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Molluscan Fauna of The“ Miocene” Namigata Formation in the Namigata Area, Okayama Prefecture, Southwest Japan

Molluscan Fauna of The“ Miocene” Namigata Formation in the Namigata Area, Okayama Prefecture, Southwest Japan

Jour. Geol. Soc. , Vol. 119, No. 4, p. 249–266, April 2013 JOI: DN/JST.JSTAGE/geosoc/2012.0048 doi: 10.5575/geosoc.2012.0048 Molluscan fauna of the“ ” Namigata Formation in the Namigata area, Prefecture, southwest Japan

Abstract Takashi The molluscan fauna of the Namigata Formation, traditionally ac- cepted to be of Miocene age, are reexamined taxonomically, and the Received 27 February, 2012 geologic age of the formation and its paleogeographic implications Accepted 12 June, 2012 are discussed. The formation is subdivided into the main part and two new members (the Senjuin Shell-Sandstone and Ônishi Con- Division of Natural History, Museum of Na- glomerate members). The Namigata Formation yielded 13 species of ture and Human Activities Hyogo, 6 Yayoiga- , 16 species of and 1 species of Scaphopoda. The oka, Sanda 669-1546, Japan occurrences of Molopophorus watanabei Otuka, Acila (Truncacila) nagaoi Oyama and Mizuno, Chlamys (Nomurachlamys?) namiga- Corresponding author: T. Matsubara, [email protected] taensis (Ozaki), and Isognomon (Hippochaeta) hataii Noda and Fu- ruichi indicate that the molluscan age should be revised to the late Late Eocene–Early . Taking account of the latest elasmo- branch data and preliminary strontium isotope ratio, the age of the formation is confined to the late Late Eocene. The present and recent results show that the First Seto Inland was actually composed of two sea areas that existed at different times: the Paleogene sea area is estimated to have been an open sea facing south to the Pacific , whereas that in the Miocene is thought to have been an em- bayment connected to the northwest to the Sea of Japan. Therefore, the concept of the so-called Miocene First should be reassessed in the Cenozoic geohistory of the Japanese Islands. Selected molluscan species including (Felaniella) nami- gataensis n. sp. are taxonomically described and/or discussed. Key words: First Seto Inland Sea, geologic age, , Namigata Formation, , paleogeography

c; D, E in Fig. 1, respectively); and the“ Bihoku Group” Introduction (Tai, 1965) below the Kojima Bay (Kurita et al., 2002; The First Setouchi Series (Kasama and Huzita, 1957; F in Figure 1). In addition, a small outcrop exposure of revised by Huzita, 1962) is known to comprise the Upper Eocene–Lower Oligocene marine sediments has Lower–Middle Miocene sediments scattered in the been found in the area (Matsubara et al., Setouchi Geologic Province (Ikebe, 1957) in the central 2004; et al., 2008; G in Fig. 1). zone of southwest Japan. Recent paleontologic and ra- The Namigata Formation (Takeyama, 1931) is known diometric studies show that the constituent formations as a Miocene fossiliferous formation in the Namigata in the coastal areas of the eastern Seto Inland Sea, locat- area, southern , southwest Japan ed in the western part of the province, were not of Mio- (e.g., Ozaki, 1956b; Itoigawa, 1983; Moriyama et al., cene age, but were of Eocene–Oligocene age (see Mat- 1994a) (Fig. 1). It was also referred to the First Setouchi subara, 2009 for review). The following marine Series on the basis of its lithology, sedimentary cycle, sediments have been revised as the Eocene–Oligocene: and molluscan and benthic foraminiferal fossils (e.g., the Tainohata Formation of the Kôbe Group in the west Itoigawa, 1983; Itoigawa and Shibata, 1992; Moriyama Kôbe area (Ozaki et al., 1996; Matsubara et al., 2010; A et al., 1994a, b; Yano et al., 1995a–c). Based on the ben- in Fig. 1); the Iwaya Formation of the Kôbe Group in thic foraminiferal assemblages, Yano et al. (1994, the north Awajishima area (Yamamoto et al., 2000b; Ya- 1995a) correlated the Namigata Formation with the maguchi et al., 2005; B in Fig. 1); the Maéjima Forma- Miogypsina kotoi-Operculina complanata japonica As- tion in the Ushimado area (Matsubara, 2002b; C in Fig. semblage Zone of Nomura (1992), indicating a latest 1); the Tonoshô Group in the Shôdoshima and Teshima Early–early Middle Miocene age. Yano et al. (1995b, c) areas (Kurita and Matsubara, 2002; Matsubara, 2002a, also correlated it with Haq et al.’s (1987) third-order cy- ©The Geological Society of Japan 2013 249 250 Takashi Matsubara 2013―4

133˚E

133°

Fig. 1. Map showing the distribution of Paleogene marine sediments in coastal areas of the eastern Seto Inland Sea and lo- cation of the study area. Modified after Matsubara (2009). A. The Tainohata Formation of the Kôbe Group in the west Kôbe area. B. The Iwaya Formation of the Kôbe Group in the north Awajishima area. C. The Maéjima Formation in the Ushima- do area. D. The Shikai Formation of the Tonoshô Group in the northwest Shôdoshima area. E. The Teshima, Iéura and Karato formations of the Tonoshô Group in the Teshima area. F. The“ Bihoku Group” below the bottom of Kojima Bay. G. Unnamed Paleogene marine sediments in the Kurashiki area.

cle TB2.3 (absolute age“ 16.6–15.5 Ma”: Haq et al., Moriyama et al. (1994a) subdivided the Namigata 1987) from the sequence chronostratigraphic point of Formation into five members (the lowest, lower, middle, view. upper, and uppermost members) based on their lithology On the other hand, recent reexaminations based on the and stratigraphic relationship. However, the nomencla- preliminary strontium isotope ratio and elasmobranch ture for these lithostratigraphic units does not follow the fauna suggest that the Namigata Formation is of Mid- International Stratigraphic Guide (Salvador ed., 1994). I dle–Late Eocene age (Yamamoto et al., 2000a; Tanaka herein propose the following subdivision: et al., 2006). However, its molluscan fauna has not been 1. Main part fully examined taxonomically, and previous molluscan The main part of the Namigata Formation is dominat- data include some discrepancies with the latest results ed by massive very fine- to fine-grained sandstone at- because it includes Miocene and younger species (Oza- tains to 100 m in maximum thickness (Moriyama et al., ki, 1956a, b; Itoigawa, 1983). The aim of the present 1994a). The basal part may be composed of boulder paper is to reexamine the molluscan fauna of the Nami- conglomerate more than 10 m in thickness (B in Fig. 3). gata Formation taxonomically, and discuss its geologic The sandstone is strongly bioturbated, and sporadically age and implications for the Cenozoic paleogeography to commonly includes shells of balanids, brachiopods of southwest Japan. such as Coptothyris sp. and Terebratalia sp., and bi- valve molluscs referred to the Acila-Mactra assemblage Geologic setting of Itoigawa (1983). It intercalates thick bioclastic con- The Namigata Formation has a limited distribution glomerate and limestone beds (= the Senjuin Shell- abutting the pre-Cenozoic basement in the mountainous Sandstone Member; see below) and is separated into the area near the boundary between City and Yakage two portions (Fig. 3). Town in the southwestern part of Okayama Prefecture, 2. Senjuin Shell-Sandstone Member (new name) (Figs. 2, 3). The type locality was designated to be Definition.̶Shell-sandstone and limestone beds in Namigata, Ibara City, Okayama Prefecture (Takeyama, the middle part of the Namigata Formation. This mem- 1931). Its lithology, stratigraphy, paleoenvironment, and ber corresponds to the“ middle member” of Moriyama sedimentary history were precisely described and dis- et al. (1994a). cussed by Moriyama et al. (1994a, b) and Yano et al. Type locality.̶An outcrop exposure at a landscape (1994, 1995a–c). garden in the precincts of the Senjuin Temple, Ibara Jour. Geol. Soc. Japan 119（ 4 ） Mollusca from the“ Miocene” Namigata Formation, SW Japan 251

