1993 Solemya (Acharax) sp.; Kanie et al. 1993: 34, fig. 2.

1993 Calyptogena sp.; Kanie et al. 1993: 34, fig. 2.

1997 Solemya sp.; Kanie and Sakai 1997: 218, fig. 8.3 (but listed as Solemya cf. angusticaudata Nagao, 1932 in their table 2).

1997 Calyptogena sp.; Kanie and Sakai 1997: 210, fig. 8.1.

2002 Solemya (Solemya) angusticaudata Nagao; Kanie and Kuramochi 2002: 51, fig. 2.1.

Etymology: For Mikasa City, place of the type locality.

Type material: Holotype: UMUT MM 29524; an internal mold showing internal features, 41mm long, 19mm high; paratype: UMUTMM 29523; a large specimen showing external features, 55 mm long, 21 mm high.

Type locality: Seep carbonate on the banks of the Ponbetsu River in Mikasa City, Hokkaido, Japan.

Type horizon: Albian (Lower Cretaceous) of the Yezo Group.

Material.—60 specimens and fragments from the type locality.

Diagnosis.—An elongate-oval Acharax with elongate and slightly concave posterior adductor scar, anterior adductor D-shaped with a close, almost vertically ascending band extending from its posteroventral corner.

Description.—Shell with elongate-oval outline, dorsal margin straight, anterior margin rounded, ventral margin slightly convex, posterior margin truncated and pointed. Sculpture of broad radial ribs, grooves between ribs ca. 1/3 of rib width, about 10 ribs on anterior part of shell, ribs weak to absent below umbo. Anterior lateral extension lacking; posterior adductor scar elongate, curved, tapering towards the umbo; posterior ligament external, distinct and strong. Anterior adductor scar broad D-shaped, from its posteroventral corner undulates an ascending band to the dorsal margin where it broadens out.

Discussion.—A very unusual feature of this species is the undulating band extending from the posteroventral corner of the anterior adductor scar: in most if not all solemyids this band ascends obliquely to the dorsal margin (Pojeta 1988; Taylor et al. in press; and observations by SK on Recent solemyids in the USNM zoology collection), whereas in Acharax mikasaensis sp. nov. it is almost perpendicular to it. In his description of Solemya angusticaudata from the Yezo Group of Hokkaido Nagao (1932) does not mention the ligament but describes the posterodorsal margin as “sloping backward and downward towards the posterior end”, and indeed, his figure shows a specimen in which the posterodorsal margin is slightly concave, which is in contrast to Acharax mikasaensis where the posterodorsal margin is clearly convex. Acharax cretacea Kanie and Nishida, 2000 is more elongate than Acharax mikasaensis and appears to have a more angular anterior margin. The posterior adductor scar of Acharax mikasaensis is more elongate than in the Recent type species Acharax johnsoni but resembles that of Zesolemya parkinsoni Smith, 1874 from New Zealand (Taylor et al. in press: fig. 1D). Clearly distinct is the lower Miocene Solemya (Acharax) tokunagai Yokoyama, 1925 as figured by Kamada and Hayasaka (1959) from the Joban coal-field, because it has a more elongate posterior end than Acharax mikasaensis.

Stratigraphic and geographic range.—Only known from the Albian type locality on Hokkaido, Japan.

2000 Acharax cretacea sp. nov.; Kanie and Nishida 2000: 82, fig. 4.

2007 Acharax cretacea Kanie and Nishida; Jenkins et al. 2007b: fig. 5.8.

Material.—Kanie and Nishida's (2000) type material was examined in the Yokosuka City Museum; ten newly collected specimens from the Yasukawa seep site.

Discussion.—Kanie and Nishida (2000: 83) suggested the presence of a long external ligament. A newly collected small specimen from the Yasukawa seep site shows that the ligament is external and rather short (Fig. 2D₁ ). Another specimen from this site shows the prismatic shell microstructure (Fig. 2E), typical for solemyids.

Stratigraphic and geographic range.—Cenomanian sulfide-rich sediments (own observation) and Campanian seep carbonates of the Yezo Group on Hokkaido, Japan.

2007 Nucinella gigantea sp . nov.; Amano et al. 2007: 85, figs. 2–7.

Material.—Twelve specimens from the Kanajirisawa seep site in Obira Town.

