Micronectes bellaturus 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. [pl. 5, figs. 13-17]
1958 Micronectes bellaturus Ichikawa & Maeda, 1958
K. Ichikawa & Y. Maeda, 1958,
plate 5. |
«Description: — Shell thin, small, to about 14 mm long, higher than long (length/height proportion: 0.81 to 0.95), suborbicular, a little inaequilateral, slightly prosocline or nearly acline in the immature stages, but a little opisthocline in mature, somewhat expanding antero-ventrally, moderately inflated, the left valve being slightly more inflated than the right. Umbo subcentral or slightly posterior to median in position, small, narrow, moderately incurved and nearly orthogyrous or slightly inclined forward. Umbonal angle about 90-100º. The antero-dorsal margin of the shell body a little concave, the corresponding submargin of the left valve distinctly costate, the ventral margin moderately rounded, the postero-dorsal nearly straight or slightly convex; hinge-line shorter than the length of the shell body. Dorsal margins of the two auricles of the left valve in a straight line, those of the right valve slightly angulated or concave upward. The anterior auricle larger than the posterior, being 1.2 to 2.0 times as long as the posterior, not producing beyond the anterior end of the shell body. Left anterior auricle well defined from the shell body, subtriangular in outline; its anterior margin a little arcuate, forming a nearly vertical angle with the hinge axis. Right anterior auricle subquadrate; its anterior margin convex and curving rapidly into the well developed byssal notch. Posterior auricles of the two valves fairly well defined from the shell body, obtusely truncated posteriorly. Resilifer trigonal and nearly vertically directed. A pair of cardinal crura, namely, narrow teeth-like transeverse ridges, running from the umbo to both sides immediately below and parallel to the cardinal axis. Sculpture consisting of regular flat-topped concentric ribs separated by very narrow, sharp incised concentric striae and very fine radial lines which are generally restricted to the submargins of the shell body and more pronounced on the auricles. Besides, microscopic, discontinuous, densely set radiating lines on the entire surface of somewhat eroded specimens. These lines suggest the radial crossed-lamellar structure of the upper ostracum, are divaricating and sometimes converging and cross the radial ljnes at the submargins, thus giving there the divaricating appearence of the radial elements.
Material: — About twenty specimens, both left and right valves, are at hand. They are mostly at the Division of Geoscience, Osaka City University. Two specimens from the Azenotani shale are on deposit at the Geological Insitute, University of Tokyo (MM 7738).
The holotype is a left valve (internal and external moulds) from the Azenotani shale at loc. 752 Kamatani-1, west of Azenotani, Izumi mountain-range (OCU MM 237) (Pl. V, Figs. 15a-c). Measurement (in mm):
Observations: — The characteristic crossed-lamellar structure of the ostracum are well impressed on the holotype. It was photographed very distinctly (Pl. V, Fig. 15b), owing to the large magnification and the oblique light, but is actually hardly discernible to the naked eye. In some other specimens the crossed-lamellar structure is suggested by very faint subradial threads.
The concentric ribs are quite narrowly spaced and mostly flat-topped, thus giving the shell smooth appearence. In some subinternal moulds the concentric sculpture appears as low concentric folds. However, these folds may not indicate the original feature of the very surface of the shell. Whether the ctenolia below the right anterior auricle are present, is not determinable at present. The length/height proportion is fairly variable among specimens. So far as the specimens now available are concerned, those from the Shichi shale are a little taller than those from the Azenotani shale. Comparison: — The present species much resembles "Pecten (Camptonectes)" hawfmanensis STEPHENSON (1941, p. 133, pl. 27, figs. 7-9) from the Maestrichtian Navarro group of Texas, which is nearly as large as the present species and has very fine and faint bifurcating radial sculpture, judging from the original description. However, this Navarro species is not so tall as the Izumi species. Unfortunately, nothing is known of the concentric sculpture of the Navarro species, so that it is not certain whether kaufmanensis belongs really to Micronectes.