25200 250

300 250

250

25200 20250

300

15100 Fig. 2. Geologic map of the 200 150 Namigata area with measured sections (A–F) and fossil lo-

150 calities (modified after Fig. 3 of Moriyama et al., 1994a).

stone. The bioclasts are originated from molluscs, bala- nids, brachiopods, bryozoans, foraminifers, and calcare- ous algae (Yano et al., 1995a). Bedding planes are generally weak due to the strong bioturbation. At the type locality, the basal part of the member is composed of an about 3 m-thick, pebble to cobble conglomerate bed consisting of subangular gravels. It is conformably covered by shell-sandstone and limestone more than 5 m in thickness (A in Fig. 3). Although the thickness of the member is generally 10 to 15 m (Moriyama et al., 1994a), it decreases to about 2 m in the northeast of Ônishi, Yakage Town (F in Fig. 3). Stratigrahic relations.̶It is intercalated in the middle part of the main part. It also abuts on the basement. 3. The Ônishi Conglomerate Member (new name) Definition.̶Thick cobble to boulder conglomerate beds occupying the uppermost part of the Namigata Formation. This member corresponds to the“ uppermost Fig. 3. Columnar sections of the Namigata Formation. member” of Moriyama et al. (1994a). Measured sections A–F correspond to those in Fig. 2. Type locality.̶An exposure at the upper portion of an abandoned sandpit in the northwest of Ônishi, Yakage Town (E in Figs. 2, 3). City (A in Figs. 2, 3). The outcrop was figured by Akagi Distribution.̶The member has small distributions on (1927: pl. 2). mountains higher than an altitude of about 300 m be- Distribution.̶The member is distributed at the type tween the north of Nanayashiki and the type locality, locality and the area between the northeast of Itami, and higher than an altitude of about 320 m in the west Ibara City and the north of Ônishi, Yakage Town via the and northwest of Senjuin Temple (Fig. 2). north of Namigata (Fig. 2). Lithology and thickness.̶This member is character- Lithology and thickness.̶This member is represented ized by massive cobble to boulder conglomerate, which by“ Namigata-iwa” (meaning wavy rocks in Japanese), is generally weathered. Its lithofacies is closely similar which is composed of shell-sandstone and impure lime- to that of the Kibi Group (Suzuki et al., 2003) composed 252 Takashi Matsubara 2013―4

Table 1. Mollusca from the Namigata Formation. R: rare; F: few; C: common; A: abundant; +: present.

