VENUS 64 (l-2): 11-21, 2005

A New Species of Japanese Chama (Bivalvia: Heterodonta) with with a Calcitic Outermost Layer

Naoto Hamada1 and Akihiko Matsukuma2

2 1Kumamoto University, 2-39-1, Kurokami, Kumamoto 860-8555, Japan めushu Universi ηMuseum, 6-10-1 Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan; matukuma@museum. わ1ushu-u.ac.j

Abstract: Abstract: A new species of the Chamidae from Japan is described under the name of Chama cerinorhodon cerinorhodon n. sp. Chama cerinorhodon has hitherto been misidentified as Chama fragum Reeve, Reeve, 1846, from its superficial morphological characters. Howev 巴r, it is here reveal 巴d that this this species is sufficiently distinct from true C. fragum in shell mineralogy and shell structure to to warrant separate status. Chama cerinorhodon has a calcitic outermost shell layer in addition to to aragonitic inner and median shell lay 巴rs, whereas C. jトagum has only aragonitic inn 巴r and outer outer shell layers. Chama cerinorhodon is closely related to the eastern Pacific species Chama arcana arcana Bern 紅 d, 1976, and Chama pellucida Broderip, 1835, in shell morphology, mineralogy, and and structure.

Keywords: Keywords: Chama, new species, calcitic lay 巴r

Introduction Introduction

The superfamily Chamoidea is represented by the single family Chamidae Lamarck, 1809, with with approximately 70 living species world-wide (Bernard, 1976). The taxonomy is confused at not not only species but also higher levels (Matsukuma, 1996). Ecologically, almost all chamids are sessile sessile in the epifaunal benthos and usually attach themselves to a hard substratum by one or the other other valve. Some exceptions exist, such as the secondary free living genus Arcinella Schumacher, 1817, 1817, of Central America, but they too undergo a brief phase of cementation at an early stage of of life. Chamid shells are highly inequivalve, with strongly prosogyrate beaks; the free valve is relatively relatively flat and smaller than the cemented valve, which is usually convex. Kennedy et al. (1970) examined examined 19 fossil and 14 living species and concluded that almost all chamid species have two aragonitic aragonitic shell layers, i.e. an inner complex crossed-lamellar layer and an outer crossed-lamellar layer, layer, and only a few species have an additional calcitic outermost shell layer. A specimen with peculiar shell characters was reported by Hamada & Matsukuma (1995) from the the southwestern coast of Japan. This seemed to have long been identified as Chama j均gum by many Japanese authors. The sp 民 imen had a calcitic outermost layer in addition to the aragonitic inner inner and median shell layers. It has hitherto been considered that chamids have two aragonitic shell shell layers, except for four species with a calcitic outer shell layer: Chama arcana Bernard, 1976, 1976, C. pellucida Broderip, 1835, Pseudochama exogyra (Conrad, 1837) and P. granti S佐ong, 1934. 1934. All four are limited in dis 佐ibution to the eastern Pacific. The specimen reported from Japan was concluded to possibly represent one of these four species or an entirely new species, because its its characters had not been observed in any speci 回 reported from the Indo-West Pacific, including Japan. Japan. Based on specimens from Japan, Taiwan, and Philippines, the reported species is here described described as new. 12 12 N. Hamada & A. Matsukuma

Materials and Methods

Specimens Specimens from the collections of the following institutions were ex 創凶ned: GK - Department of Earth and Planetary Sciences, Faculty of Sciences, Kyushu University, Fukuoka, Fukuoka, Japan; NSMT - Department of Zoology, National Science Museum, Tokyo. Specimens Specimens from the collections of the following institutions were examined for comp 訂 ison: LACM - Los Angeles County Museum of Natural History, Los Angeles, California; NHM - Natural History Museum, London; SBMNH - Santa Barbara Museum of Natural History, Santa Barbara, California; SDNHM-S 姐 Diego Natural History Museum, San Diego, California. Morphological Morphological observation was carried out with an optical microscope and WET-SEM. The WE 下SEM observation was performed using a Topcon Model ABT-32 at the Center of Advanced Instrumental Instrumental Analysis, Kyushu U 凶versity. The microstructure of the shell was observed optically using acetate peels prepared from polished polished O. lN HCl etched sections.