"Pecten" occulte-striatus ZITTEL (1866, p. 109 [= p.33 of separate issure], pl. 17, figs. 6a-c) from the Gosau formation is also a comparable form, but is larger, provided with more arcuated and more inclined antero-dorsal margin of the shell body of the right valve and appears to lack fine radial lines on the submargins and auricles. The incised concentric sculpture is not assured also for this Gosau species. But exceedingly fine oblique lines are present, under magnifying glass, at least on the right anterior submargin of the shell body and on the left anterior auricle (ZITTEL 1866). The general shell form of the Gosau species is more like that of Micronectes than that of Eburneopecten. Incidentally it is mentioned that Pecten (Camptonectes) hectori WOODS (1917, p. 26, pl. 11, figs. 6-9; pl. 12, fig. 1) from the Upper Cretaceous of New Zealand bears some resemblance to P. occulte-striatus, as remarked by WOODS. Some Late Cretaceous camptonectids bear regular concentric sculpture, but none of them is safely referable to Micronectes. Camptonectes bellisculptus CONRAD, 1869 (p. 99, pl. 9, fig. 11) from the Upper Cretaceous of New Jersey may appear to resemble the present species. CONRAD noted that the corcentric lines are extremely thin and minute. They are separated by regular and wide interspaces. According to CONRAD, however, the interior hinge-line is crenulated in bellisculptus. This is not the case with the Izumi species. WHITFIELD (1885, pp.53, 54) and some others synonymized C. bellisculptus with "Pecten" burlingtonensis GABB, 1860. But the type specimen of the latter species has evidently a broader outline, judging from the illustration. In having the regular concentric sculpture the present species may be apparently compared also with some Late Cretaceous camptonectids such as "Pecten" burlingtonensis GABB in WADE (1926, p. 63, pl. 20, figs. 5, 6, 10, 11) and "Pecten" argillensis CONRAD in WADE (1926, p. 62, pI, 20, figs. 8-9), the latter of which is, according to STEPHENSON (1941, p. 132), more like "Pecten" bellisculptus CONRAD. The nature of the concentric sculpture of these species (in WADE) is, however, different from that of the Izumi species. In the former it is concentric ridges or lamellae with wide interspaces, whereas it is flat-topped broad ribs separated by very narrow incised concentric striae in the latter. In this respect and also in being larger than the Izurni species, these forms in WADE are nearer to the second of the three groups of Camptonectes in COX (1952, p. 22); this group is represented by C. annulatus (SOWERBY) from the Jurassic and comprises forms with erect concentric larninae at more or less regular intervals. It is, however, noteworthy that the cardinal crura are developed in "Pecten" burlingtonensis of the Latest Cretaceous (WADE, 1926, pI. 20, fig.5) as in the Izumi species. The so-called Camptonectes-striations are said to be exceedingly fine in that American species (WELLER, 1907). The concentric sculpture of Pecten (Camptonectes) bubonis STEPHENSON (1941, p. 131, pl. 21, figs. 3-6; 1955, p. 113, pl. 17, figs. 1-4) from the Maestrichtian Navarro group and the contemporaneous formations is more resembling that of the Izumi species than that of WADE's forms, mentioned above, do. But bubonis has evidently coarser Carnptonectes-striations and must be included within that group of Camptonectes, which is characterized by the comparatively symmetrical shell body. Occurrence: Hetonaian.
Izumi mountain-range: Azenotani shale at loc. 152 Kamatani-2, west of Azenotani (type locality), loc. 151 Azenotani-3, loc. 149 Azenotani-1, etc. Awaji Island: Minato shale at loc. 55 Kiba near Minaguchi, Seidan-cho. Shichi shale at loc. 51 Hansanji. Kitaama sandstone and shale at loc. 50 Mitsugawa.» KOICHIRO ICHIKAWA & YASUO MAEDA, 1958
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