of fluvial“ mountain gravels” (Suzuki et al., 2009). impression material (Provil Novo® Putty Fast Set, Her- However, Yano et al. (1994) and Moriyama et al. aeus-Kluzer Co., Ltd.) was used during sampling in the (1994a) included it in the Namigata Formation because field and preparation in the laboratory. a marine bivalve“ Cardium” sp. was discovered from Results and discussion this member (Yano et al., 1994). The thickness is evalu- ated to be more than 10 m, although its upper limit is 1. Geologic age of the Namigata Formation not observed. Thirteen species of Gastropoda, 16 species of Bival- Stratigraphic relations.̶It disconformably overlies via, and one species of Scaphopoda were discriminated the main part of the Namigata Formation, and abuts on from the Namigata Formation (Table 1, Figs. 4–7). The the basement. following species are biostratigraphically important: Isognomon (Hippochaeta) hataii Noda and Furuichi, Material and method Acila (Truncacila) nagaoi Oyama and Mizuno, Chlamys Molluscan material has been collected from three lo- (Nomurachlamys?) namigataensis (Ozaki), and Molo- calities in the lower portion of the main part, two locali- pophorus watanabei Otuka. I. (H.) hataii was described ties in the Senjuin Shell-Sandstone Member, and three from the Eocene Teshima Formation of the Tonoshô localities in the upper portion of the main part (Figs. 2, Group on Teshima Island, northern , 3). Among these, the material from two localities southwest Japan (Noda and Furuichi, 1972; Noda, (NON-01 and NON-02) is that collected by Mr. Satoshi 1982). Subsequently, it was also reported from the Eo- Morinobu of the City Horseshoe Crab Muse- cene Shikai Formation on Shôdoshima Island and from um. The molluscan specimens from the main part were the Middle Eocene–Oligocene Maéjima Formation poorly preserved except for those in Loc. NON-01, and (Noda, 1982; Matsubara, 2002b). A. (T.) nagaoi was they occurred as molds in brittle sandstone. To examine considered to be one of the index species of the Maze the precise shell characteristics, a silicon vinyl dental Stage (Mizuno, 1962) of uppermost Eocene–lowest Oli- Jour. Geol. Soc. Japan 119（ 4 ） Mollusca from the“ Miocene” Namigata Formation, SW Japan 253 gocene age (Oyama and Mizuno, 1958; Oyama et al., Howarth and MacArthur, 1997) for these formations. 1960; Mizuno, 1964; Kamada, 1980). However, its up- Tanaka et al. (2006) taxonomically reexamined elasmo- per limit reaches to the Nishisonogi Stage of Oligocene branch fauna in the Namigata Formation, and concluded age (Fuse and Kotaka, 1986; Tomita and Ishibashi, this formation is of Middle–Late Eocene age on the ba- 1990). M. watanabei occurs in the Iwaki and Asagai sis of the occurrences of Carcharocles auriculatus (Bla- formations of the Shiramizu Group, and is known as inville), Physogaleus secundus (Winkler), and Hemi- one of the representative elements of the Asagai Fauna pristis curvatus Dames. These data suggest that the (Makiyama, 1934). As summarized by Matsubara et al. upper limit of the Namigata Formation is confined to (2010), the geologic age of the Shiramizu Group is Late the Eocene. In conclusion, the geologic age of the Eocene–Early Oligocene on the basis of planktonic mi- Namigata Formation is late Late Eocene, on the basis of crofossils and a mammal Entelodon cf. orientalis integrated mollusc, elasmobranch and strontium isotope Dashzeveg from the upper part of the Iwaki Formation ratio data. (Tomida, 1986; Yanagisawa and Suzuki, 1987; Kurita, 2. Paleogeographic implication 2004). M. watanabei was also recorded from the upper It has been known that the molluscan assemblages in Lower–lower Upper Oligocene Ashiya Group in the the western part of the Setouchi Geologic Province dif- Chikuhô Coalfield, northern Kyûshû, southwest Japan fer significantly between the west-northwestern area (= (Hirayama, 1956; Tomita and Ishibashi, 1990). The Bihoku Belt: Yano et al., 1995b) and the east-southeast- planktonic foraminifer and calcareous nannofossil as- ern area (= Setouchi Éngan Belt: Yano et al., 1995b). semblages indicate the geologic age of the Ashiya Faunal differences occur not only in terms of the generic Group to be the late Early–early Late Oligocene (Saito and specific compositions but also in terms of paleoen- and Okada, 1984; Okada, 1992). Ch. (N.?) namiga- vironmental nature (e.g., Itoigawa, 1969, 1971, 1983; taensis was originally described from the Namigata Okamoto, 1974; Itoigawa and Shibata, 1992). The west- Formation, and was subsequently recorded from the Eo- northwestern area is represented by the Cyclina-Vicarya cene or Oligocene Maéjima Formation on Maéjima Is- and Vasticardium assemblages, which represent brack- land (Matsubara, 2002b). Matsubara (2002b) also iden- ish intertidal sandy bottom and upper sublittoral sandy tified Chlamys ashiyaensis (Nagao) and Patinopecten bottom environments in an embayment, respectively. sp. of Nemoto and O’Hara (2001) from the uppermost On the other hand, the east-southeastern area is charac- Eocene–lowest Oligocene Iwaki Formation of the Shi- terized by the Ostrea and Mactra-Acila assemblages, in- ramizu Group to be the present species. dicating open, rocky or gravelly bottom and sublittoral Nishimoto and Itoigawa (1977) pointed out that the sandy bottom environments, respectively (Itoigawa, Namigata Formation yields the Odontaspis-Heterodon- 1983; Itoigawa and Shibata, 1986, 1992; Ueda, 1991). tus elasmobranch assemblage including Carcharodon Three major opinions have been proposed to explain angustidens Agassiz. As this assemblage is shared with the differences. The first was proposed by Itoigawa and those in the Ashiya Group in the Chikuhô area in north Shibata (1992), who considered that the difference is Kyûshû and the Hioki Group in the Kottoi and Yuyawan due to the restricted inflow of open sea waters into the areas in westernmost Honshû, southwest Japan, Nishi- southeastern part of the First Seto Inland Sea from the moto and Itoigawa (1977) considered that the geologic Pacific Ocean, and that two sea areas were connected age of the Namigata Formation is probably older than through Early–Middle Miocene time. The second is ad- that of the major constituent formations of the Miocene vocated by Ueda (1991) and Takayasu et al. (1992), First Setouchi Series. The geologic ages of the Ashiya who thought that the difference is due to a geographic and Hioki groups were believed to be Early Miocene at barrier formed by tectonic movement in the Early–Mid- that time, they were later revised as Oligocene based on dle Miocene. The third is a compromise between Itoiga- planktonic microfossils and radiometric dating (Saito wa and Shibata’s (1992) and Ueda’s (1991) opinions. and Okada, 1984; Fuse and Kotaka, 1986; Okada, 1992; Yano et al. (1995a–c) considered that the First Seto In- Ozaki and Hamasaki, 1991; Ozaki, 1999). Subsequently, land Sea was divided initially into two sea areas by the Yamamoto et al. (2000a) preliminarily assessed the geo- geographic barrier, and two areas were connected by the logic age of the so-called Miocene First Setouchi Series subsequent transgression. These three opinions are in the coastal area of the eastern Seto Inland Sea, in- based on the premise that all constituent formations of cluding the Namigata Formation based on the strontium the First Setouchi Series are of Early–Middle Miocene isotope. They reported a 87Sr/86Sr ratio of ca. 0.7077‰, age. indicating a Late Eocene or older age (e.g., Ito, 1993; On the other hand, as a result of reexamination of the 254 Takashi Matsubara 2013―4

molluscan fauna from the Maéjima Formation, Matsub- phyngus Gabb, 1869. Allmon (1990) followed Nuttall ara (2002b) considered that the difference can be attrib- and Cooper’s (1973) view. However, Squires (1997) did uted to differences in the geologic age of the First not concur with their opinion because B. (M.) striata Setouchi Series between the two areas. The result from differs significantly from Brachysphingus spp. being the Namigata Formation supports my previous view much smaller and having a subsutural band. I agree with (Matsubara, 2002b), and shows that the so-called Mio- Squires’s (1997) view because the early whorl sculpture cene First Seto Inland Sea was a combination of two of B. (M.) striata seems to be different from that of B. geochronologically distinct sea areas. One is of late sinuatus. Early–early Middle Miocene age in the Bihoku Belt, Nuttall and Cooper (1973) also believed that Molopo- and the other is of late Middle Eocene–Early Oligocene phorus striata [sic] of Dickerson (1915: pl. 8, fig. 7) age in the Setouchi Éngan Belt. As suggested by Yano from the Tejon Formation of California is not conspecif- et al. (1995c), the Paleogene sea area in the Setouchi ic with Gabb’s (1869) species, and regarded it as an in- Éngan Belt was an open sea facing south to the Pacific determinate species in the Colwellia Nuttall and Ocean. On the other hand, the Miocene sea area in the Cooper, 1973. However, I consider that the type species Bihoku Belt was a bay facing northwest to the Sea of of Colwellia, Cominella flexuosa Edwards in Lowry, Japan (Itoigawa and Shibata, 1992; Takayasu et al., 1866, is not congeneric with the North Pacific species 1992; Yano et al., 1995c). Therefore, the concept of the because it has shouldered whorls, a finer, less granulated “First Seto Inland Sea” should be reassessed in the Ce- subsutural band, and dentitions on the inner side of the nozoic geohistory of the Japanese Islands. As Eocene– outer lip. In my opinion, Dickerson’s (1915) figured Oligocene marine sediments in the coastal area of the specimen is correctly identified, and the obsolete axial eastern Seto Inland Sea are sporadic and their precise ribs in the lectotype and an additional specimen figured geologic age has not fully been dated, further biostrati- by Nuttall and Cooper (1973: pl. 8, figs. 1a–b, 2) are graphic and geochronologic studies are necessary to re- due to abrasion of the shell surface. Therefore, I herein construct the precise paleogeography of the Paleogene treat the genus Molopophorus Gabb, 1869 as a valid ge- sea area and its temporal change. nus in the family Nassariidae.

