Two New Species of Venericardia and Crassatella from the Eocene Formations in Amakusa Island

Two

New

Species of Venericardia

and Ｃｒａｓｓａtella

 from the Eocene

Formations

in Amakusa

Island

         Ｉ Masayuki Tashiro

Ｄｅｐａｒtｍｅｎt ｏ/ Ｇｅｏｌｏｇｙ, Ｆａｃｕltｙ ｏ/ Ｓｃｉｅｎｃｅ ， Ｋｏｃｈｉ Ｕｕiｕｅｒｓiり

      INTRODUCTION

 This paper deals with the description of two new species belonging to VenericaΓｄに and Crassa￡ella, i. ｅ.，  Ｖｅｒieｒｉｃａｒｄｉａ（？ Ｐａｃｉｆｉｃａ『』ushibukensis sp. nov. and Ｃｒ。ｓａ￡ella (Ｅｕｃｒａｓｓａtella）んａ￡aei sp. nov., from the Paleogene strata of Amakusa-Shimojima Island, Kyushu｡

 The formation yielding a new species of Venericardia, is composed of dark-gray siltstone inserting with several calicarious sandstone lenses which inclusive of Nummulites

ushibukensis and Ｄｙｓｃｏｃ＾iclina ｓp‥ Although the formation was called the Fukuregi

Formation ( Matsushita. 1949 ； Matsushita et al., 1959 ； Miki, 1975 ； Miki and Uematsu, 1973) or Akashimisaki Formation ( Hatae, 1959), this was correlated with the“Akasaki”Formation by Tashiro and Otsuka ( 1979 ) . The geological age of the formation had been determine to lower part of the Middle Eocene judging from the occurrence of calicarious nannofossils (￡iiscoas￡er rodoensis Zone : Tashiro et al., 1979)｡

Text-fig. 1. Map showing fossil localites.

  A. locality of VenericaΓdia(？Ｐａｃｉｆｉｃａｉ^ ｕｓｈｉｂｕｆｅｅｎｓiｓTashiro, sp. riov.   B. localities of Crassa￡elLaCＥｕｃｒａｓｓａtelld>ｈａtａｅｌTashiro, sp. nov.

64 高知大学学術研究報告 第32巻 自然科学

CｒａｓｓＱtclla’（Ｅｕｃｒａｓｓtella) ｈａtａｃｉsp. nov. occurs from the fine grained glauconite sandstone of the upper-most part of the Kyoragi Formation which lays comformably on the “Shiratake” Formation.

Before going into my study, I wish to express my hearty thanks to Dr. Kenshiro Ogasawara of Tohoku Univ, for his kind advice.

  The specimens described in this paper are preserved in tねｅ Faculty of Science, Kochi University (Kochi, 780) .

      Systematic description

       Order Veneroida ゛

      Superfamily Carditacea Fleming

       Family Carditidae Fleming

      Genus Venericardia Lamarck, 1801

       Subgenus 7)ａｃｉｆｉｃａｒVerastegui, 1953

      Veｒieｒｉｃａｒｄｉａ(？Ｐａｃｉ/ｉｃａｒ■)ushibukensis, sp. nov.

      Plate l， Figs. 13-15, Text-fig. 2

  Ｍａ￡erial : − Holotype, KSG 0900, a conjoined valves, from ＵＳｈりima islet, Ushibuka City, Kumamoto Prefecture.

  Ｄｉｆｌｇｏａｏｓiｓ: − Shell large, well inflated, subtriangular in outline, a little longer than high ； test very thick ； umbo large, well prominent, pointed at a little anterior than mid-point of the dorsal margin ； beak nearly orthogyrous, very incaved, situated nearly mid-length of the valve; anterior margin weakly concave, with about two thirds of the valve height ； ventral margin broadly arched, with full length of the valve ；

Two New Species of Ｖｅｎｅｒｉｃａｒｄｉａand Ｃｒａｓｓａtella（田代） ₆₅

posterior margin nearly straight, a little ･longer than the. anterior margin ； umbonal angle is about 80 °； lunular area flat, lanceorated in outline, nearly smooth ； escutcheonal area indistinct ； surface ornamented with 35 0ｒ more radial ribs ； the ribs strong, flat-topped and narrower than their interspaces on the umbonal part,

gradually changing into broad and strongly granulated on anterior ventral part, and broaded and low or nearly effaced with only finely waved growth lines on median and posterior ventral parts ； the interspaces of the ribs deep and broad on umbonal and

anterior ventral parts, but gradually became narrow and shallow grooves on the median and posterior ventral parts｡

