Parvamussium kyushuense Shuto, 1960
SHUTO, T. 1960. On Some Pectinids and Venerids from the Miyazaki Group (Palaeontological Study of the Miyazaki Group - VII). Memoirs of the Faculty of Sience, Kyushu University, [Serie D - Geology], 9 (3): 119-149, pls. 12-14, text-figs. 1-14. [p. 127, pl. 12, figs 4, 7; text-figs. 7, 14]
1960 Parvamussium (Parvamussium) kyushuense Shuto, 1960
T. Shuto, 1960, plate 12,
text-figures 7-14. |
«Material.— Holotype: GK-L 4863, paratypes: GK-L 4708, 4864, 4865, 4866, 4867, 4868, 4869, 4870, and 4871. All specimens came from one and the same locality. The matrix is massive grey sandy siltstone.
Measurements.-- Diagnosis.— The shell is rather small in size, almost orbicular, a little higher than long, compressed, almost equilateral and inequivalve. The left valve is somewhat larger than the right one and has more produced ventral part which extend to some extent beyond the ventral margin of the left valve. The anterodorsal margin is slightly excavated and the postero-dorsal one almost straight or slightly convexed. They are almost equal in length and form an angle of about 105 degrees. The anterior, posterior, and ventral margins are regularly and broadly rounded. The exterior sculpture is the very fine incremental lines and the minute and irregular radial striae in the left valve, and the more distinct and regular concentric lines in the right valve.
The interior surface is ornamented with eight radial ribs, which start from the umbonal part and reach about three-quaters from the umbo toward the ventral margin in the lett valve and attain near that margin in the right valve. The interior ribs are weak in the young stage and become large and prominen as the shell grows. The hinge line short, about fourty percent of the shell-Iength. The ears are different in size; the anterior one is somewhat larger than the posterior one. The weak but apparent byssal notch is present at the upper pan of the antero-dorsal margin where the ear joins the dorsal slope in the right valve. Comparison.— Comparing with the type species of tee genus, the present new species has larger shell with smaller ears and less numerous inner ribs. Parvamussium (Parvamussium) rubrotinctum OYAMA (1951, p. 81, pI. 13, f. 8-10), a living species in south Japan, agrees with the present species in its size, but has lower shell with more numerous inner ribs and larger apical angle. P. (P.) maorium DELL (1956, p. 20, f. 30-31) also resembles the present species, but the former has much larger shell with larger anterior ear than the latter. P. (P.) ina (DAUTZENBERG and BAVAY) (1912, p.33, pI. 38, f. 18-21) is also an ally, but it has smaller shell with more numerous inner ribs than the present species. Furthermore the right anterior ear of the former is remarkably larger than that of the latter. P. (P.) spendulum WOODRING (1925, p. 75, pI. 9, f. 3-4) agrees with the new species in its ribbing, but is much smaller in size and stronger in the concentric sculpture of the shell-surface. Horizon.— The lower part of the Tsuma member (Upper Miocene). Locality.— Nagano (Ml-5109), Mino mura, Koyu gun, Miyazaki Prefecture.» TSUGIO SHUTO, 1960
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