Discussion.—The specimens from Kanajirisawa are similar in size (18 mm in length) and outline to the type material of Nucinella gigantea. In the right valve, a central tooth thin and oblique anteriorly, and three small teeth are recognized before the central tooth (Fig. 3C). One posterior tooth can be recognized behind the central tooth. In the left valve, three anterior teeth are present, among which the anterior-most tooth is shaped like a reverse “S” and is parallel to the hinge base (Fig. 3D). The two central teeth are large but thin, and connected to a small posterior tooth. A narrow flat area is also present below the dentition. The number of teeth in the specimens found at the Kanajirisawa site (five in the right valve and six in the left valve) is slightly lower than in the type specimens (six in the right valve and nine in the left valve) of Nucinella gigantea. However, the large size, nuculid shell shape and one or two large central teeth allow us to safely identify the specimens as N. gigantea.

One specimen has a circular drill hole (outer diameter 1.5 mm; inner diameter 1.4 mm) and a healed shell injury that is commarginal, short, and straight, and resulted in a ridge-like trace in the remainder of the shell, indicating that this injury affected not only the shell but also the shell-forming mantle margin (Fig. 3A). The shape of drill hole is similar to that made by naticids, but naticids have so far not been found at this locality (own observations and Andrzej Kaim, personal communication, 2007). Thus the predator that caused the drill hole is unknown. The sharp injury at the central part of the valve may be attributed to the attack of a crustacean. This is by far the oldest drill hole reported from a seep deposit, being almost two-and-a-half times as old as the naticid drill holes recognized at a Late Eocene cold seep site on Hokkaido (Amano and Jenkins 2007).

Stratigraphic and geographic distribution.—Late Cretaceous (Cenomanian to Campanian), Tenkaritoge and Omagari Formations of the Yezo Group, Hokkaido, Japan.

1932 Nucula (Acila) hokkaidoensis sp. nov.; Nagao 1932: 28–30, pl. 5: 17, 18.

1936 Acila (Truncacila) hokkaidoensis (Nagao); Schenck 1936: 52.

1938 Nucula (Acila) hokkaidoensis Nagao; Nagao and Otatume 1938: 37, pl. 1: 1.

1941 Acila (Truncacila) hokkaidoensis (Nagao); Nagao and Huzioka 1941: 118–119.

1975 Acila (Truncacila) hokkaidoensis (Nagao); Hayami 1975: 21.

non 1958 Acila (Truncacila) hokkaidoensis (Nagao); Ichikawa and Maeda 1958: 79–80, pl. 3: 9–11, 14.

non 1962 Acila (Truncacila) hokkaidoensis (Nagao); Saito 1962: 59–60, pl. 1: 1.

non 1976 Acila (Truncacila) hokkaidoensis (Nagao); Tashiro 1976: pl. 1: 12–16.

non 2007 Acila hokkaidoensis (Nagao); Tsujino and Maeda 2007: figs. 6C, D.

Material.—One specimen from the Omagari seep site, 27 specimens from the Yasukawa seep site (figured: UMUT MM 29531).

Discussion.—Nagao (1932: 29) indicated that Acila hokkaidoensis is easily separable from other Recent and fossil species in Japan “in that the line, from which the radial ribs diverge downward, is placed much anteriorly to the median vertical”. This applies not only for the Japanese species but also for most species from the American side of the North Pacific, including the nine species of Truncacila reported by Weaver (1942) from the Cenozoic of Oregon and Washington, and the seven Cretaceous species of Truncacila described by Squires and Saul (2006) from British Columbia to Baja California. However, Squires and Saul (2006) noted that these North American Cretaceous species were from warm, shallow water sediments and that the deep-water species from the same area need critical evaluation. A specimen with a similar anterior line of divergence was figured as Acila (Truncacila) haidana Packard in Schenck, 1936 from the Redding Fm, Frazier Siltstone Member (Turonian) in northern California (Squires and Saul 2006: fig. 15). The holotype of Acila (Truncacila) haidana figured by Squires and Saul (2006: figs. 13, 14, and 16) has the line of divergence in a central position. The specimen from the Californian Turonian Redding Formation might thus belong to Acila (Truncacila) hokkaidoensis.

Tsujino and Maeda (2007: fig. 6C, D) figured a specimen from the lowermost Campanian of the Upper Haborogawa Formation in west-central Hokkaido as A. (T.) hokkaidoensis which has the line from which the radial ribs diverge in a central position; this specimen most probably belongs to A. (T.) himenourensis Tashiro, 1985 (discussed below).

Stratigraphic and geographic range.—Turonian of California, USA (uncertain); Coniacian to Campanian of the Yezo Group, Japan.