 Ｒｅｍａｒhｓ: − This specimen is measured 74.6 mm in length, 65.6 mm in height and 26.5 mm in thickness. The hinge structure is unknown.  This species is characterized by

very weak or nearly effaced radial ribs on the median and posterior ventral parts on the flank. The feature of the ribs is one of the important characters of the subgenus Ｐａｃｉ/ｉｃａｒ Verastegui ( 1953 ) . Such the features are also known in the species

of the subgeneｒａ Ｖｅｎｅｒｉｃａｒ Stewart ( 1930) and Ｌｅｕｒｏａｃtiｓ Stewart ( 1930 ) . This species, however, may cannot be refer to Venericar and Ｌｅｕｒｏａｃtiｓ, because of its

flat lunular area and strongly prominent ｕｍｂ０｡

  The type species of Ｐａｃボｃａｒ， Venericarぷａ ( Pacボｃａｒ） mulleri Verastegui  (1953), from the Paleogene of western North America is discriminated from this

species in having its rounded outline.  This species resembles Ｖｅｒieｒ ｉｃａｒｄｉａ  （j）ａｃｉｆｉｃａｒ）んornii(Gabb ) ( Verastegui, 1953 ； Stewart, 1930 ； Givens, 1974) from

the Eocene of California, in its strongly inflated valve and large umbo, but differs in its subtriangular outline and numerous ribs. ■ Maoricardium Ｓｐａ￡iosum  ( Hutton ) ( Marwick, 1944 ； Boreham, 1965) from the Paleogene of New Zealand, is similar to this species in its large umbo and subtrigonal outline. Maor.

ｓｐａ￡iosum is, however, characterized by the distinct radial ribs on the ventral

part of the disk. ｖｅ几ｅｒｉｃａｒｄｉａ（Ｖｅｎｅｒｉｃａｒ) nipponica Yokoyama ( 1911 ； Nagao, 1928 ； Oyama, Mizuno and Sakamoto, 1960 ； Otsuka, 1978 ) , from the Eocene Formation of North Kyushu and Amakusa-Shimojima, is clearly･ discriminated from this species in its less prominent umbo and less distinct lunular area than those of this species｡

  Ｏｃｃｕｒｒｅｎｃｅ: − Dark gray siltstone of the .lower part of the“Akasaki" Formation  「= Fukuregi Formation by Matsushita,」949, or upper part of the Akashimisaki

Formation by Hatae, 1959), at northern seashour ･of the Ushijima islet, Ushibuka City ( Amakusa-Shimojima ) , Kumamoto Prefecture ； lower part of the Middle Eocene.

Superfamily Crassatellacea Ferssac

  Family Crassatellidae Ferssac

 Subfamily Crassatellinae Ferssac

66        ，尚刈入字字爾研児報舌 弟32巷 目然科字

      Genus Crassa￡ella Lamarck, 1799        Subgenus ＥｕｃｒａｓｓａtellaIredale, 1924        Cｒａｓｓａtella(Ｅｕｃｒａｓｓａtella)ha￡dei, sp. nov.       Plate l，Figs. 1-12, Teχt-fig. 3

  A＆z￡erial: − Holotype, KSG 0901, left valve, from Medake of Miyanokawachi, Amakusa-Shimoiima ； paratypes, KSG 0902 −KSG 0901, iｒom the same locality of the holotype ； the other paratypes, KSG 0908 ，KSG 0909, from the western seasour of the Ubushima islet of Miyanokawachi, Amakusa-Shimojima･ .

  ﾌ:^ｉａｇｏｎｏｓiｓ: − Shell roundly ovate, longer than high, moderately inflated ； umbo small, weakly prominent, nearly orthogyrous or ，slightly･ prosogylate, located at

about two fifths from front of the valve ； anterior dorsal margin short, weakly concave ； posterior dorsal margin slightly conveχ with about ａ fourth of the valve length ； umbonal angle about 100 °； anterior margin semicircular, gradually changing into broadly arched ventral margin ； posterior ・ margin slightly convex, obliquely subtruncated from the posterior dorsal margin ；  lunule lanceorated, moderately

Ｊ』匹

Tｅχt-fig. 3. Ｃｒａｓｓａtella(Eucrassa￡ella) ha￡aei, sp. now.   Ａ: Internal view of left valve; Ｂ: External view of left valve;   Ｃ: Dorsal view of conjoint valves      '