Taxonomy

Family Family Chamidae Lamarck, 1809 Genus Chama Linnaeus, 1758 Chama cerinorhodon n. sp. (Figs. (Figs. I-4)

Chama fragum Reeve, auctt. non Reeve, 1846, pl. 9, sp. 48: Okutani & Soyama, 1987, p. 83; Uozumi, 1998, 1998, p. 137, fig. 102, no. 3 (sp. 384).

Chama japonica Lam 訂 ck, sensu Fukuda et al., 1992, p. 88, pl. 32, fig. 493, non Lamarck, 1819. Chama sp.: Hamada & Matsukuma, 1995, pp. 95-96.

Holo 砂'Pe: GK.N10108 ・2 (ex Sato collection). Type local 砂： Off Him 回 hima Islet, Shima-machi, Itoshima-gun, Fukuoka Prefecture, Japan (Long. (Long. 130 。03' 10" E, Lat. 33° 33' 30" N). Para 砂'Pes: GK.N20018, Sumiyoshi, Hakodate, Hokkaido, ex Igarashi coll.; GK.N20010, Jogashima Jogashima Islet, Miura, Kanagawa Prefecture; NSM 下Mo 73694, Inatori, Higashi-Izu, Shizuoka Prefecture; Prefecture; GK.N20008, Kushimoto, Wakayama Prefecture, ex Sato coll.; GK.N20014, Sakai, Minabe, Wakayama Prefecture, ex Ikebe coll.; GK.N20016, Shiono-misaki, Kushimoto, Wakayama Prefec 佃re, ex Ikebe coll.; GK.N20015, Nogita, Shima-machi, Itoshima-gun, Fukuoka Prefecture; Prefecture; GK.Nl0103 (specimen reported by Hamada & Matsukuma, 1995), 10109, 20009, and 20012, 20012, Fukuyoshi, Nijo ・machi, Itoshima-gun, Fukuoka Prefecture, ex Sato coll.; GK.N20011, Kamae, Oita Preti 氏側re, ex T. Hamada coll. E 砂mo logy ：’ The speci 回 name is derived from the Greek adjective κ1]pt υoσ （waxy) and noun po8ov (rose), referring to the pinkish translucent appearance and radial lamellate spines of 白is species. species. Description: Description: Shell thick, small, rather to 23.6 mm in length (Table 1), cemented to hard substratum substratum by wide anterior section of left valve. Outline of both valves circular or ovate, depend- ing ing upon habitat. Exterior sculpture of both valves consisting of close-set lamellate spines, with sever ョI folds and larger spines. Exterior waxy, translucent, reddish pink to white. Shell margins minutely minutely crenulate. Dentition of adult shells as in Chama of Matsukuma (1996), not highly developed. developed. Prodissoconch with only commarginal growth lines (Figs. 7-8); early dissoconch A New Species of Japanese Chama 13