Systematic description of selected species Molopophorus watanabei Otuka, 1937 Abbreviations.̶GSJ: Geological Survey of Japan, (Figs. 4.4–4.6) AIST; IGPS: Tohoku University Museum (formerly housed at Institute of Geology and Paleontology, To- ? Nassa sp. Nagao, 1928, p. 112, pl. 17, figs. 26, 26a. hoku University); MNHAH: Museum of Nature and Molopophorus watanabei Otuka, 1937, p. 170–171, pl. Human Activities, Hyogo; NMNS: National Museum of 16, figs. 5–7; Hirayama, 1955, p. 119, pl. 4, figs. 21, Nature and Science. 22; Oyama et al., 1960, p. 74, pl. 14, figs. 4a–c; Ka- mada, 1962, p. 170–171; Shikama, 1964, p. 108, pl. Class Gastropoda 31, fig. 25; Tomita and Ishibashi, 1990, pl. 15, fig. 10. Family Nassariidae Molopopholus [sic] watanabei Otuka. Kamada, 1962, Genus Molopophorus Gabb, 1869 pl. 20, fig. 8; Kamada, 1972, pl. Pg-6, fig. 19; Nemoto Type species.̶Bullia (Molopophorus) striata Gabb, and O’Hara, 1979, pl. 2, figs. 16a–b; Nemoto and O’ 1869 by original designation. Hara, 2001, pl. 5, fig. 16; Nemoto and O’Hara, 2007, Discussion.̶The taxonomical status of the genus pl. 2, fig. 9. Molopophorus Gabb, 1869 is controversial (e.g., Nuttall ?“ Nassarius” sp. Itoigawa, 1983, p. 32. [listed only] and Cooper, 1973; Cernohorsky, 1984; Allmon, 1990; Type material.̶UMUT CM13143 (holotype); UMUT Squires, 1997; Haasl, 2000), because the type species CM13143–CM13145 (paratypes). All the type material Bullia (Molopophorus) striata Gabb, 1869 was de- is missing (Ichikawa, 1983). scribed based on a few, minute, incomplete specimens Material examined.̶MNHAH D1-31932 (from Loc. from the Upper Eocene Tejon Formation of California, NM01); D1-31936–31938 (from Loc. NM02). U.S.A. Cernohorsky (1984) and Haasl (2000) treat Mo- Discussion.̶The species from the Namigata Forma- lopophorus Gabb, 1869 as a nomen dubium. Nuttall and tion can be safely referred to Molopophorus watanebei Cooper (1973) thought it to be invalid, because they as- Otuka, 1937, from the lowest Oligocene Asagai Forma- sumed that B. (M.) striata is a juvenile of Brachysphin- tion in the Jôban area, Prefecture, northeast gus sinuatus Gabb, 1869, the type species of Brachys- Japan, on the basis of a rather small ovoidal shell with Jour. Geol. Soc. Japan 119（ 4 ） Mollusca from the“ Miocene” Namigata Formation, SW Japan 255

Fig. 4. Gastropoda from the Namigata Formation. 1. Margarites? sp. indet. MNHAH D1-031918. Non-apertural view, ×2.5. 2. Turbonilla? sp. indet. MNHAH D1-031942. Apertural view, ×3. Silicon vinyl cast. 3. Ficus sp. indet. MNHAH D1- 031941. Non-apertural view, ×2. Silicon vinyl cast. 4–6. Molopophorus watanabei Otuka. 4. MNHAH D1-031935. Non-ap- ertural view, ×3. 5. MNHAH D1-031936. Showing the penultimate and earlier whorl sculpture, ×2. 6. MNHAH D1-031937. Non-apertural view of body and penultimate whorls, ×2. All specimens are silicon vinyl casts. 7. Mohnia? sp. indet. MN- HAH D1-031935. Apertural view, ×2. 8–10. Turritella (Kotakaella) sp. indet. 8. MNHAH D1-031932. Non-apertural view, ×2. 9. MNHAH D1-031931. Non-apertural view, ×2. 10. MNHAH D1-031930. Non-apertural view, ×1.5. All specimens are silicon vinyl casts. 11, 12. Turritella (Kotakaella) sp. aff. T. (K.) infralirata Nagao. 11. MNHAH D1-031926. Non-apertural view, ×2.5. 12. MNHAH D1-031925. Non-apertural view, ×1.5. All specimens are silicon vinyl casts. 13. Exilia sp. indet. MNHAH D1-031940. Non-apertural view, ×2. Silicon vinyl cast. 14. Sinum sp. aff. S. zonale (Quoy and Gaimard). MN- HAH D1-031922. 14a. Apical view; 14b. Basal view, ×2. 15. Cellana? sp. indet. MNHAH D1-031913. 15a. Apical view. 15b. Side view. Both views are ×1.5. 16, 17. Lottia? sp. indet. 16. MNHAH D1-031913. 16a. Apical view. 16b. Side view. Both views are ×2. 17. MNHAH D1-031916. 17a. Apical view. 17b. Side view. Both views are ×1.5. 18. Hipponix? sp. in- det. MNHAH D1-031915. 18a. Apical view. 18b. Side view. Both views are ×1.5. 256 Takashi Matsubara 2013―4