Two New of Ｖｅｎｅｒｉｃａｒｄｉａand Ｃｒａｓｓａtella（田代） ₆₇

depressed, ornamented with numerous concentric ribs which conjoint with the ribs on the flank ； escutcheon narrow, deeply depressed ； flank ornamented with concentric ribs and riblets ； the ribs narrow, round-topped, crowded regularly near the umbo but broadly spaced on the later ； the riblits occupying on each interspace between ａ rib and ａ next rib, number about ４ in each interspace; posterior Carina not angulated

only ａ roundly elevated ridge ； ligamental resilifer entirely internal, moderately in size, located under the umbo, not reaching the lower periphery of the hinge plate ； hinge plate wide with anterior and posterior lateral teeth a‘nd two cardinal teeth on each valve; hinge formula is as follows: −

     All 2 4b PII ／ AI 3b 5b PI

cardinal 2 is very narrow ； 4b large, nearly vertical ； 3b strong, triangular ； 5b short and small ； lateral teeth narrow ； nymph rather broad ； inner surface smooth ； inner margin smooth; ａ strong radial inner buttres extending from the umbo to posterior of the anterior adductor scar but not reach ，the･ pallial line ； both lateral adductor scars strongly impressed ； anterior ono somewhat larger than posterior one ； pallial line simple, weakly impressed.

Mｅａｓｕｒｅｍｅｎtｓ（ｉｎｍｍ）:−

Specimen

KSG

KSG

0904, conjoint internal moulds

Le gth 5 ３ ８ ７ ６ ３ Ｕ Ｄ   Ｉ   Ｉ   Ｉ   Ｉ   Ｉ   Ｉ   一 6 8 5 0 4 9 C N ] １ １ Ｃ -a Ｃ Ｏ C O C N 3 C M Height -14. 4 ］５．０ 21.7 24.0 + 24.9十 28.0 16.8十 Thickness 2.8 4‥３ ５．７ ７．5 7.0 ち Ｓ ゝ ♂ ｊ 1 1 1 1 1

Text-fig. 4. Comparison of the outlines betｗｅｅｎＣｒａｓｓａtella（Ｅｕｃｒａｓｓａtella）   んα￡aei,sp. nov. (full line) andＣｒａｓｓａtella（Ｅｕｃｒａｓｓａtella）・

68

高知大学学術研究報告 第32巻  自然科学

 Ｒｅｍａｒfeｓ: − This species is probably identical with “ Crassa￡eli￡es n. sp.”which was listed from so-called Tomiyama fossil Bed of the Kyoragi Formation of the

Amakusa-Shimojima, by Hatae (1960).

 The concentric ribs on the flank number about 20. 10 0ｒ so ribs near the umbo are closely spaced. But the other ribs are irregular in the stoutness and gradually broaded in their interspacess towards the venter. The ratio of length to height of the valve is rather uniform with about 0.88.

 Ｃｏｍｐａｒiｓｏｎ ". − Although this species differs fromミCrassa￡ella (Eucrassatelld) ｎｉｐｐｏｎｅｎｓiｓＹｏｋｏyａｍａ（＝Ｃ. fｕｓｃａYokoyama, 1981 ; Nagao, 1928 ； Oyama, Mizuno and Sakamoto, 1960), from the Eocene Formations of North Kyushu and Amakusa Island, in having its rounded outline and more posterior location of the umbo, the

hinge stracture of this species is nearly identica! with that of Ｃ.（￡），zφρonensis in the almost features. Ｃｒａｓｓａtella(Spissa￡ella) aus￡ralis (Hutton) (Boreham, 1965)

from the Middle Eocene of New Zealand is similar to this species in its rounded outline and the arrengiment of the ribs and riblets on the surface. The foregin species is, however, characterized by more depressed escutcheon and narrower cardinal 3b and distinct 3a than those of this species. Several species which were d:escribed from the Paleogene

○○☆

○く

○∧卜∧

Tｅχt-fig.5. Comparison of the   Ｃｒａｓｓａtella、

outlines among the species of Japanese Paleogene

1: Ｃｒａｓｓａtella（Ｃｒａｓｓａtelは)komodai Oyama et Mizuno; 1:  Ｃｒａｓｓａtella（ＣｒａｓｓａtiｒxＱ）ｐａｕｘillｕｓ（Ｙｏｋｏｙａｍａ） 3: Ｃｒａｓｓａtella（Ｅｕｃｒａｓｓａtella) ｎｉｐｐｏｎｅｎｓiｓYokoyama