Table 1. Chama cerinorhodon n. sp. Measurements (mm) of selected material. Valve Valve L H Cs Cb lx h GK.Nl0103 -1 Left 16.1 8.0 ・2 L 巴ft 11.8 13.1 5.9 8.7 0.40 × 0.38 Right Right 11.3 10.7 3.7 GK.Nl0108 -1 Left 20.2 28.5 12.7 16.9 Right Right 18.2 19.4 6.7 -2 -2 Left 16.8 22.3 9.8 13.7 0.41 × 0.40 Right Right 16.0 16.9 5.6 GK.Nl0109 -1 Left 18.4 20.3 10.1 14.7 0.54 x 0.48 Right Right 16.1 15.8 6.3 -2 -2 Left 29.l 14.5 GK.N20008 -1 Left 20.9 20.3 11.5 15.7 Right Right 19.2 16.5 6.8 0.58 × 0.44 -2 -2 Left 16.3 9.8 10.2 Right Right 14.2 11.5 3.8 -3 -3 Lft 14.0 18.2 9.1 12.5 Right Right 14.6 15.1 5.7 -4 -4 Left 10.4 14.1 6.1 8.4 Right Right 10.1 11.5 4.1 GK.N20009 Left 23.6 27.0 12.5 16.5 Right Right 21.2 22.0 6.2 GK.N20010 Left 18.2 18.7 9.2 12.6 0.45 × 0.39 Right Right 15.2 17.7 6.5 GK.N20011 -1 Left 16.0 15.8 7.6 9.1 0.48 × 0.44 Right Right 13.0 11.1 4.1 -2 -2 L 巴ft 11.2 12.7 7.6 9.5 0.46 × 0.42 Right Right 10.3 10.6 4.8 GK.N20012 -1 Left 14.6 13.8 10.5 11.7 Right Right 11.9 11.8 4.6 ・2 Left 10.6 13.4 7.4 8.9 0.46 × 0.36 Right Right 10.3 9.5 3.8 GK.N20014 ・1 Left 14.8 16.8 6.7 10.1 0.46 × 0.41 Right Right 15.0 13.9 4.3 -2 -2 Left 15.6 15.6 8.3 9.8 Right Right 14.2 13.0 3.8 GK.N20016 Left 19.1 21.9 8.2 12.4 0.54 × 0.48 Right Right 17.7 16.2 6.3 GK.N20018 Left 15.6 17.5 Right Right 14.9 14.5 5.4

Mean S.D. N 0.48 0.48 0.06 10 h 0.42 0.04 10 Abbreviations Abbreviations Cb: Cb: Convexity of 加由valves measured vertically to the commissure plane. Cs: Cs: Convesity of odd valve measured vertically to the commissure plane. H: H: Shell height measured vertically to the anterior-posterior adductors line. h・ h・ Height of early dissoconch. L: L: Shell length measured in p釘 allel to the anterior-posterior adductors line. Length Length of early dissoconch 14 14 N. Hamada & A. Matsukuma

Figs. Figs. la-d. Holotype of Chama cerinorhodon n. sp. from Himeshima Islet, Shima-machi, Fukuoka Prefecture, Prefecture, GK.N10108-2. la-b. The right valve. Shell length 16.0 mm. lc - d. The left valve . Shell length 16.8 16.8 mm. Figs. Figs. 2a-c. Paratype of Chama cerinorhodon n. sp. from Sumiyoshi, Hakodate, Hokkaido, GK.N20018-l. 2a-b. 2a-b. The right valv 巴 Shell length 14.9 mm. 2c. The left valve . Shel! length 15.6 mm. Figs. Figs. 3a-c. Paratype of Chama cerinorhodon n. sp. from Sakai, Minabe, Wakayama Prefecture, GK.N20014 -2. 3a. The left valve . Shel! length 15.6 mm . 3b-c. The right valve . Shell length 14 .2 mm. Fig. Fig. 4. Paratype of Chama cerinorhodon n. sp. from Fukuyoshi, Nijo - machi, Fukuoka Prefecture, GK.N10108 -l. A conjoined specimen. Shell length 20.2 mm . A New Species of Japanese Chama 15

