The malacologicalsocietymalacological society of Japan Res VENUS (jap. jour. Malac.) Vn]. 45, Ne,4 (1987):231 -244 pat13}e}kteigeeAIfRo?iifN= tz p ci . yopeF5 rv. -( -[) =?- s Jif ( F}, gt ze -e:- a) ew re vc L li ilZ5 lfN'{ wr if Cpalii･:7･Wvaew) Studies on the Kawamura Colleetion (Mollusea) Stored in the National Seience Museum, Tokyo IV. The Family Plaeunidae (Bivalvia) with SpecialReference to theirLigament" Akihiko MATsuKuMA (National Science Museum, 3-23-1, Hyakunin-cho, Shinjuku-ku, Tokyo, 160> A comprehensive survey of the Anomiacea based on the anatomy and shell morphology, including the ligament, has been made by Yonge (1977). He eon- eluded that the superfamily Anomiacea of the Pteriomorphia consists of two different lines of deseent from primitive anomiacean stock: the Placunanomia- Plaeuna (Placunidae) and Poclodesmus-Anomia-Enigmonia (Anomiidae) lines. The right valve of juvenile Placuna has a very small, degenerated byssal foramen, However, Placuna species spend unattaehed free-living life, beeause the byssal gland is barely functional during the pQstlarval stage (Yonge, 1977). They live on the surface of mud flat lying on their right valve. They are eharacterized in having very thin, strengly compressed, suborbicular, translucent shell with a minute, closed, byssal noteh, diverging hinge-ridges and only one small, orbicular, post･erior adduetor sear at the center of sheli (Kuroda, 1932). Hinge-teeth, an- terior adductor, hypobranchial gland, byssal apparatms and crural stalk have disappeared. The shell of Placunidae is composed of foliated ea]eite and lacks a complex crossed lamellar, aragonitic, inner layer which commonly occurs in speeies of Anomiidae, e.g. Anomia･ ephippium Linnaeus, 1758 (Taylor et al., 1969). The rather weak ligament ef Placuna consists of an amphidetic dorsal ligament and a semieylindrieal internal ligament. They semicylindrical ligament of the Placunidae is unique in the Pteriomorphia and is apparently similar to the transverse ligament of the Mytilidae and Pinnidae among the Pteriomorphia and the parivineular ligament of the Heterodonta. The morphology and function of " Accepted: August 28, 1986 NII-Electronic Library Service The malacologicalsocietymalacological society of Japan 232 VENUS: VoL 45, No.4(1987) the Placuna ligament are diseussed in the following section. Although fossil Placuua are known frem the Eocene to Oligocene of Europe (Janssen, 1979) and nerthern Africa, living speeies are restricted to the tro- pieal Indo-West Paeific. Another genus of the Placunidae, i.e. Placzananomia. Broderip, 1832, is confined to the tropical east Pacifie (Olsson, 196I; Bernard, 1983). Linnaeus (1758) described Ano7nia plaeenta [=PIacuna placenta] from an "Pelago" uneertain loeality and then about ten species names have been pro- posed by Philipsson (1788), Gmelin (1791), R6ding (1798), Lamarck (1819), Blainville (1826), Gray (1849> and Sowerby (1871). They included many synonyms and non-plaeunid species. For example, Lamarek (1819) enumera+ed the following four speeies: Plac･Mna sella (Gmelin, 1791) [=:P. ephippium Phil- ipsson, 1788], P. papyracea Lamarck, 1819 [=P. ephippiorm], P. placenta (Iin- naeus, 1758), and P, peetinoides Lamarek, 1819. Lamarck (1819) cited figures of Bruguiere et al. (1792, pl. 175, figs. 1-4) as references to P. pectinoides. The figures illustrate thick, solid shells with the isedont-type hinge structure and the posterior adductor scar placed near the postero-dorsal margin. These fea- tures elearly prove that it should be placed in the genus Pl･icatuta Lamarck, 1801 of the Plicatulidae Watson, 1930, Gray (1849) deseribed a new species of Placenta Philipsson, 1788 f'=Placuna Lightfoot, 1786] and divided the genus Placenta into two greups, namely Placenta eph-ippium-group and P. placenta-group. Accerding te Gray (1849) P, ephippt'um- group contains P. ephippium, P. papyracea･ and P. Iincolnii Gray, 1849, and they have hinge-ridges of nearly equal length rapidly diverging from one another, The Placenta plaeenta-group ineludes only one species. In the shell ef P. placenta the nymphs in the right valve gradually diverging from each other and the posterier Iimb of nymphs is longer than the anterior, Plaeenta placenta-group and P, ephippiu･m-group eorrespond with subgenera Plac2tna (s.s.) rLightfoot, 1786] and PIacuna (Ephippium) [R6ding, 17981 of medern authors, respectively. Two Placuna species, namely Placuna･ (Placuna) placenta, (Linnaeus, 1758), and P. (Ephippium) ephippium (Philipsson, 1788) are found in the Kawamura Collection, and an observation of Placuna ligament is inspired by t.he eNcellent specimens in the collection. Another two species, viz. Placzana (Ephippium) lineotnii (Gray, 1849) and Placitna (E.) lobata Sowerby, 1871 are stored in the