４: Ｃｒａｓｓａtella（？Ｅｕｃｒａｓｓａtella）ｍＱ￡suraensisNagao｡ 5: Ｃｒａｓｓａtella（Ｅｕｃｒａｓｓａtella）ａｓahｕｒａｅｎｓiｓNagao ６: Ｃｒａｓｓａtella（Ｅｕｃｒａｓｓａtella）ｍｃｏｎｓpici↓ｓNagao ７: Ｃｒａｓｓａtella（Ｅｕｃｒａｓｓａtella）ｙｅｓｓｏｅｎｓiｓMizuno et Kumano 8: Ｃｒａｓｓａtella（Ｅｕｃｒａｓｓａtelln）ｙａｂｅｉNagao

Two New Ｓ of Venericardia and Ｃｒａｓｓｄtella（田代） ･69

of Japan as the members of genus Crassatella, are easely distinguishable from this species in their features of each outline of the valve as showing in Text-fig. 5｡

  Ｏｃｃｕｒｒｅｎｃｅ: − Fine grained glauconite sandstone of the upper-most part of the

Kyoragi Formation at the roadside eχposure of Nonaka, Medake peninsula, and the western seasore of Ubushima islet, both of Miyanokawachi, Kawaura-Machi, Amakusa-gun ( Amakusa-Shimojima ) , Kumamoto Prefecture ； Middle Eocene.  This species occurs together with Venerkarぶａ（Ｖｅｒieｒｉｃａｒ）y1叩ponica Yokoyama, Aces￡a kumasoana  (Nagao) etc.

REFERENCES

Boreham, A. U. E. (1965): A revision of F. W. Hutton's pelecypod species described in   the Catalogue of Tertiary mollusca and echinodermata (1873). N. Z.Ｇｅｏｌ. Ｓｕｒ，Ｐａｌｅａｎt．   Ｂｕll. 37, 84 p･， 20 pis.

Givens, C. R. ( 1974): Eocene molluscan biostratigraphy of the Pine Mountaine Area,

   Ventura County, California.び ｢ひ. Cal.Ｐｕbl.Ｇｅｏｌ.  Ｓｃｉ.,vol. 109, 107 p., 11 pis Hatae, Ｎ. (1959):On the stratigraphic boundary between the Upper Cretaceous and Paleogene   strata of Amakusa-Shimoshima, Kumamoto Prefecture (in Japanｅｓｅ with English abstract).   Ｓｃｉ.Ｒｅｐ. Ｋａｇｏｓhlma Ｕｎｉｕ.,no. 8, p. 101-113.

(1960):The geology and geological stracture of Amakusa-Shimo-shima, Kumamoto Prefecture    （ｉｎJapanese with English abstract). Ibid, no. 9, p. 61-107.

Matsushita, H. (1949):Geology of the coal field in northern Kyushu （ｉｎ Japanese). Sci.   Ｒｅｐ. Ｆａｃ.Sci.Ｋｙｕｓhｕ Ｕａｉｕ.,Geol., vol. 3, no. 1, p. 1-57.

一 一

. Takahashi. R., Urata. H., Iwahashi, T., Ohara, F.. Tomita, S. and Ota.K. (1959):On the boundary between the Cretaceous and Paleogene sediments in Amakusa-  Shimoshima (in Japanese).Ｙｕfeochｕ, no. 10, p. 30-41.

Marwick, J. (1944): New Zealand fossil and recent Cardidae (Mollusca )｡ Ｒｏｙal:. Soc. N.   Z. Tra几Ｓ。vol. 74, p. 255-272, pi. 25-37.

Miki, T. (1975): Formation and development of sedimentary basins during the Paleogene   in Amakusa and adjacent areas, western Kyushu. Ｍｅｍ.. Fac. SciＫｖｕｓfiｕ Ｕｎｉｕ.ser. D,   Geol. vol. 23, no. 2, p. 165-209.

一一 and Uematsu, M. (1973): The Paleogene and Upper Cretaceous formation in the    southern part of Shimoshima, Amakusa Coal-field, with special emphasis on their   stratigraphic relation and petrographic characters of sandstone （ｉｎJapanese with English   abstract)｡対決決ｇ GeoL, vol. 23, no. 3, p. 227-236.

Nagao, T. (1928): Palaeogene fossils of the island of Kyushu, pt. l.Sci. Rep.Ｔｏｈｏｋｕ   Ｉｍｐ. Ｕｎｉｕ。ser. 2, vol. 9, no. 3, p. 97-128, 5 pis., pt. 2, Ibid., vol. 12, no. 1, p. 11 -140,   17 pis.

０t･suka, M. (1978): Ａ new species of Bankia in paleogene fossil wood, from Amakusa,    Kumamoto Prefecture, Japan.  Ｔｒａｎｓ.Ｐｒｏｃ. Ｐａｌａｅｏｎt. Soc.Ｊａｐａｎ.,n. s･, no. 112,   ｐ. 417-423, pis. 52, 53.