Fig. 5. Bivalvia from the Namigata Formation (1). All specimens are silicon vinyl casts, unless otherwise stated. 1–4. Sac- cella sp. indet. A. 1. MNHAH D1-031970. Left valve, ×2.5. 2. MNHAH D1-031971. Internal view of right valve, ×2. 3. MNHAH D1-031968. Left valve, ×2. 4. MNHAH D1-031975 Left valve, ×2. 5. Saccella sp. indet. B. MNHAH D1-031976. Internal view of left valve, ×2. 6, 7–9, 12. Acila (Truncacila) nagaoi Oyama and Mizuno. 6. MNHAH D1-031955. Internal view of left valve. 7. MNHAH D1-031964. Internal view of left valve, ×1.5. 8. GSJ F5019 (Holotype). Left valve. 9. MNHAH D1-031943. Left valve, somewhat compressed. 12. MNHAH D1-031944. Left valve. All views are ×1.5. 10. Crassostrea sp. indet. MNHAH D1-032006. Internal view of left valve, ×1.5. 11. Modiolus sp. indet. MNHAH D1-031997. Inner mold of left valve, ×1.2. 13. Acila (Acila) ashiyaensis (Nagao). IGPS 35963 (Holotype). Right valve, ×1.5. Shown for comparison. 14, 15. Isognomon (Hippochaeta) hataii Noda and Furuichi. 14. MNHAH D1-031983. External view of right valve, ×1. 15. MNHAH D1-031979. Right valve. 15a. External view. 15b. Internal view. Both views are ×1. rounded axial ribs which tend to become obsolete and Korobkov in Zhidkova et al. (1994) “[ 1992”] de- broader with shell growth, two strong keels on the fasci- scribed a new subspecies, Molopophorus watanabei ole, and the lack of a distinct spiral sculpture except for costulatus, from the Upper Oligocene Nevelskaya Suite a weakly granulated subsutural band. in the Makarov area, South Sakhalin. This subspecies is Jour. Geol. Soc. Japan 119（ 4 ） Mollusca from the“ Miocene” Namigata Formation, SW Japan 257 easily distinguished from M. watanabei by its less de- Acila (Truncacila) sp. Itoigawa, 1983, p. 32. [listed veloped subsutural band and its body whorl sculptured only] with feeble axial ribs in greater numbers. It should be Acila (Truncacila) cf. nagaoi Oyama and Mizuno. Fuse treated as a distinct species belonging to the genus Nas- and Kotaka, 1986, pl. 17, figs. 2–4. sarius Duméril, 1806, because it seems to lack a few Acila (Truncacila) cf. nagaoi Oyama and Mizuno. Mat- strong keels on the fasciole. Molopophorus ornatus subara, 2002b, p. 134, fig. 5.4. Devjatilova in Devjatilova and Volobueva, 1981, from Material examined.̶GSJ F5019 (holotype); GSJ the Eocene Getkilninskaya Suite in the Panzhina Inlet F5020 (paratype). MNHAH D1-031943–D1-031946 coastal area, Russia, closely resembles M. watanabei. (from Loc. NM01); D1-031947–D1-031949 (from Loc. However, it is distinguished from M. watanabei by hav- NM02); D1-031950–D1-031954 (from Loc. NON03); ing a smaller shell with more rounded, broader axial D1-031955–D1-031963 (from Loc. WON01); D1- ribs. Molopophorus ovumiformis Oleinik in Sinelnikova 031964–D1-031967 (from Loc. WON02). et al., 1991 form the Eocene Snatolskaya Suite is anoth- Discussion.̶Ozaki (1956a, b) reported Acila (Trun- er allied species. However, this species differs from M. cacila) gottschei (Böhm, 1916) from the Namigata For- watanabei in having greater numbers of finer axial ribs mation. The named species was originally described on the body whorl. M. watanabei is easily distinguished from the“ ” [= Middle Miocene Sertunayska- from Molopophorus denselineatus (Nagao, 1928), from ya Suite: Kogan, 1939] in Sakhalin. However, the illus- the Oligocene Ashiya Group in the Chikuhô area, north- tration of the lectotype (Böhm, 1916: pl. 29, fig. 6) des- east Kyûshû, southwest Japan, by having a lower, larger ignated by Schenck (1936) seems to be different from A. shell with smaller numbers of axial ribs, and lacking a (T.) nagaoi in having a more rounded shell. According distinct spiral sculpture. As the anterior part of the shell to Kafanov et al. (1999), the type material of Nucula is unknown, its generic position remains uncertain. (Acila) gottschei Böhm, 1916 is missing today. Distribution.̶Iwaki and Asagai formations of Shi- The Namigata species closely resembles Acila (Trun- ramizu Group (Otuka, 1937; Hirayama, 1955; Kamada, cacia) nagaoi Oyama and Mizuno, 1958 (Fig. 5.8) from 1962; Nemoto and O’Hara, 1979, 2001, 2007); Nami- the uppermost Eocene–lowest Oligocene Kishima For- gata Formation (this study); Ashiya Group (Tomita and mation in the Karatsu area, Prefecture, southwest Ishibashi, 1990). Latest Eocene–early Late Oligocene. Japan, but has a slightly larger shell and may have a feeble rostral sinus (Fig. 5.9). However, the younger in- Class Bivalvia dividuals have the same shell outline and lack a rostral Family sinus. Therefore, I consider the Namigata specimens to Subfamily Nuculinae fall within the intraspecific variation of A. (T.) nagaoi. Genus Acila H. Adams and A. Adams, 1854 in Acila (Truncacila) nagaoi closely resembles Acila H. Adams and A. Adams, 1853–1858 (Truncacila) schumardi (Dall, 1909), the type species of Subgenus Truncacila Grant and Gale, the subgenus Lacia Slodkewitsch, 1967. However, A. 1931 ex Schenck MS (T.) nagaoi is distinguished from A. (T.) schumardi by Type species.̶Nucula castrensis Hinds, 1843 by having a shorter, more inflated shell with coarser exter- original designation. nal sculpture. As pointed out by Moore (1976), the sub- genus Lacia Slodkewitsch, 1967 is hardly distinguished Acila (Truncacila) nagaoi Oyama and Mizuno, 1958 from Truncacila Grant and Gale, 1931. Acila (Truncaci- (Figs. 5.6–5.9, 5.12) la) decisa (Conrad, 1855) from the Eocene Ardath Shale of California, U.S.A., differs from A. (T.) nagaoi the Nucula (Acila) mirabilis var. ashiyaensis Nagao, 1928, shell being smaller and more equilateral. Acila (Trun- pl. 7, figs. 8, 9. [non Nagao, 1928] cacila) oyamadensis Hirayama, 1955 from the Oligo- Acila nagaoi Mizuno (MS). Mizuno, 1956, p. 270, pl. 2, cene Asagai Formation in the Jôban area, northeastern fig. 1. [nomen nudum] Honshû, Japan, is distinguished from Acila (Truncacila) Acila (Truncacila) gottschei (Böhm). Ozaki, 1956a, p. 7 nagaoi by having a smaller, more rounded, higher shell [listed only]; Ozaki, 1956b, p. 12. [non Böhm, 1916] with a less developed escutcheon. Acila (Truncacila) Acila (Truncacila) nagaoi Oyama and Mizuno, 1958, p. nagaoi is distinguished from Acila (Truncacila) pictura- 595–596, pl. 1, figs. 14, 15; Oyama et al., 1960, p. ta (Yokoyama, 1890) from the upper Middle–Upper Eo- 101, pl. 21, figs. 1a–b; Kamada, 1980, pl. Pg-19, figs. cene Poronai Formation in Hokkaidô, northeast Japan, 1, 2; Tomita and Ishibashi, 1990, pl. 14, figs. 7a–b. by having a shorter posterior dorso-ventral margin. 258 Takashi Matsubara 2013―4

Fig. 6. Bivalvia from the Namigata Formation (2). All views are in natural size, unless otherwise stated. 1–4. Chlamys (No- murachlamys?) namigataensis (Ozaki). 1. MNHAH D1-032001. Right valve. 2. MNHAH D1-032004. Left valve, ×1.5. Sil- icon vinyl cast. 3. MNHAH D1-031999. Left valve. 4. MNHAH D1-032001. Right valve. 5–8. Acesta (Plicacesta) takeya- mai (Ozaki). 5. MNHAH D1-031986. Left valve. 6. MNHAH D1-031987. Left valve. ×1.5. 7. MNHAH D1-031989. Right valve. 8. MNHAH D1-031985. Left valve. Jour. Geol. Soc. Japan 119（ 4 ） Mollusca from the“ Miocene” Namigata Formation, SW Japan 259