1958 Acila (Truncacila) hokkaidoensis (Nagao); Ichikawa and Maeda 1958: 79–80, pl. 3: 9–11, 14.

1962 Acila (Truncacila) hokkaidoensis (Nagao); Saito 1962: 59–60, pl. 1: 1.

1976 Acila (Truncacila) hokkaidoensis (Nagao); Tashiro 1976: pl. 1: 12–16.

1985 Acila (Truncacila) himenourensis sp. nov. (in MS); Tashiro 1985: 56.

1992 Acila (Truncacila) himenourensis Tashiro; Tashiro 1992: 26, pl. 3: 4.

2007 Acila hokkaidoensis Nagao; Tsujino and Maeda 2007: fig. 6C, D.

Material.—One specimen from the Kanajirisawa seep site, three from the Yasukawa seep site (figured is UMUT MM 29532).

Original description.—“Shell ovate with short posterior margin and strongly arched ventral margin. Ribs on surface fine, but strong. Lines connecting tops of v-shaped ribs gently sloping anteriorly, attaining to slightly anterior part to center of ventral margin.” (translated from the Japanese by KA).

Discussion.—Acila (Truncacila) himenourensis has a narrower umbonal angle than A. (T.) hokkaidoensis, and the line from which the radial ribs diverge is not as far anterior as in the specimen of A. (T.) hokkaidoensis figured by Nagao (1932: pl. 5: 17, 18). Acila (T.) picturata Yokoyama, 1890 from the late Eocene Poronai Formation differs from A. (T.) himenourensis by having a broader apical angle and finer, and more numerous ribs (Yokoyama 1890; own observations). Also Acila brevis Nagao and Huzioka, 1941 from the Miocene of Hokkaido has a broader apical angle and finer ribs (Kanno and Ogawa 1964). A similar species from the North American Pacific Coast Cretaceous is A. (T.) grahami Squires and Saul, 2006, which has usually a more elongate shape and thicker ribs with narrower interspaces than A. (T.) himenourensis (Squires and Saul 2006: 95, figs. 27–38).

Stratigraphic and geographic range.—Japan; Cenomanian to Campanian of the Yezo Group, Hokkaido; Coniacian to Santonian of the Futaba Group, northeast Honshu; Campanian part of the Izumi Group, Kinki District of Honshu; Coniacian to Maastrichtian of the Himenoura Group, western Kyushu.

Type species: Tellina flexuosa Montagu, 1803; Recent, North Sea.

Discussion.—A Recent molecular analysis using 18S and 28 S rRNA sequences showed that Thyasira can be subdivided into two clades; the T. flexuosa and T. sarsi clades (Taylor et al. 2007). Morphologically, the former clade is characterized by having an ovate shell and a deep sulcus in the posterior area while the latter has a subcircular shell as well as a shallow sulcus (Oliver and Killeen 2002).

1993 Conchocele? sp.; Kanie et al. 1993: fig.2.

2008 Thyasirid bivalve; Kiel, Amano et al. in press: fig. 3L, M.

Etymology: For Prof. Kazushige Tanabe (The University of Tokyo), a specialist on ammonoids from Hokkaido, who also brought our attention to several cold seep deposits there.

Type material: Holotype (UMUT MM 29533: articulated specimen, length 10.4 mm, height 10.3 mm, width 6.1 mm) and paratypes (UMUT MM 29534, articulated specimen, length 10 mm, from the Kanajirisawa site; UMUT MM 29535: articulated specimen, length 9.2 mm; UMUT MM 29536: specimen with preserved shell material from the Omagari site, height 11.2 mm; UMUT MM 29537: specimen with preserved hinge; UMUT MM 29539: articulated specimen from the Ponbetsu site, length 7.1 mm).

Type locality: Yasukawa seep site in Nakagawa Town, Hokkaido.

Type horizon: Early Campanian, Omagari Formation, Yezo Group.

Material.—29 specimens from the type locality, five from the Kanajirisawa seep site, nine from Ponbetsu River.

Diagnosis.—Medium-sized Thyasira with narrow posterior area and sharp 1^st posterior fold.