Fig. Fig. 5. One of the possible syntypes of Chamafragum Reeve , 1846 .BMNH1995182. Sa, b. Right valve, shell shell length 21 mm. Sc. Left valve, shell length 28 mm. Fig. Fig. 6. One of the possible syntypes of Chama fragum Reeve, 1846. BMNH1995182 . 6a : Right valve, shell shell length 32 mm. 6b. Left valve , shell length 40 mm. ornamented ornamented with minute sculpture of closely spaced radiating striae and commarginal lamellae (Figs. (Figs. 7, 9). Early dissoconch small, 0.48 mm in average length (Table 1) . Dentition of nepionic shell shell unknown. Shell Shell structure consisting of an outermost calcitic layer , a median aragonitic crossed-lamellar layer, layer, and an inner aragonitic complex crossed -lamellar layer (Fig . 8). Irregular prismatic myos- 仕acum present between median and inner layers. Outermost calcitic thick layer and translucent , built built up of “ minute blade -like prisms ” of Taylor & Kennedy (1969 ）.“ Prisms ”variable in shape and size , and arranged into longer irregular blocks, especially near point of contact between outermost outermost and median layers (Figs. 9-11 ). Longer axis of each block usually set parallel to shell interior interior margin at time of secretion. Median layer formed with crossed-lamellar structure (Fig. 13). 13). Line of contact between outermost and median layers highly irregular (Fig. 12). Inner layer basically basically formed with complex crossed-lamellar structure, but pillars of myostracal type prisms occasionally occasionally occurring (Fig. 14) . Measurements : See Table 1. Distribution: Distribution: Pleistocene - Miyata Formation, Tsukui , Yokosuka, Kanagawa Prefecture, Japan. Recent Recent - Southern coast of Hokkaido to Kyushu (Fig. 15) . Comparisons: In this study, it was revealed that the outermost layer of C. cerinorhodon n. sp. has a similar structure to that of C. arcana . Taylor & Kennedy (1969) called the layer “prismatic ”； however they pointed out a marked difference from the calcite prismatic layer of most other bivalves , in that thick conchioline walls were not developed between the prisms. Further Further study will be necessary to fully determine the s住ucture of the calcitic outermost layer of chamid species . Chama cerinorhodon n. sp. may be compared with a number of other species. Although Although Chama cerinorhodon n. sp . has previously been misidentified as Chama fragum Reeve, 1846 , originally described from Mindoro ，出巴 Philippines, it is clearly a different species . The Natural History History Natural Museum, London, has possible syntypes of C. fragum (BMNH 1995182) from an unknown locality . Observations of these demonstrated that there are some common morphological characters characters with C. cerinorhodon n. sp., i.e . size, color , crenulate margin s, and ornamentation of 16 16 N . Hamada & A. Matsukuma

Fig. Fig. 7. SEM micrograph of the prodissoconch and 巴arly dissoconch of Chama cerinorhodon n . sp. on the right right valv 巴 GK.N10103 . Scale bar= 80 μm. Fig. Fig. 8. SEM micrograph of the prodissoconch of Chama cerinorhodon n. sp. on th 巴 right valv 巴. Note the concentric concentric growth lines . GK .Nl0103. Scale bar= 20 μm . Fig. Fig. 9. SEM micrograph of the early dissoconch of Ch αmα cerinorhodon n. sp. on the right valve. Note the the minute sculpture of clos 巴ly spaced radiating striae and concentric lam 巴llae. GK.N10103. Scale bar= 20 μm .

outermost calcitic layer

outer crossed-lamellar layer

inner inner complex crossed-lamellar layer

pallial pallial myostracum

Fig. Fig. 10. Shell layers in Chama cerinorhodon n. sp. Radial section of right valve . GK.N10103. A New Sp 巴cies of Japanese Chama 17

Fig. Fig. 11. Transverse sectio n of th 巴 outermost calcitic layer (abov 巴） and median crossed lamellar layer (bottom) (bottom) of Chama cerinorhodon n. sp. Irregular outline and size of blocks are shown. Finer units are seen seen in the upper margin of this layer. Acetate peel. Scale bar = 100 μm. Fig Fig 12. Planar section of the outermost calcitic layer of Chama cerinorhodon n. sp. showing the irregular blocks blocks and finer units (in the lower left hand corner). The growt h direction is leftwards. Acetate peel. Scale Scale b紅＝ 100 μm. Fig. Fig. 13. Radial section of the outermost calcitic layer of Chama cerinorhodon n. sp. The line of contact between between this layer and the median crossed-lamellar layer (bottom) is also s巴en Inclined larg er irregular blocks blocks and finer units (in the upper margin of this layer) are shown . The growth direction is leftwards. Acetate Acetate peel. Scale bar = 100 μm. Fig. Fig. 14. Line of contact between th 巴 outermost calcitic layer (abov 巴） an d the median crossed-lamellar layer layer (below) of Chama cerinorhodon n. sp. Radial section. The crossed-lamellar layer projects into the outermost outermost calcitic la yer . The growth direction is leftwards. Acetate p巴el. Scal 巴 bar= 100 凶n. Fig Fig 15. Radial s巴ction of the media layer of Chama cerinorhodon n. sp. showing the cross 巴d-lamellar structure; structure; inner surface is also shown (bottom) . Th 巴 growth direction is leftwards. Acetate p巴el. Scale bar = 100 阿n. Fig. Fig. 16. Radial section of the inner layer of Chama cerinorhodon n. sp. showing the complex crossed- lamellar lamellar and myostracal structure . Inner surface is also shown (bottom). Ac 巴tate peel. Scale bar = 100 戸n. 18 18 N. Hamada & A. Matsukuma

