Amussiopecten hyugaensis, Shuto, 1955
SHUTO, T. 1955. Amussiopecten from the Miyazaki group, Miyazaki Prefecture, Japan. Transactions and Proceedings of the Palaeontological Society of Japan [N. S.], 20: 101-110, pls. 16-17. [p. 105, pl. 16, figs. 1-5; pl. 17, figs. 2, 4, 5; text fig. 3]
Amussiopecten hyugaensis n. sp.
Pl. 16, figs. 1-5, Pl. 17. figs. 2, 4 and 5, text fig. 3 Material.— Holotype-GK-L 4368, loc. MI-3592: paratypes-GK-L 4150-53, 4155, 4159, 4172, 4364, 4418, 4674, 4676 and 4695.
Measurements.— Shown in the table 2. Diagosis.— Shell medium to large in size, rather solid, compressed, orbicular in outline, somewhat broader than high, slightly inequivalve. Right valve a little convex, slightly curved near the beak, provided with radial ribs; exterior ribs about 11, low, broad, flat-topped, straight, becoming gradually lower towards the ventral and lateral margin; interstices narrower than ribs themselves; interior ribs wide and flat at the begining, later becoming concave to have prominent ridges on both sides, and interstices becoming wider near the ventral margin. Concentric lines of growth weak but distinct. Ears subequal; anterior portion being somewhat wavy at anterior border with acute corner and with weak but distinct byssal notch. Left valve planoconvex; ornamented with exterior and interior ribs and growth lines; exterior ribs rather round-topped, as wide as interstices; interior ribs like those on the right valve. Ears equal. Hinge line horizontal, about fourty percent of shelllength in the adult stage. Comparisons.— The present species is closely allied to A. praesignis (YOKOYAMA), but the former has smaller and thicker shell than the latter and is ornamented with less numerous ribs which never split into riblets.
A. ? yabei NOMURA, which may be included in A. praesignis, is also allied to this new species, but the former has more numerous ribs and large apical angle than the latter. Amussiopecten flavellum [sic] UGOLINI (Flavellipecten [sic] pasinii MENEGHINI), reported from Sardaigne, closely resembles the young specimens of the present new species in the general outline and in size, but the former has somewhat more numerous ribs, which are narrower and slightly weaker than the latter. "Pecten (Vola)" sinkirensis [sic] MARTIN has much more numerous ribs, which are rather roof-shaped on the left valve. Oopecten rotundatus (LAMARCK) is similar to the present species, but is distinguished by its larger size, more convex valves and its ribs are slightly narrower than the interstices. Furthermore the internal ribs are not prominent in the former. Variation.— As indicated in table 3, this new species is remarkably variable in the size, depth of the valve, number and strength of the ribs etc. The specimens from the lowest horizon attain to 70 mm, those from the middle horizon are less than 50 mm, and those from the highest horizon are 100 mm or more in the length of the shell. It is neccessary to determine whether or not the larger specimens in each locality are really the adult shells. For the discrimination of this point the relief of the ribs may be useful. The exterior ribs of Amussiopecten are distinctly high in relief in young and become lower and obscure as the shell grows. The growth-stage of each individual is not well indicated by size of the shell but by the relief of the ribs at the margin. The ribs are generally almost equal in relief at the shell-margin among the specimens of equal size, regardless of the horizon of the specimens. ln other words in the present new species there may be merely the difference in growth stage. Therefore the fact that the size is dissimilar among different localities or horizons may be due to such factors as ecological and sedimentary environments, mode of preservation and or others, although the number of the adult shells are not so numerous as to confirm this interpretation. Conceming the ecology of the following fact should be noted. Apart from the above mentioned difference in size, some other characters also vary by horizon. For instance the specimens from the basal part of the Tano and the Boroishi member show the wide variation in the number and the strength of the ribs, the depth of the valve and the ratio of height to width. Those from the upper part of these members (Clementia-Paphia horizon) are rather invariable, the shell of about 50 mm in length, being provided with 11 to 12 ribs of moderate relief and with rather deep valves. Furthermore the shells from the highest horizon attain large size and have the thick shell, the deep valves and the ribs of rather variable number. The lithology of the lower horizon is silty sandstone, coarse sandstone or granule-bearing sandstone and remarkably variable laterally and vertically, and that of the middle horizon is rather monotonous massive silty sandstone or sandy siltstone. In the upper horizon the lithology is again coarse sandstone but is less variable. It has been well known that many Pectinids are the active swimmer and accordingly necessitate much oxigen. Thereupon they favour the coarse sandy, gravelly or rocky bottom, where water is generally agitated and contains plenty free oxigen. If Amussiopecten has similar habit, it is reasonably explained that the large and solid specimens occur in the coarse sediments and the small specimens in the muddy sediments. Much more evidence is required to settle the question. The variations in the important characters range as follows. 1) The ratio of height to width of the valve varies between 89 and 102 percent and most frequently between 96 and 99 percent. 2) The ratio of depth of the valve to width varies between 11 and 19 percent, and most frequently between 12 and 17 percent in the right valve; between 4 and 8 percent in the left. 3) The ratio of hinge-line to shelllength varies between 40 and 50 percent in the adolescent and adult shells, but the young specimens show larger value, for example, it is 60 percent in a specimen whose shell-length is 23.2 mm. 4) Apical angle varies between 103 and 129 degrees and most frequently between 110 and 117 degrees in the adolescent shells and between 122 and 127 in the adult. 5) The number of the exterior ribs varies between 10 and 15, and most frequently between 11 and 12. Horizon.— Lower part of the Miyazaki group (late Middle Miocene to middle Upper Miocene). Locality.— Waritsuke and Mokudo, Aya Machi*; Morinaga, Honjo Machi**; Yanaze, Akatani and Kano, Takaoka Machi***; Yusunokihashi and Oyamada, Mukasa Mura****; Kariyabaru, Haigano, Hokobo. Horiguchi and Obira, Tano Machi.*****; lyeichigo, Maruno and Kagamisu, Miyazaki City******; Inohaye, Kitago Mura*******. TSUGIO SHUTO, 1955
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T. Shuto, 1955, plate 16.
T. Shuto, 1955, plate 17.
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