Description.—Shell moderate size attaining 12.9 mm in length, thin-walled, subcircular, equivalve and inequilateral. Convexity of shell variable, from moderately inflated (Fig. 5A₁, G₁) to strongly inflated (Fig. 5C₃). Beak projecting above dorsal margin, prosogyrate, situated at anterior one-third of shell length. Antero-dorsal margin concave, making right angle with ventral margin at anterior end; ventral margin well rounded. Posterior area narrow; posterior sulcus rather deep, but posterior sinus shallow; 1^st posterior fold elevated and sharp, rib-like; submarginal sulcus relatively shallow with very shallow marginal sinus; auricle narrow. Lunule lanceolate, concave, bounded by sharp ridge. Surface sculptured only by growth lines or irregular growth ridges in some specimens. In right valve, no evidence of cardinal tooth (Fig. 5E). Palliai line entire. Anterior muscle scar large, elongate ovate, weakly impressed, separated from palliai line ventrally (Figs. 5A₂, 6A); posterior muscle scar small, subquadrate, in front of 2nd posterior fold (Figs. 5D, 6A). Radiating interior striae distinct.

Discussion.—Kanie et al. (1993: fig. 2) illustrated a shell silhouette as Conchocele? sp. from the Albian Ponbetsu seep site in Mikasa City, central Hokkaido. Their figure of Conchocele? sp. does not show a posterior fold and an auricle. However, these features can be seen in several specimens that we collected at this site. Thus we assume that Kanie et al. (1993) overlooked these features and that their Conchocele? sp. is in fact Thyasira tanabei described herein.

Thyasira tanabei sp. nov. resembles Thyasira sp. nov. described by Tashiro (2004) from the Albian to Cenomanian sediments of the Yezo Group in Horokanai Town in its general outline and size. But judging from the figure (Tashiro 2004: pl. 1: 3, 4) the Horokanai specimen seems to lack a posterior fold and auricle. Thyasira tanabei also resembles T. nakazawai Matsumoto, 1971 from the early Miocene Wappazawa Formation in Shizuoka Prefecture, central Honshu, by having a sharp rib-like 1^st posterior fold. However, T. tanabei reaches twice the size of T. nakazawai (maximum length = 21.0 mm) and the wide posterior part of T. nakazawai enables us to separate it from T. tanabei. The thyasirids from the Campanian Western Interior Seaway of the USA are different from T. tanabei by having a wider posterior area, a shallowly depressed lunule, and a posterior adductor scar astride the posterior sulcus (Kauffman 1967). Thyasira tanabei can be distinguished from Recent Thyasira species by its narrow posterior area, the posterior adductor scar in front of the posterior sulcus, and the sharp rib-like 1^st posterior fold.

Hikida et al. (2003) illustrated two fragmentary specimens from the Campanian Omagari seep site as Thyasira sp. Because the type locality and formation of the new T. tanabei is the Omagari Formation near their site, these specimens may be identical with T. tanabei, but better preserved material from the Omagari seep site is necessary to confirm this suggestion.

Measurements.—See Table 2.

Stratigraphic and geographic range.—Albian to Campanian, Yezo Group, Hokkaido Japan; so far found at the Ponbetsu site (Albian), the Kanajirisawa seep site (Cenomanian), and the type locality (Campanian, Omagari Formation), as well as associated with Coniacian sunken wood in the Nishichirashinai Formation (Kiel, Amano et al. in press).

Material.—One articulated specimen (though most of the left valve is missing) from the Kanajirisawa site in Obira Town; length 11.7 mm, height 12.4 mm, thickness 6.7 mm, UMUT MM 29540.

Description.—Shell moderate size, thick-walled, triangular in shape, swollen and slightly inequilateral. Umbo prominent, beak slightly prosogyrate, situated at central part of shell. Antero-dorsal margin steeply sloping, making angle with well rounded ventral margin. Posterior area wide; 2nd posterior fold very strong, posterior sulcus very deep, but without posterior sinus; 1^st posterior fold elevated and sharp, rib-like; submarginal sulcus also deep without marginal sinus; auricle slightly convex and wide. Lunule double; outer lunule wide, bounded by crude ridge; inner lunule small, aubcircular, deeply depressed and bounded by distinct incision. Surface sculptured only by growth lines. Internal features unknown.

Discussion.—This species is unique in having a double lunule, a strongly inflated shell, and a deeply grooved posterior sulcus. These features separate this species from T. tanabei sp. nov. found in the same seep site. Because it is only known from a single specimen with an incomplete left valve, we do not establish a new species for it.

Stratigraphic and geographic range.—Known only from the Cenomanian Kanajirisawa seep site.

Type species: Thracidora gigantea Shikama, 1968; Middle Miocene, central Japan.