Oyama, K., Mizuno, A. and Sakamoto, T. (1960): Illustrated handbook of Japanese Paleogene   molluscs. ＧｅｏＬSur.Ｊａｐａｎ,224 p. 71 pis.

Stewart, R. B. (1930):Gabb's California Cretaceous and Tertiary type lamellibranches. Acad.   Ｎｎt..Sci。Ｐｈｉｌａｄｅｌｐｈｉａ,Spec. Publ., no. 3, 314 p， 17 pis.

Tashiro, M. and Otsuka, M. (1978):Stratigraphical study on the boundary between the Upper   Cretaceous and Paleogene strata of Amakusa-Shimojima Island, Kumamoto Prefecture,   Kyushu. Ｒｅｓ，Ｒｅｐt，ＫｏｃｈｉＵｎｉｖ。vol. 27, nat. sci., p. 113-]34, 2 pis.

一一, Okada, H., Taira, A. and Otsuka, M. (1980):Middle Eocene calcareous nannofossils   from the basal part of the Tertiary strata in Amakusa-Shimojima, Kyushu (in Japanese).   Ｊｏｕｒ. Ｇｅｏｌ，Ｓｏｃ.Ｊａｐａｎ,vol.86, no. 2, p. 139-141.

Verastegui, P. (1953): The pelecypod genus Venericardia in the Paleocene and Eocene of   western North America. Palaeontographica Americana, Vol. 3, no. 25, 112 p. 22 pis.

Yokoyama, M. (1911): Some Tertiary fossils from the Miike coal fieldJoｕｒ Ｃｏｌｌ Ｓｃｉ.   Ｉｍｐ.び ｢lﾉ｡７０ん■yo,vol. 27, art, 20, 16p, 3 pis.

7 0 高知大学学 告  第32巻・ 自然科学

(Manuscript

Received: July 5，1983)

(Published: March

15パ984)

Explanation

of plate l

Cｒａｓｓａtella(Eucrassatella)んａ￡aei sp. now.

 Fig. 1: Lateral view of left valve, KSG 0902, χ1.5; loc. Nonaka of    Medake, Miyanokawachi, Kawaura-Machi, Amakusa-Shimoiima.

 Fig. 2: Lateral view of left valve (holotype), KSG 0901, χ 1.5; loc.:ditto.  Fig. 3: Lateral view of left valve, KSG 0907, χ 1.5; loc.:ditto.

 Fig. 4: Lateral view of right valve, KSG 0903, ｘ 1.2; loc.:ditto.

 Fig. 5: Lateral view of left valve, same specimen with KSG 0903, ｘ 1.2.  Fig. 6: Dorsal view of the same KSG ０９０３，ｘ 1.2.

 Fig. 7: Lateral view of right valve, same with KSG 0903, ｘ 1.5.

 Fig. 8: Lateral view of internal mould of left valve, KSG 0904，ｘ l; loc.:ditto.  Fig. 9: Lateral view of left valve, gum cast うfexternal mould, KSG

 ・ 0905，χ 1; loc. ditto.

 Fig. 10: Inner umbonal view of right valve,･KSG 0906，ｘ l; showing hinge    structure;loc. ditto.

 Fig. 11: Lateral view of left valve, KSG 0908，ｘ 1.5; loc, south-wesstern    seashore of Ubushima islet, Miyanokawachi, Kawaura-Machi.

 Fig. 12: Lateral view of right valve, same specimen with KSG 0908, ｘ 1.5.

Vｅｎｅｒｉｃａｒｄｉａ（？Ｐａｃポｃ＆，ｕｓhi.bｕfeenｓiｓ,sp. now.

  Fig. 13. Lateral view of left valve (holotype), KSG 0900，ｘ l; loc.:   eastern seashore･of Ushiiima, Ushibuka City, Amakusa-Shimojima.   Fig. 14. Dorsal view of the same specimen, ｘ l.

  Fig. 15. Anterior marginal view of right valve, same with KSG 0900, X 1

-｜ ｀ ゝ １５ い , ４ ｊ ≒ − ” − − ・ ・ ・ 。 汲 コ 難 ’ Ｓ ゛ へ ・ ｙ ｆ ６ − − ・ − − １３ ２ ・ ． ５ − ・ ● φ ・ Ｗ ” ︲ＩＩ − ４   ・ ● ●     ゝ ぐ ／ 〆 ８ １ １ ３ １１ １２ １４ 4 ･ ● -９