Mizuno (1964: p. 55) proposed Acila (Truncacila) hi- Chlamys ashiyaensis (Nagao). Nemoto and O’Hara, rayamai with the following remarks:“ A. hirayamai is 2001, pl. 2, fig. 2. [non Nagao, 1928] anew named for the Nishisonogian shell which has large Patinopecten sp. Nemoto and O’Hara, 2001, pl. 2, fig. 3. angle of bifurcation of ribs”. Although the type material Chlamys (Leochlamys) namigataensis (Ozaki). Matsub- was neither designated nor illustrated, this name is ara, 2002b, p. 139–140, figs. 5.13, 5.14, 6.12, 6.15. available according to ICZN Art. 13.1.1 because it in- Material examined.̶NSMT P1-4379 (holotype); cludes the diagnostic characters. In addition, he noted MNHAH D1-031999–D1-032000 (from Loc. NM03; that“ Acila sp. reported by Hirayama (1956) from the presumable topotypes); D1-032001–D1-032003 (from Ashiya Group on Hikoshima (Hirayama, 1956, p. 102, Loc. NON02); D1-032004 (from Loc. NON03). pl. 6, figs. 2–4) is surely referable to this new species.” Discussion.̶The present species was originally de- However, Hirayama’s (1956) illustrated specimens are scribed from the Namigata Formation and is character- too poorly preserved to discern its shell characteristics, ized by its rather large shell with 14–15, highly elevated and are missing today. In addition, I could not find any radial ribs with fine striations with a scaly sculpture on type material of A. (T.) hirayamai in the GSJ collection. the back, and a large anterior auricle with a deep byssal Therefore I herein treat this taxon as a nomen dubium. notch. Matsubara (2002b) thought this species to be a Although Acila (Acila) ashiyaensis (Nagao, 1928) from member of the subgenus Leochlamys MacNeil, 1967 the Oligocene Yamaga Formation of the Ashiya Group (type species: Chlamys (Leochlamys) tugidakensis Mac- has a nearly same shell size to A. (T.) nagaoi, it has a Neil, 1967) in the genus Chlamys [Röding], 1798. How- distinct rostral sinus even in the younger shell (Fig. ever, it is tentatively transferred to the subgenus Nomu- 5.13). rachlamys Kurihara and Matsubara in Kurihara, 2010 Distribution.̶Maze, Itanoura, Kakinoura formations (type species: Pecten kaneharai Yokoyama, 1926) on in Sakito-Matsushima area, Prefecture; Kishi- the basis of the shell sculpture consisting of subregular, ma Formation in Karatsu area, ; Ashiya stout radial ribs with a few fine, scaly ribs on the radial Group in the Umashima area, Prefecture; Ki- ribs, an interstitial rib and shagreen microsculpture, and wado Formation of Hioki Group in Yuyawan area, Ya- a large anterior auricle. A single left valve (Fig. 6.2) has maguchi Prefecture; Maéjima Formation in Ushimado eleven highly elevated radial ribs. The numbers are area, Okayama Prefecture. Latest Eocene–Late Oligo- smaller than the typical specimens, including the holo- cene. type. However, it is thought to fall within intraspecific variation, because other shell characters including a Family Pectinidae large anterior auricle and scaly fine ribs on the radial Subfamily Chlamydinae ribs are identical to Ch. (N.?) namigataensis. Genus Chlamys [Röding], 1798 Distribution.̶Namigata Formation (Ozaki, 1956a, b; Subgenus Nomurachlamys Kurihara and this study); Maéjima Formation (Matsubara, 2002b); Matsubara in Kurihara, 2010 Iwaki Formation (Nemoto and O’Hara, 2001, as“ Chla- Type species.̶Pecten kaneharai Yokoyama, 1926 by mys ashiyaensis Nagao” and“ Patinopecten sp.”). Late original designation. Eocene–Early Oligocene.

Chlamys (Nomurachlamys?) Family Limidae namigataenis (Ozaki, 1956a) Genus Acesta H. Adams and A. Adams, (Figs. 6.1–6.4) 1857 in H. Adams and A. Adams, 1853–1858 Subgenus Plicacesta Vokes, 1963 Pecten (Chlamys) namigataensis Ozaki, 1956a, p. 7–8, Type species.̶Lima smithi Sowerby, 1888 by original pl. 2, fig. 4; Ozaki, 1956b, p. 12. designation. Non Chlamys (Mimachlamys) namigataensis (Ozaki). Masuda, 1962, p. 188, pl. 21, fig. 1. [= Chlamys (No- Acesta (Plicacesta) takeyamai (Ozaki, 1956a) murachlamys) aff. meisensis (Makiyama, 1936)] (Figs. 6.5–6.8) Chlamys (Mimachlamys) namigataensis (Ozaki). Masu- da and Noda, 1976, p. 128; Kaikiri and Nishimoto, Lima smithi Sowerby. Takeyama, 1933, p. 434. [non 1995, p. 204. Sowerby, 1888] Chlamys namigataensis Ozaki [sic]. Itoigawa, 1983, p. Lima takeyamai Ozaki, 1956a, p. 8, pl. 2, fig. 3; Ozaki, 38. [listed only] 1956b, p. 11, 12; Masuda and Noda, 1976, p. 85–86. 260 Takashi Matsubara 2013―4

