636
Journal of Paleontology 92(4):634–647
(>600m in thickness) according to the original definition by Oda et al. (1959), which corresponds to the ‘Katsuhira Mudstone Member’ of the Tomikawa Formation by Nishijima (1964). The formation consists of dark gray mudstone fre- quently yielding calcareous concretions with plant debris (~30cm in diameter). The age of the Katsuhira Formation herein, near the bound-
Type species.—Pecten (Propeamussium) ceciliae de Gregorio, 1884 (by original designation); Miocene, Italy, Sicily, Terrebianche.
Propeamussium yubarense (Yabe and Nagao, 1928) Figure 2.1–2.4, Table 1
ary with the overlying Tokomuro Formation, was confirmed as Selandian by planktonic foraminifers, and the lower part of the formation also as Selandian by calcareous nannofossils (Kaiho, 1984). However, as a result of our examination of many dinoflagellate cysts from the dark gray mudstone of the middle part of the formation at Loc. 1, from which many molluscan fossils have been collected, the following species were identified: Senegalinium microspinosum (Boltenhagen, 1977), Palaeocystodinium golzowense Alberti, 1961, Eisenackia margarita (Harland, 1979), Eisenackia sp.,Glaphyrocysta spineta (Eaton, 1976), G. divaricata (Williams and Downie, 1966), and Hystrichosphaeridiumtubiferum(Ehrenberg, 1838), aswell as the acritarch species Paralecaniella indentata (Deflandre and Cook- son, 1955). This association indicates that the sediments belong to the PalaeocystodiniumgolzowenseBiozone and the Senegalinium microspinosum Biozone (Kurita, 2004), ranging from late Selan- dian to Thanetian (middle to late Paleocene) in age. Furthermore, Eisenackia margarita has a narrow stratigraphic range from the late Selandian to the earliest Thanetian (Luterbacher et al., 2004).
Material and methods
Most specimens were observed by stereoscopic microscope (Olympus SZ-2-W). Because astartid specimens are very small size but paleontologically important (as discussed below), we have observed and photographed them by scanning electron microscopy (JEOL model JSM-6010LV) at Kanazawa Uni- versity. We also measured the specimens by digital calipers (Mitsutoyo Company, model CD-20) to the first decimal place. Abbreviations used throughout the text include: AL/L=
ratio of anterior length and shell length; D=shell diameter; H=shell height; H/L=ratio of shell height and shell length; L=shell length; SH=spire height; W=shell width; W/L= ratio of shell width and shell length.
Repositories and institutional abbreviations.—Type, figured, and other specimens examined in this study are deposited in the following institutions: GMH=Institute of Geology and Miner- alogy, Faculty of Science, Hokkaido University, Sapporo; IGPS=Institute of Geology and Paleontology, Faculty of Science, Tohoku University, Sendai; JUE=Joetsu University of Education, Joetsu.
Systematic paleontology Class Bivalvia Linnaeus, 1758
Subclass Pteriomorphia Beurlen, 1944 Order Pectinoida Gray, 1854
Family Propeamussiidae Abbott, 1954 Genus Propeamussium de Gregorio, 1884
1928 Pecten (Propeamisium) cowperi var. yubarensis Yabe and Nagao, p. 88, pl. 16, figs. 17–19.
1932 Pecten (Propeamusium) cowperi var. yubarensis; Nagao, p. 38, pl. 6, figs. 7, 8, 12, 13.
1938 Pecten (Propeamusium) cowperi var. yubarensis; Nagao, p. 129, pl. 16, fig. 3.
1956 Pecten (Propeamisium) cowperi var. yubarensis; Amano, p. 73, pl. 1, fig. 12.
1976 Parvamussium cowperi yubarensis; Tashiro, p. 52, pl. 4, figs. 20–24.
1992 Parvamussium yubarense; Tashiro, p. 112, pl. 32, fig. 1. Type specimen.—IGPS 22599.
Occurrence.—Cenomanian to Campanian formations from Hokkaido to Kyushu; upper Selandian to lowermost Thanetian, Katsuhira Formation, eastern Hokkaido, Japan.
Original description.—“Shell very compressed, small, sub- orbicular, nearly as high as long; subequivalve, almost equi- lateral, except for the ears; the dorsal third of the disc triangular, with the apical angle about 120°; remaining part of shell semi- circular; antero-dorsal margin almost straight, the postero-dorsal faintly concave, scarcely longer than the anterior; posterior end bluntly angulated. Test thin. Right valve: Posterior ear shorter, with the outer angle slightly obtuse; anterior ear distinctly notched below, with an acute outer angle; both ears ornamented
with lines of growth. External surface covered with numerous, fine, equally spaced, concentric lines and numerous very fine radial lines scarcely visible under a lens: inner surface bearing 7 strong, equally spaced round ribs, extending from umbo nearly to the main margin and abruptly truncated at their low ends. Left valve: Ears subequal, with the outer angles slightly obtuse. Outer surface ornamented with numerous, slightly raised, fine radial striae and lines of growth; inner surface bearing strong radial ribs of unknown number” (Yabe and Nagao, 1928, p. 88).
Materials.—Six specimens from the floor of Urahoro River at Katsuhira (JUE 16012-1, 16012-2, Loc. 1), Oppushinai-zawa (JUE 16010, Loc. 7; 16013, Loc. 45), Katsuhira-kita-zawa (JUE 16011, Loc. 21), and Ponsetarai-zawa (JUE 16009, Loc. 42); see Table 1 for dimensions.
Remarks.—The Katsuhira specimens have a slightly inflated, small, thin shell, to 4.9mm in length and eight internal radial ribs, not reaching the ventral margin. The shell surface is sculptured with very fine radial threads on the left valve and many distinct commarginal lines on the right valve. The left anterior auricle is slightly larger than the posterior one. From these characters, the Katsuhira species is identified as Propeamussium. Tashiro (1976, 1992) allocated this species to Parvamussium Sacco, 1897, however, because P. yubarense
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