ノ 130° 130° E 140° E &¥

Aら~う ':,'

ζフ・ " － ・ol 30 。N

Fig. Fig. 17. Geographical distribution of Chama cerinorhodon n sp. Solid dots: specimen examined. Open circles: circles: information from literature.

the adult the adult shell. However, the outer layer of C. j均gum is not translucent, but porcellaneous, with a cross 巴d-lamellar structure. The appearance suggests that the outer shell layer is possibly arago- nitic. nitic. Because these specimens are type material, we could not confirm the absence of a calcitic shell shell layer by X-ray diffraction technique or by making thin sections, but we are convinced that C. fragum lacks an outermost calcitic layer. Chama japonica Lamarck, 1819, is thought to be one of the most similar species to C. cerinorhodon cerinorhodon n. sp. among Japanese chamids. Chama japonica is attached by the left valve and its shell margins are crenulate. However, C. japonica can be easily distinguished from C. cerinorhodon cerinorhodon n. sp. by its somewhat larger size, reddish brown color, more minute and delicate spines, spines, and lack of a calcitic outermost shell layer. Pseudochama exogyra (Conrad, 1837), P. granti Strong, 1934, Chama arcana Bernard, 1976 and C. pellucida Broderip, 1835 are the four species that have an outermost calcitic shell layer (Kennedy et al., 1970; Hamada & Matsukuma, 1995). Pseudochama exogyra and P. granti in 出e collections collections of LACM, SBMNH, and SDNHM are considerably different from C. cerinorhodon n. n. sp. These two species are attached by the right valve, and their shell margins are smooth. Pseudochama exogyra is larger, up to 30-50 mm long, and its shell is ornamented by prominent, A New Speci 巴s of Japanese Chama 19

deeply deeply lobed lamellae. Pseudochama granti is a smaller sp 目 ies and its shell is ornamented by regular regular commarginal lamellae, occasionally with a few spines. Although both species have a cal- citic citic outermost shell layer, no other characters have been found in common with C. cerinorhodon. Chama arcana of the northeastern Pacific was described by Bernard (1976), and is consid 巴red to to be closely related to C. pellucida Broderip, 1835, of the southeastern Pacific. Chama arcana and C. cerinorhodon have the following points in common in shell morphology: An attached left valve valve ornamented by commarginal lamellate spines, especially in case of younger (approximately 20- 30 mm in length) individuals; a round outline and waxy translucent exterior; a prodissoconch ornamented ornamented by growth lines; an early dissoconch (the “dissoconch ” of Taylor & Kennedy, 1969) ornamentβd by regular commarginal lamellae and fine radial s佐iae (Taylor & Kennedy, 1969), and an early dissoconch of approximately 0.5 mm length (Hamada & Matsukuma, 1995). Chama arcana arcana is similar to C. cerinorhodon in shell structure and mineralogy, as well as general mor- phology. phology. It features a three-layered shell and a thick calcitic outermost layer built up of irregular blocks blocks (Taylor & Kennedy, 1969). However, th 巴re are some points of difference: Chama arcana is is significantly large in size (55 mm in length in the holotype), with closely set foliations on adult shells shells developed from lamellate spines, a whitish shell color and pink or red streaks on the right valve. valve. Chama pellucida Broderip, 1835, is similar to C. arcana, but is restricted to the southeastern Pacific Pacific (Bernard, 1976). Chama pellucida also has a larger shell than C. cerinorhodon. After After comparing C. cerinorhodon with a number of other species, it was concluded that C. arcana arcana and its close relative C. pellucida have the largest number of characters in common with C. C. cerinorhodon. It is nevertheless our conclusion that C. cerinorhodon is a di 能 rent species from C. C. fragum, and is closely related to 白e eastern Pacific species C. arcana and C. pellucida. In In this S旬 dy, it was revealed that the outermost layer of C. cerinorhodon has a similar struc- ture ture to that of C. arcana. Taylor & Kennedy (1969) called the layer “prismatic ”； however, they pointed pointed out a mar~巴d di 妊erence from the calcite prismatic layer of most other bivalves, in that thick thick conchioline walls were not developed between the prisms. Further study will b巴 ne 氾ess ぽ y to fully fully determine the structure of the calcitic outermost layer of chamid species.