Lima (Acesta) takeyamai Ozaki. Okamoto and Nakano, southwest Japan, by its higher, less inflated shell with 1967, p. 188. smaller anterior auricle and larger posterior auricle, and Acesta (Plicacesta) takeyamai (Ozaki). Nakano and smaller numbers of radial ribs. Okamoto, 1982, p. 359–360. Distribution.̶Known only from the Namigata For- Acesta (Acesta) takeyamai (Ozaki). Kaikiri and Nishi- mation. Late Eocene. moto, 1995, p. 205. Material examined.̶NMNS P1-4380 (holotype); Family Carditidae GSJ F15891-1 (= F16689-2) and F15891-2 (K. Okamo- Subfamily Carditimerinae to Collection; plaster replicas); MNHAH D1-0031985– Genus Cyclocardia Conrad, 1867 D1-031988 (from Loc. NM03; presumable topotypes); Type species.̶Cardita borealis Conrad, 1832, by D1-031989–D1-031994 (from Loc. NON02); D1- original designation. 031995–D1-031996 (from Loc. NON03). Description.̶Shell large (shell height to 115 mm), Cyclocardia sp. indet. rather thin, higher than long, inequilateral, posteriorly (Fig. 7.6) oblique ovate, weakly inflated; anterior auricle small but thick; posterior auricle rather long, indistinctly defined Cyclocardia siogamensis (Nomura). Itoigawa, 1983, p. from disc; anterior dorsal margin long, nearly straight; 38. [listed only] [non Nomura, 1935] hinge line straight, moderately long; posterior margin ?“ Cardium? sp.” Yano et al., 1994, figs. 4, 6. gently curved; ventral margin arcuate; shell surface Material examined.̶MNHAH D1-032007 and D1- sculpture of 40–59 strong, rounded or flat topped radial 032008 (both from Loc. WON02). ribs and irregularly strengthened commarginal growth Discussion.̶The species from the Namigata Forma- lines; weak scaly sculpture may be present on radial tion has a small (shell length to 10 mm), circular, mod- ribs. erately inflate shell with 27–31, narrow, roof-topped ra- Discussion.̶Takeyama (1933) identified the present dial ribs and a small, deeply concave lunule. species as Lima smithi Sowerby [= Acesta (Plicacesta) Cyclocardia siogamensis (Nomura) listed by Itoigawa smithi (Sowerby, 1888)] living off Japan at 100–600 m (1983) is probably the present species. However, C. sio- depth, although he pointed out that the specimens from gamensis (Nomura, 1935), originally described from the the Namigata Formation have smaller numbers of radial Lower Miocene Ajiri Formation in the area, ribs (ca. 40) than those (47–56: Oyama, 1943) of the , northeast Japan, differs from the Recent species. Subsequently, Ozaki (1956a) thought Namigata species in having a smaller numbers of, more this difference to be sufficient to separate the Namigata rounded radial ribs (see Sonpirom and Ogasawara, species from the Recent one, and proposed a new spe- 1997). Cardium? sp. of Yano et al. (1994) from the“ up- cies, Lima takeyamai. Nakano and Okamoto (1982) permost member” [= the Ônishi Conglomerate Member treated it as a valid species, and transferred it to the sub- in this study] of the Namigata Formation is probably the genus Plicacesta Vokes, 1963 in the genus Acesta H. present species. However, it is represented only by Adams and A. Adams, 1854 in H. Adams and A. Adams, small inner molds derived from a single specimen and 1853–1858. precise determination can not be made. As a result of reexamination of the type and additional specimens, it became obvious that the number of radial Family Ungulinidae ribs ranges from 40 to 59. This range overlaps those of Genus Felaniella Dall, 1899 the Recent Acesta (Plicacesta) smithi. However, A. (P.) Subgenus Felaniella Dall, 1899 takeyamai is distinguished from A. (P.) smithi by having Type species.̶Mysia (Felania) usta Gould, 1861 by a higher, less inflated shell with a larger posterior auri- original designation. cle, and stronger, flat- or round-topped radial ribs. This species differs from Acesta (Plicacesta) amaxensis (Yo- Felaniella (Felaniella) namigataensis new species koyama, 1911) from the Middle Eocene Sakasegawa (Figs. 7.1–7.4, 7.10a–b) Group in the Amakusa Islands, Kyûshû, southwest Ja- ? Cycladicama takeyamai (Otuka). Itoigawa, 1983, p. pan, in being higher in outline. A. (P.) takeyamai is also 31. [listed only] [non Otuka, 1938] separated from Acesta (Plicacesta) watanabei Nakano ? ? sp. Itoigawa, 1983, p. 32. [listed only] and Okamoto, 1982 from the Middle Miocene Taisha or Type material.̶MNHAH D1-032009 (holotype; from Tadaúra Formation in the western Shimane Peninsula, Loc. NM01); D1-032010 (paratype from Loc. NM01); Jour. Geol. Soc. Japan 119（ 4 ） Mollusca from the“ Miocene” Namigata Formation, SW Japan 261

Fig. 7. Bivalvia from the Namigata Formation (3). All specimens are silicon vinyl cast, unless otherwise stated. 1–4, 10. Felaniella (Felaniella) namigataensis n. sp. 1. MNHAH D1-032014. Internal view of left valve. 2. MNHAH D1-032019. Internal view of right valve. 3. MNHAH D1-032018. Internal view of right valve. 4. MNHAH D1-032013 (Paratype). Right valve. 10. MNHAH D1-032009 (Holotype). Left valve. 10a. Internal view, 10b. External view. All views are ×2.5. 5. Felaniella (Felaniella) confusa (Nagao). IGPS 36358 (Holotype). Left valve, ×2. Shown for comparison. 6. Cyclocardia sp. indet. MNHAH D1-032007. Left valve, ×2.5. 7. Liocyma sp. indet. MNHAH D1-032023. Internal view of left valve. ×2.5. 8. Cultellus sp. indet. MNHAH D1-032021. Internal view of fragmental right valve, ×1.5. 9, 12, 13. Mactromeris sp. indet. 9. MNHAH D1-032026. Cardinal properties of right valve. 12. MNHAH D1-032024. Inner mold of left valve. 13. MNHAH D1-032031. Left valve. All views are ×1.