ordinary mollusk eollection of the Nationa! Science Museum, Tokyo and in Dr. Horikoshi's private colleetion, Other species, e,g. Ptacuna pa･pyracea Lamarck, 1819, P. sella (Gmelin, 1791), P. poloniczam (R6ding, 1798) and P, quadrang2da (Philipsson, 1788), were considered te be synonyms of P. ephippium. Acknowledgements:-This paper is dedieated to Dr. Tokubei Kuroda, an Honorary President of the Malaeologieal Society of Japan, in commemoration of his 100th anni- versary of his birth. I am grateful to Professor Tadashige Habe of Tokai University, NII-Electronic Library Service The malacologicalsocietymalacological society of Japan Matsukttma: Family Placunidae (Bivalvia) 233 Professor Takashi Okutani of Tokyo University of Fisheries, Dr. Frank R. Bernard the ?acific Biologieal Station, Canada, and Dr. Katura Oyama, Toba Aquarium, foi their fruitful diseussions and eomments on the subjeet for improving the manuseript. Dr. Masuoki Horikoshi in Tokyo kindly allowed me to study his Ptacttna speeimens. Ligarnent of the Placunidae Shells of Placuna species are very thin and brittle. Their ventral margins usually open narrowly and their ligament is small. The ligament of the Plaeuni- dae is situated between the valves dersally and extends inwards on the reversed V-shaped ridges Cnymphs) of the right valve. It consists of two main regions, viz. the internal and marginal ligaments (Fig. 1), They are Yonge's (1977) primary and secondary (periostracal) ligaments, respectively. The glossy dark brown, amphidetic, marginal ligament unites the dorsal mar- gins of the valves. The supposed position of the pivotal axis may extend along this marginal ligament. The location, appearance and funetion of the marginal ligament of Plaeorna ephipptzem, P, lincolnii, and P. Iobata are very similar to the outer layer of Pecten ma-xi･mzas (Linnaeus, 1758) illustrated by Trueman (1953). Both anterior and posterior parts of the marginal ligament of Placuna. A mt me .pll ,'･,･/'･i,-･,..,'･,･-- Fig. 1. Hinge region of the left valve of Placuna (Placuna) placenta (A) and Placuna (EphiPPium) ePhiPPium -e '.i '? (B). ti h'((A), a li.T (B)Oewas. ail: anterior inner ligarnent IUsuSagurT}t/rJwr, ml: mar- ginal Iigamznt wa enSew･'nt", pil :posterior inner ligament fisusewewenwt NII-Electronic Library Service The malacologicalsocietymalacological society ofJapanof Japan 234 VENUS: Vol. 45, No.4(1987) ``teeth" plaeenta has small, periostraeal of Hornell (1909), which are distantly spaeed and fall beakwards (Fig. IA). 'the The relatively large and thick, internal ligament is attached to valves in Iigamental pits of the left valve and tooth-like ridges (nymphs) of the right valve. Both anterior and posterior extremities of the ligament are long and 'two narrow and semicylindrieal. The internal ligament may be divided inte parts, outer and inner layers (Fig. 2C). [Vhe thin, glossy dark brown to yel- lowish brown outer layer is apparently homogeneous, laeks any struetural pattern, and is curved outwards covering the thicker inner layer. The outer layer is possibly referable to the lamellar layer of Newell (1938), The lamellar layer is eomposed of protein hardened by aromatic tanning, giving molecular eross linkage of adjacent polypeptid chains (Andersen, 1967), and is elastic under both ten- sienal and eompressional stresses. The thiek, lustrous, yellowish brown, inner layer shows fine growth lines and a fibrous texture. This is the fibrous layer of Newell (1938). The inner fibrous layer is elastie under compressional stress A c alp LV plp -t /i" .; pftttpll alltttafl acr taRV pcr la--e...-J'''..'''1'y.l --. acr RV ',r Fig. 2. Ligumenta･1 area of Ptacuna (EPhiPPiun-t) ephiPpium, P i F' i)' d O ew reo A, B :,location of cross section maveirin' (C) P)fiZW, C:cross sectl,on of ligarnental area ua"rtimaMgi im, D : lateral view of anterior crur21 ridge en 4']treMMU wh, acr : anterier crural ridge fu/u"va ee, afl : fibreus layer of anterior inner ligament ptilM;ew･"ff'fifJwr a) wakE ejdimpvasul all: lamellar layer of anterier inner ligament FtlsuStwurF/trMONt)( apfiiMS, alp:anterior ligamental pit eU/'IEmaxx, 1: inner ligarnent [N'MSmpW, la: location of ligamental attachment mpmeN'tsMS, LV:left valve Eif, pcr:posterior crural ridge thtivava, pfl: fibrous layer of posterier inner ligament iNMSew･'rt'iacvaa)wamettew ue-I, p]1: lamellar layer of posterior inner ligament Nutmpeec wta)Ealtas･'fi'rr' zz, plp : posterior ligamental pit ftEZtftrr", RV : r;Lght valve aue NII-Electronic Library Service The malacologicalsocietymalacological society of Japan Matsukuma: Family Placunidae (Bivalvia) 235 'the but not tensional one. A set of the lamellar and fibrous layers of internal ligament forms a shallow C-spring or semicylinder. The internal ligament is chiefly respollsibile