Acknowledgements

We are grateful to Dr. Masanori Shimamoto of Tohoku University and Dr. Seiichi Suzuki of Fukuoka Univ 巴rsity of Education for valuable advic 巴 about shell structure in the Ch 創凶dae. We also express our sincere sincere thanks to Dr. Takashi Okutani of the Japan Agency for Marine-earth Science and Technology, Yokosuka, Yokosuka, Dr. Eugene V. Coan of California Acad 巴my of Sciences, Dr. Paul Valentich-Scott of Santa Barbara Barbara Museum of Natural History, and Mr. Paul Callomon of the Academy of Natural Sciences, Philadelphia Philadelphia their for comments and suggestions. The following collectors kindly offered us a number of chamid specimens: th 巴 late Mr. Katsuyoshi Sato Sato of Fukuoka Pref1 巴cture, the late Mr. Shigeo lgarashi of Hokkaido Prefecture, Mr. Shin' ichi Ikebe of Wakayama Prefecture, and Mr. Tamotsu Hamada of Oita Pref1 巴cture. This This study was financially supported by the fund of TIC Co. Ltd., Tokyo, Professor Emeritus T. Matsumoto Matsumoto scholarship fund and 血e Takachiho scholarship fund.

References References

Bernard, Bernard, F. R. 1976. Living Charnidae of 出 e eastern (Bivalvia: Pacific Heterodonta). Contributions in Science, Science, Natural History Museum of Los Angeles County (278): 1-43. Broderip, Broderip, W. J. 1835. Description of some species of Chama. Proceedings of the Zoological Society of London for 1834: 148 151. Conrad, Conrad, T. A. 1837. Descriptions of marine shells from Upper California, collected by Thomas Nuttall, Esq. Esq. Journal of the Academy of Natural Sciences of Philad 中 hia 1: 227 268, pls. 17-20. 20 N. Hamada & A. Matsukuma