D1-032011 and D1-032012 (paratypes from Loc. Table 2. Measurements of Felaniella (Felaniella) namiga- NM02); D1-032013–D1-032017 (paratypes from Loc. taensis n. sp. (in mm) WON02). Other material examined.̶MNHAH D1-032018– D1-032020 (from Loc. WON02; all specimens are sili- con vinyl casts lacking original molds). Type locality.̶Loc. NM-01. A small roadside cliff at Namigata, Nogami-chô, Ibara City, Okayama Prefecture (34˚37’31”N, 133˚31’19”E). Diagnosis.̶Felaniella (Felaniella) with small, circu- lar, nearly equilateral, weakly inflated shell, narrow hinge plate, and small teeth. Etymology.̶Named after Namigata, the type locality. ventral margin weakly curved, posterior margin Description.̶Shell small (length to 12 mm), circular, subtruncated; shell surface sculpture of very fine, irreg- weakly inflated; beak small, weakly prosocline, situated ular, commarginal growth lines; posterior ridge indis- rather anteriorly to mid-point of shell length; anterior tinct; hinge plate narrow, with two cardinal teeth; left dorsal margin short, weakly recurved; posterior dorsal valve anterior tooth nearly perpendicular, trigonal, bifid; margin nearly straight; anterior margin gently arcuate; left valve posterior tooth anteriorly oblique, narrow; 262 Takashi Matsubara 2013―4 right valve anterior tooth perpendicular, narrow; right (No. 12740293, Leader: T.M.; No. 21540472, Leader: valve posterior tooth anteriorly oblique subtrigonal, bi- Prof. Emeritus Yoshihiro Sawada of Shimane Universi- fid; nymph narrow; adductor muscle scars and pallial ty) line indistinct; ventral margin smooth. References Discussion.̶Felaniella (Felaniella) namigataensis n. sp. closely resembles Felaniella (Felaniella) confusa Adams, H. and Adams, A., 1853–1858, The Genera of Recent (Nagao, 1928) from the Oligocene Yamaga Formation Mollusca; Arranged According to Their Organisation. Vols. 1–3. J. v. Voorst, London, xl + 484 [Vol. 1], 661 p. [Vol. 2], of the Ashiya Group in the Chikuhô Coalfield, Kyûshû, 138 pls. [Vol. 3]. southwest Japan (Fig. 7.4). However, F. (F.) confusa Akagi, T., 1927, Explanatory Text of Geological Map of Japan, Scale 1:75,000, Okayama. Min. Comm. & Ind., , 36 differs from F. (F.) namigataensis n. sp. in having a + 3 p.＊2 larger shell (shell length to 20 mm) with a longer poste- Allmon, W. D., 1990, Review of the Bullia Group (Gastropoda: rior dorsal margin, a broader hinge plate and larger car- Nassariidae) with comments on its evolution, biogeography, and phylogeny. Bull. Amer. Paleont., 99, 5–179. dinal teeth (see Okamoto and , 1995: pl. 9, figs. 6, Böhm, J., 1916, Über Kreideversteinerungen von Sakhalin. 7). Felaniella (Felaniella) usta (Gould, 1861), living in Jahrb. Königl. Preuss. Geol. Land. Berlin (Jg. 1915), 36, ＊5 the Northwest Pacific, is another allied species. Howev- 551–558. Cernohorsky, W. O., 1984, Systematics of the family Nassari- er, F. (F.) usta is easily distinguished from F. (F.) nami- idae (Mollusca: Gastropoda). Bull. Auckland Inst. Mus., no. gataensis n. sp. in having a larger, more inflated, more 14, 1–356. anteriorly oblique shell with stronger cardinal teeth. Conrad, T. A., 1832, American Marine Conchology; or De- scriptions and Coloured Figures of the Shells of the Atlantic Itoigawa (1983) reported two ungulinid species,“ Cy- Coast of . T. A. Conrad, Philadelphia, 72 p. cladicama takeyamai (Otuka)” and“ Diplodonta? sp.”, Conrad, T. A., 1855, Report of Mr. T. A. Conrad on the fossil from the Namigata Formation. Among them, C. takeya- shells collected in California by Wm. P. Blake, geologist of the expedition under the command of Lieutenant R. S. Wil- mai (Otuka, 1938), originally described from the Mio- liamson, United States topographical engineers. In Blake, cene Bihoku Group in the Shôbara area, Pre- W. P., ed., Appendix to Preliminary Geological Report of William P. Blake, Geologist of the Survey in California Un- fecture, southwest Japan, differs from F. (F.) der the Command of Lieut. R. S. Williamson. Palaeontolo- namigataensis n. sp. in having a larger, more inequilat- gy, Washington, 5–20. eral and more inflated shell with a more concave Conrad, T. A., 1867, Description of a new genus of Astartidae. Amer. Jour. Conch., 3, 191. nymph. The two species reported by Itoigawa (1983) Dall, W. H., 1899, Synopsis of the American species of the may be conspecific with F. (F.) namigataensis n. sp., al- family Diplodontidae. Jour. Conch., 9, 244–246. though these two species are only listed and its shell Dall, W. H., 1909, The Miocene of Astoria and Coos Bay, Ore- gon. U. S. Geol. Surv. Prof. Paper, 59, 1–278. characters are not known. Devjatilova, A. D. and Volobueva, V. I., 1981, Atlas Fauni Pa- Measurements.̶Table 2. leogena i Neogena Severo-Vostoka SSSR. Nedra, Moskva, 219 p.＊3 Acknowledgments Dikerson, R. E., 1915, Fauna of the type Tejon: Its relation to the Cowlitz Phase of the Tejon Group of Washington. Proc. I would like to express my appreciation to Prof. Ka- Calif. Acad. Sci., 4th Ser., 5 (3), 33–98. zutaka Amano (Joetsu University of Education, Jô-étsu) Duméril, A. M. C., 1806, Zoologie Analytique, ou Méthode Na- turelle de Classification des Animaux, Rendue Plus Facile a and an anonymous reviewer for their critically reading l’Aide de Tableaux Synoptiques. Allais, Paris, xxxii + 344 ＊ manuscript and providing helpful comments. I am deep- p. 7 Fuse, K. and Kotaka, T., 1986, The molluscan assemblages ly grateful to Mr. Kazuya Yamaoka (Yakage Town) for from the Hioki Group, , southwest Ja- his facilities for my field survey and collecting fossil pan. Monogr. Mizunami Fossil Mus., no. 6, 119–141.＊2 specimens. Thanks are also due to Mr. Satoshi Morino- Gabb, W. M., 1869, Section 2. Cretaceous fossils. Part 1. De- scription of new species. In Gabb, W. M., Palaeontology of bu (Kasaoka City Horseshoe Crab Museum, Kasaoka) California, Vol. 2. Geol. Surv. Calif., Sacramento, 127–205. for his offering some fossil specimens from the Nami- Gould, A. A., 1861, Descriptions of shells collected by the gata Formation. I would like to appreciate to Prof. To- North Pacific Exploring Expedition (continued). Proc. Bos- ton Soc. Nat. Hist., 8, 14–40. moki Kase (National Museum of Nature and Science, Grant, U. S., IV and Gale, H. R., 1931, Catalogue of the marine ); Dr. Rei Nakashima, Dr. Seiichi Toshimitsu, Pliocene and Pleistocene Mollusca of California and adja- and Dr. Tomoaki Sumii (Geological Survey of Japan, cent regions, with notes on their morphology, classification, and nomenclature and a special treatment of the Pectinidae Tsukuba); and Dr. Reishi Takashima and Mr. Jun Nemo- and Turridae (Including a few Miocene and Recent species), to (Tohoku University, ), for their cooperation in together with a summary of the stratigraphic relations of the accessing some specimens in their care. This study was formations involved. Mem. San Diego Soc. Nat. Hist., 1, 1–1036. supported by the Grants-in-Aid for Scientific Research Haasl, D. M., 2000, Phylogenetic relationships among nassariid from the Japan Society for the Promotion of Sciences gastropods. Jour. Paleont., 74, 839–852. Jour. Geol. Soc. Japan 119（ 4 ） Mollusca from the“ Miocene” Namigata Formation, SW Japan 263

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（要 旨）

Matsubara, T., 2013, Molluscan fauna of the“ Miocene” Namigata Formation in the Namigata area, Okayama Prefecture, southwest Japan. Jour. Geol. Soc. Japan, 119, 249–266．（松原尚志，2013，岡山県浪形地域の“中新統”浪形層の貝類化石群．地質雑， 119, 249–266.） 岡山県井原地域に分布する“中新統”浪形層の貝類化石群について分類学的な再検討を行 い，本層の地質年代と古地理学的意義について議論した．浪形層は，主部と千手院貝殻砂 岩部層（新称），大西礫岩部層（新称）の 2 部層に区分される．本層から８種の腹足綱，13 種 の二枚貝綱および１種の掘足綱が得られた．Molopophorus watanabei Otuka, Acila （Truncacila）nagaoi Oyama and Mizuno, Chlamy（s Nomurachlamys?）namigataensis （Ozaki）および Isognomon（Hippochaeta）hataii Noda and Furuichi の産出から，本層の 貝類化石年代は後期始新世後期～前期漸新世に改訂される．最新の板鰓類化石のデータお よび予察的なストロンチウム同位体比を考慮すると，本層の年代は後期始新世後期に限ら れる．本研究および最近の研究から，第一瀬戸内海は異なる時代に存在した２つの海域， すなわち，南で太平洋に面した古第三紀の外海と北西部で日本海と接続していた内湾の中 新世の海域，から構成されていたことが示される．したがって日本の新生代地史における「第 一 瀬 戸 内 海 」の 概 念 に つ い て は 再 評 価 さ れ る 必 要 が あ る．Felaniella（Felaniella） namigataensis n. sp（. 和名：ナミガタウソシジミ）を含むいくつかの種について，分類学的 な記載・議論を行った．

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