may be derived the genus Micronectes through the diminution of the Carnptonectes-type of the shell structure of the upper ostracum. Incidentally it may be mentioned that an exceedingly fine, sometimes bifurcating, radial shell structure is known also in some specimens of Palliolum (Delectopecten) from the Japanese Neogene (UTASHIRO, 1958, pl. 14). It is interesting that the radial structure in these younger forms is still finer than that of Cretaceous Micronectes. STEWART (1930, p. 118), who established the subgenus Delectopecten, remarked: "It may be that Delectopecten will eventually be placed near Camptonectes —" However, it is not yet certain whether the Cenozoic and recent pectinids with microscopic "Camptonectes-striations" are phylogenetically closely related to the Mesozoic camptonectids. Systematic positions, within the superfam. Pectinidae, of Camptonectes, Micronectes and other genera with "Camptonectes-striations", either macroscopic or microscopic, are not clear. In this paper Micronectes is provisionally placed under the Pectinidae, until more will be known of the phylogeny of the camptonectids. Distribution: — Late Cretaceous (Campanian and Maestrichtian), Japan.»
KOICHIRO ICHIKAWA & YASUO MAEDA, 1958
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Micronectes bellaturus Ichikawa & Maeda, new genus and specie; K. Ichikawa & Y. Maeda, 1958, Late Cretaceous pelecypods from the Izumi group. Part 2. Orders Taxodontida, Prionodontida, Dysodontida, Desmodontida and Adapedontida, plate 5,figures 13-17.
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