Propeamussium awajense 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.
1958 Propeamussium awajense Ichikawa & Maeda, 1958
K. Ichikawa and Y. Maeda, 1958, plate 5.
|
«Description — Left valve: shell small, more or less inflated, a little higher than long, subequilateral except for auricles; the anterior side slightly longer than the posterior. The antero-dorsal and the postero-dorsal margins of the shell body nearly straight, the ventral margin evenly rounded. Umbo subcentral, nearly orthogyrous, hardly rising above the hinge-line. The umbonal angle about 90º ±. Auricles unequal; the anterior one larger, better defined from the shell body than the posterior, its anterior margin nearly verticai, but slightly convex; the posterior auricle more or less obtusely subtruncated with the slightly concave posterior margin. Dorsal margins of the two auricles in a straight line. Cardinal crura distinct. Auricular crura present, but short. Surface of the shell body sculptured with very fine, equally spaced concentric lamellae and stitt finer close-set radial striae. The anterior auricle with fine lattice sculpture. Nine short, narrow and widely spaced internal ribs beginning at considerable distance from the umbo, nearly uniform in strength through growth, and ending at the radial position well above the ventral margin.
Material: — Three left valves are at the Division of Geoscience, Osaka City University. Two of them, including the holotype, are represented by the internal mould and a part of the external mould. The holotype (OCU MM 234) came from the Shichi shale at loc. 6 north of Hansanji. A right valve of propeamussiids is also at hand (OCU MM 258); it came from the Shichi shale at loc. 51 Hansanji. But it is not certain whether it belongs to the present species. It is apparently longer than high. This broad outline must be a result of the secondary deformation, if it really belongs to this species. In the foregoing descrlption this right valve was not taken into account.
Measurement (in mm):
Observations and remarks: -- The radial striae on the anterior ear are well exhibited in a specimen from the Minato shale (OCU MM 235). In this specimen, however, the radial striae on the shell body are not preserved, while the concentric lamellae are distinctly visible.
The aforementioned right valve is fairly flat and has nine internal ribs as in the left valve of P. awajense, but the internal ribs reach until the ventral margin. The anterlor margin of the anterior auricle is moderately and evenly curving downward and backward. The byssal notch is distinct but not very deep. Comparison: -- Although a definite right valve of this species is not known at present, this species is sufficiently distinct from Propearnussium cowperi var. yubarense YABE & NAGAO, 1928, which is the only species of propeamussiids hitherto known from the Japanese Upper Cretaceous. Campared with the lzumi species, P. yubarense YABE & NAGAO (1928, p. 88, pl. 16, figs. 17-19; NAGAO, 1932, p. 38, pI. 6, figs.7, 8, 12, 13; NAGAO, 1938, p.129, pl. 16, fig.3) from the Upper Yezo group of Hokkaido and Saghalien is much less inflated, becomes larger and a little broader, with the larger umbonal angle; the internal ribs are distinctly longer; its left valve has subequal auricles and bears, on the shell body, radial riblets with narrower interstitial grooves (NAGAO, 1932), whereas radial striae on the left valve of awajense are nearly as narrow as, or even narrower than, the interspaces.
Recently Prop. cowperi var. yubarense was reported by AMANO (1956, p. 73, pl. 1, fig. 12,) from the Goshora (= Goshonoura auct.; fide MATSUMOTO et al., 1954, p. 145) group of Kyushu. As remarked by AMANO, the Goshora form has the smaller umbonal angle than the typical yubarense from Hokkaido, but its internal ribs are quite long as in yubarense. Thus P. awajense is distinct also from the Goshora form. P. yubarense YABE & NAGAO is perhaps referable to Pseudopalliorum ÔYAMA, 1944 (p. 244) (type-species: "Pecten" interradiatus GABB, 1869; Eocene or Oligocene?, vide GRANT & GALE, 1931, p. 233, California), which was established as a subgenus of Propeamussium. The more exact generic or subgeneric status of the Awaji species, here described, is uncertain. At least it does not belong to the subgenus Propeamussium (Propeamussium) in ÔYAMA sense (1944), because it has distinctly unequal auricles and the short internal ribs, which appear only in the later stages of ontogeny. In the last mentioned feature it is distinct also from Ctenamusium lREDALE, 1929 and its allided genera or subgenera. Occurrence -- Early Hetonaian.
Awaji Island: Shichi shale at loc. 6 north of Hansanji, loc. 51 Hansanji. Minato shale at loc.58 Shojigamaru, north of Nakano, Seidan-cho.» KOICHIRO ICHIKAWA & YASUO MAEDA, 1958
|