638
Journal of Paleontology 92(4):634–647
Table 1. Measurements of Propeamussium yubarense (Yabe and Nagao, 1928). H=height, L=length, – =not available.
Locality Loc. 42
Loc. 7
Loc. 21 Loc. 1 Loc. 1
Loc. 45
Specimen JUE 16009
JUE 16010 JUE 16011
JUE 16012-1 JUE 16012-2 JUE 16013
L (mm) H (mm) 4.0
3.0 –
3.9 3.9 4.9
4.4 3.3 4.9 2.9 2.8 5.4
Order Lucinida Gray, 1854
Family Lucinidae Fleming, 1828 Subfamily Myrteinae Chavan, 1969 Genus Myrtea Turton, 1822
Type species.—Venus spinifera Montagu, 1803 (by original designation); Recent, British Islands.
Myrtea ezoensis (Nagao, 1938) Figure 2.5–2.8, 2.11, 2.14, Table 2
1890 Lucina cf. fallax Forbes; Yokoyama, p. 176, pl. 18, fig. 9. 1938 Lucina (Myrtea) ezoensis Nagao, p. 136, figs. 4–6. 1962 Lucinoma ezoensis; Saito, p. 70, pl. 2, fig. 6. 1976 Myrtea (sensu lato) ezoensis; Tashiro, p. 60, pl. 9, fig. 5.
Type specimen.—GMH 8234.
Occurrence.—Cenomanian to lower Campanian formations from Hokkaido to Kyushu; upper Selandian to lowermost Tha- netian, Katsuhira Formation, eastern Hokkaido, Japan.
Original description.—“Shell rather small, somewhat penta- gonally ovate in outline, slightly longer than high, sub- equilateral, moderately convex from the umbo toward the ventral margin, compressed and attenuated both anteriorly and posteriorly; antero-dorsal margin nearly horizontal, slightly excavated beneath the umbo, convex in the anterior length; postero-dorsal margin straight, inclined backward and down- ward; ventral margin broadly and evenly curved, passing gradually into the anterior one, which is arcuate and slightly oblique; antero-dorsal end a little produced and rounded along the margin; posterior end vertically truncated by a straight and rather long margin that forms an obtuse angle with the ventral. Umbones small, subcentral, not prominent, with a round pos- terior umbonal angle running from it to the postero-ventral end, the surface behind this angle compressed and flattened; anterior umbonal angle not well defined, antero-dorsal area bordered by a distinct groove that originates from the umbo and runs to the antero-dorsal end. Lunule small, very short, deep, bounded by sharp ridges. Ligamental groove deeply depressed; escutcheon well defined. Test relatively thin. Surface with numerous, pro- minent and widely spaced concentric lamellae and flat inter- spaces in alternation, the latter having a few fine concentric lines. These lamellae prolonged on the ridge at the anterior margin of the escutcheon and also near the antero-dorsal margin. The lamellae and lines distinctly wavy on crossing the groove near the anterior margin. A few specimens have been examined. One from the Obirasibe district in the province of Tesio is well
Valve left
right left left left
unknown
Table 2. Measurements of Myrtea ezoensis (Nagao, 1938). H=height, L= length,W=width, – =not available.
Locality Specimen Loc. 1
Loc. 1 Loc. 1 Loc. 1 Loc. 1 Loc. 1 Loc. 1 Loc. 1 Loc. 3 Loc. 3
JUE 16000-1 JUE 16000-2 JUE 16000-3 JUE 16001 JUE 16002
JUE 16004-1 JUE 16004-2 JUE 16004-3 JUE 16005-1 JUE 16005-2
Loc. 41 JUE 16003
L (mm) H (mm) W(mm) Valve 15.7
15.5 13.5 13.5 20.8 20.3 20.7 18.7 6.4 4.3
11.5
13.8 12.5 10.8 11.0 17.4 15.0 16.2 14.2 5.9 3.9 9.3
6.1 – – –
7.4 –
7.3 5.7 – –
5.0
both right right left
both right both both both both both
preserved and accordingly selected as the holotype. However, the inner features are not observable in it. One of the two specimens collected in the Abesinai district that are considered conspecific with the holotype, shows, some of the inner features impressed on a cast of the right valve. In this specimen, the lunule is very deeply impressed and very short and the hinge plate narrow, with a distinct and more or less elongated anterior lateral tooth. The posterior cardinal is narrow and slightly oblique backward. The socket in front of the posterior cardinal is also narrow and separates the latter from the anterior margin. The anterior cardinal is apparently obsolete. Anterior adductor muscle scar narrow, elongated, and the pallial line rather distant from the shell margin. The inner margins are smooth. The inner surface has numerous fine radial lines that are distinctly preserved on the cast” (Nagao, 1938, p. 136–137).
Materials.—Eight specimens from the floor of Urahoro River at Katsuhira (JUE 16000-1–3, 16001, 16002, 16004-1–3, Loc. 1) and near the Asahi Bridge (JUE 16005-1, 2, Loc. 3), and one specimen from Ponkatsuhira-minami-sawa (JUE 16003, Loc. 41); see Table 2 for dimensions.
Remarks.—The maximum size of the collected specimens is 20.8mm in length. Their shells are longer than high (H/L=0.76–0.88; type specimens of M. ezoensis, 0.79–0.88), weakly inflated (W/L=0.30–0.39; holotype of M. ezoensis, 0.37), subquadrate or quadrate-ovate with a posterior blunt ridge and a subtruncated posterior margin. The beak is pointed and centrally or anteriorly located (AL/L= 0.37–0.57). The surface is sculptured with regular commarginal ribs with fine, weak interstitial riblets. The anterior adductor muscle scar is elongate- ovate in outline, detached from the entire pallial line and larger than the ovate posterior one. The lunule and escutcheon are narrow, sunken, and demarcated by a sharp ridge. In the left valve, the anterior tooth (2a) is thin and vertical whereas the posterior one (2b) is rather thick and inclined posteriorly. Based on these characters, the specimens from the Paleocene Katsuhira Formation can be safely identified as Myrtea ezoensis, which was described from the Yezo Group in northern and central Hokkaido and South Sakhalin.
Family Thyasiridae Dall, 1900 (1895) Genus Thyasira Lamarck, 1818 Subgenus Thyasira Lamarck, 1818
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