Discussion.—Nipponothracia was initially introduced as a large, potentially symbiont-bearing thraciid from a Miocene and an Early Cretaceous methane seep deposit (Kanie and Sakai 1997). Kase et al. (2007) recently documented an elongate anterior adductor muscle scar that is ventrally detached from the palliai line in the Miocene type species, showing that Nipponothracia belongs to the Lucinidae rather than the Thraciidae.

1996 Thracia yezoensis sp. nov.; Kanie and Kuramochi 1996: 64, fig. 1.

Material.—Several specimens from the type locality at Obira.

Supplementary description.—Hinge plate narrow and smooth; ligament rapidly widening posterior of umbo, supported by narrow nymph. Internal radial striations well developed; palliai line distant from shell margin, marked by a groove; anterior adductor muscle scar elongate, about 1/3 of shell length, tongue-shaped, marked by a groove, ventrally detached from palliai line by a thin margin for about 4/5 of its length; posterior adductor muscle scar less distinct, apparently subcircular with a short ventral protrusion.

Discussion.—Kase et al. (2007) reported that a specimen of Nipponothracia yezoensis from the type locality stored in the National Museum of Nature and Science, Tokyo, has “an elongate anterior muscle that is more elongate than the posterior one and seemingly detached from the palliai line”. This observation is here confirmed. The anterior muscle scar of N. yezoensis is more elongate than that of N. gigantea, the posterior muscle scar is more rounded, and the palliai line is more distant from the shell margin than in N. gigantea (Fig. 6B). In addition, the hinge plate of N. gigantea is deeply excavated below the umbo, a feature not seen in N. yezoensis. Thus N. yezoensis can clearly be distinguished from the type species. Because no internal features except radial striations are known for N. ponbetsensis (see below), the question whether N. ponbetsensis and N. yezoensis are synonymous remains unresolved.

The Indonesian Nipponothracia hetzeli (Martin, 1933) from a late Miocene asphalt deposit at Buton, Indonesia, also has an excavated hinge plate, a more elongate ligament, and a broader nymph than N. yezoensis (Beets 1942; Kase et al. 2007; SK, own observations). The same applies to Crypto-lucina elassodyseides Saul, Squires, and Goedert, 1996 from the Eocene of Washington State, USA. Species of the Recent Anodontia clade generally have a long and narrow ligament that slowly tapers toward the umbo (Taylor and Glover 2005). This is in contrast to the ligament of N. yezoensis which appears to attain its maximum width just posterior to the umbo.

Stratigraphic and geographic range.—Only known from the Cenomanian type locality on Hokkaido, Japan.

1993 Thraciidae gen. et sp. indet.; Kanie et al. 1993: 33, fig. 4.

1997 Nipponothracia ponbetsensis Kanie and Sakai, 1997: 214, figs. 6, 7.

1997 Yoldia (Megayoldia) cf. thraciaeformis Kanie and Sakai, 1997: 210, fig. 4.

Material.—Several specimens from the Ponbetsu site.

Discussion.—Kase et al. (2007) indicated that Nipponothracia ponbetsensis has a long ligamental nymph posterior to the beaks and that it appears to belong to the Lucinidae. Here we document radial internal striations in this species that suggest its position within Lucinidae. The specimen that Kanie and Sakai (1997) figured as Yoldia (Megayoldia) cf. thraciaeformis is clearly a deformed specimen of N. ponbetsensis; we have several specimens from the Ponbetsu site that experienced a similar kind of deformation.

Stratigraphic and geographic range.—Only known from the Albian type locality on Hokkaido, Japan.

Material.—Three specimens from the Kanajirisawa seep site.

Description.—Shell oval in outline, compressed, slightly wider than high, becoming more circular during ontogeny; sculpture of irregular, fine commarginal ribs, fine radial ribs in larger specimens; radial striations on inner side of shell, one anterior lateral tooth; anterior adductor muscle scar very weakly impressed, elongate, tongue-shaped, apparently detached from the palliai line. The smaller of the two figured specimens is 26 mm wide and 21 mm high, the larger one is 43 mm wide, 40 mm high, and a single valve is 8 mm thick.

Discussion.—Fossil bivalves with circular, compressed shells like the one described here have frequently been placed in the genus Miltha H. Adams and A. Adams, 1857, without knowledge of internal features like hinge dentition and muscle attachment scars. Chavan (1969), Vokes (1969), and Bretsky (1976) diagnosed that lateral teeth are absent from Miltha, hence the Kanajirisawa species can not be placed in this genus. Because other internal features such as the cardinal hinge dentition and the shape of the ligament are not preserved in the available specimens, they are described here only in open nomenclature.