Fukuda, H., Mashino, K. & Sugimura, T. 1992. A review of the molluscan fauna of Yamaguchi Prefecture, Prefecture, western Japan. 99 pp., 50 pls., Yamaguchi Museum, Yamaguchi. (in Japanese) Hamada, N. & Matsukuma, A. 1995. Bivalve family Chamidae and evolutionary paleontology, with special special reference to the shell mineralogy and transposition. Science Reports of Department of Earth and Planetary Sciences ，め ushu University 19: 93-102. (in Japanese) K 巴nnedy, W. J., Mo 汀 is, N. J. & Taylor, J. D. 1970. The shell structur ，巴， mineralogy and relationships of the the Chamidae (Bivalvia). Palaeontology 13: 379-413, pls. 70 一77. Lamarck, J. B. P. A. de Monet de 1809. Philosophie zoologique, ou exposition des considerations relatives relatives a l'histoire naturelle des animaux, l α， diversite de leur organization et des facultes qu'ils en obtiennent, obtiennent, aux causes physiques qui main tiennent en eux la vie, et donnent lieu aux mouvements qu ’its executent; enjin, a celles qui produisent les unes les sentiments, et les autres l’intelligence de ceux ceux qui en sont doues. 2 vols., 422 pp.; 473 pp., Paris. Lamarck, Lamarck, J. B. P.A. de Monet de 1819. Histoire naturelle des animaux sans vertebres. 6(1): v vi, 343 pp. pp. Linnaeus, Linnaeus, C. 1758. Systema naturae per regna 加 a naturae. Ed. 10, vol. 1 (Regnum animale), 824 + iii pp., pp., Stockholm (Salvii). Matsukuma, A. 1996. A new genus and four new species of Chamidae (Mollusca: Bivalvia) from the Indo Indo West Pacific, with reference to transposed shells. Bulletin du Museum national aHistoire naturelle, naturelle, Paris, ser. 4, sec. A 18: 23-53. Okutani, Okutani, T. & Soyama, I. 1987. Marine shell-b 巴aring mollusks. 237 pp., Tokai University Pu 由民 Tokyo. (in (in Japanese) Reeve, Reeve, R. A. 1846 1847. Monographs of genus Chama. Conchologia Iconica, vol. 4, Chama pls. 1-5, spp. spp. 1-28 (Dec. 1846); pls. 6 9, spp. 29 55 (Jan. 1847). Schumacher, Schumacher, C. F. 1817. Essai d’un nouveau syst 色me des habitations des vers testacees. 287 pp., 22 pls., Copenhagen. Copenhagen. Strong, Strong, A. M. 1934. A new California Pseudochama. Nautilus 47: 137, pl. 8. Taylor, Taylor, J. D. & Kennedy, W. J. 1969. The shell structure and mineralogy of Chama pellucida Broderip. Veliger Veliger 11: 391-398. Uozumi, K. 1998. Shells of Fukuma machi, Northern Coast of Kyushu, Japan. viii + 180 pp., Fukuma- machi, machi, Fukuoka Pref. (in Japanes 巴）

(Received (Received Novemver 27, 2004 I Accept 巴d February 14, 2005) A New Species of Japanese Chama 21

方解石質最外殻層を持つ日本産キクザルガイ科二枚貝の 1 新種

漬田直人・松隈明彦

要約

国立科学博物館及び九州大学理学部地球惑星科学教室所蔵のキクザルガイ科二枚貝の標本群を検討し たところ，未記載の新種が l 種発見されたのでここに記載，報告する。

Chama cerinorhodon n. sp. イチゴキクザルガイ（黒田；新種） 殻は厚く，殻長は約 20 mm と小型，輪郭は円形，または楕円形で，やや不規則。底質に左殻で固着す る。両殻とも外表面は板状突起によって同心円状に密に装飾され，独特の蝋状のつやのある半透明の質 感を持つ。殻色は赤色がかった桃色から白色。内面は白色で，陶質。閉殻筋痕は背腹方向に長い。腹縁 は細かく刻まれる。胎殻は小型（長さの平均 0.48 mm ）。こう歯の特徴はキクザルガイ科のそれに準ずる が，強くは発達しない。原殻は同心円状の成長線，初期後生殻は細かい放射肋と等間隔の同心円肋で装 飾される。 貝殻構造はアラレ石質の内殻層（複合交差板構造）と中殻層（交差板構造）及び方解石質の最外殻層 の 3 層構造。最外殻層は不規則な形状のブロックの集合によって形成され，北東太平洋の Chama arcana Bernard, Bernard, 1976 のものに似る。 ホロタイプ： GK.N10108-2 ，九州大学総合研究博物館所蔵。 タイプ産地：福岡県糸島郡志摩町姫島周辺。 分布： 更新世：宮困層，神奈川県横須賀市津久井。 現生：北海道南部から九州。

Chama fragum R 巴eve, 1846 に同定されてきたが， Chama fragum のシンタイプを検討した結果，貝殻構 造及び貝殻鉱物の違いから別種であることが分かつた。また，東太平洋に Chama arcana Bernard, 1976 と Chama pellucida Broderip, 1835 は殻のサイズが大きく違うが，形態及び殻構造，殻鉱物の特徴が良く一致 するため，近縁であると思われる。