Contr. Tert. Quatern. Geol. 37(r-2) 3-21 6 pls. kiden, July 2000

REVISION OF PLIOCENE GLYCYMERIDIDAE(MOLLUSCA, BTVALVIA)

FROM TFIE NORTH SEA BASIN

PETER W. MOERDIJKMiddelburg, the Netherlands

and

FREDDY A.D. VAN NIEULANDENieuw- en Sint Joosland, the Netherlands

Moerdijk, Paer W. & Freddy.4.D. van Meulande. Revision ofPliocene Glycymerididae (IvÍolusca, Bivalvia) fiom thc North ScaBasio" - Contr. T€rL Quaem. Geol., 3(l-2): 3-21, 6 pls. kiden, July 2000.

The syst€matic staÍus of Pliocene glyclrncridid bivalves frcrn the Nqth Sea Basin is dis(,sssed' Chevronia n si'lDgfr-, Glwneris(Chevronia) obovata ringelei n. sóspec, G, (Glycyneris) radiofuata funn pseudodcslwyesi n. forma, ed G. (G) radiolyroafumr aaggeruta n- furma. are proposed as new. Lcctotlpcs rc designared fu G. (Ch.) obowta obovsls Q-ou.Drc.dr" 1tl9) and G.(Ch) varbbilis Q, e C. Sowerby, 1824).

Key wcds - Bivalvia, Glysymerididaê, Pliocene, NoÍth S€8 BasirL syst€mdica, new tna.

P.W. Mo€rdijlq Kingsftad 14, NL4336 IÍi Middclbury the Nethcrlands [c-mail: pw.moerdijk@rcelan<Lnl]; F.A-D. van Meulan<te,Sch€ldepmrtstrast 56, NL4339 BN }lieuw- en Sint Jooclmd, the Nctherlands.

CONTENTS

Inhoduct ion . . . . . . p . 3Systematicdescript ions .. . . . . .p. 3Acknowledgements .. . . .p. 9Refere l rces . . . . . . . . p . 9

INTRODUCTION

In the literature on Neoge,ne molluscs from the NorthSea Basin, there is little agreement over the qrste,maticstatus of Plioce,ne Glycymerididae. Moerdijk & vanNieulande (1995) have rec€Nrtly noted that most previousauthors had lumpd Glycymeris variabilis and G. radi-olyrata.In the prese,lrt paper, results of a study of repre-se,ntatives of Glycymens from the North Sea Basin Pliocene, originating from te,mporary excavations in Bel-ginm (see Marquet,1995 for details), borehole materialfrom the Nettrerlands, dredged and beach fossil materialas reworked from especially Plioce,ne strata in theprovince of Zeeland (the Netherlands), as well as fromthe Pliocene Coralline and Red crags (England). Speci-mens from the Mioce,ne (Helvetian, Redonian) and Plio

cene (Redonian) of France have been studied for com-parison.

Moerdijk & van Nieulande (1995) concluded that,apart from external orname,nt, details of the ligamentarea are important within the Glyrymerididae. In theprese,nt paper, a new subgeÍrus, Chevronia, is introducedfor those shells that show combined featues of ligame,ntand external orname,nt. Other fossil species from west-ern, cental and southerÍl European localities have beenstudied; a few are here referred to the new subge,Írus.Stratigraphic data concerning Belgian material des-cribed in the prese,nt paper are according to De Meuter&Laga (1976).

The very coÍnmon glycymoidids from the LowerPlioce,ne (Kattendijk Formation) of Belgium have turnedout to be related to the Miocene G. obovata baldii Glibert & van de Poel, 1965 and are here described as a newsubspecies, G. obovata ringelei. Lectotlpes aÍe desig-nated for G. obovata obovata and G. variabilis.

SySrguerlC DESCRIPTIONS

Abbreviations To denote the repositories of

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specime,ns illustrated and/or referred to in the text thefollowing abbreviations are used:

BMNH The Natural History Museum, London (De-partment of Palaeontology) (formerly BritishMuseum of Natural History);

RGM Nationaal Natuurhistorisch Museum (De-partment of Palaeontology, Cainozoic Mol-lusca), Leiden (the Netherlands; formerlyRijksmuseum van Geologie e,n Mineralogie);

NITG Nationaal Instituut voor Toegepaste Geo-wetenschappen-TNO, Utrecht, the Nether-lands;

KZGW Koninklijk Zeeuwsch Genootschap derWete,nschappen, Middelburg, the Nether-lands.

Order Arcoida Stoliczka, 1871Superfamily Limopsacea Dall, 1895Family Glycymerididae Newton, 1922Subfamily GlyrymeridinaeNewton, 1922

Remarks - Moerdrjk & van Nieulande (1995) notedthat two groups of glycymeridids were represented inthe Pliocene North Sea Basin material, which could bedistinguished by using the following characters:

- external ornament;- ornament of ligament area.

Useful, but less important, characters are:

- shell form, l.e. outline and convexity;- shell thickness;- dimensions;- form of ligament area;- form and position of adductor muscle scars;- characters ofjuvenile shells.

Ivith respect to the importance of characters of externalshell ornament, we agree with earlier authors, e.g. Nicol(1956); however, features of the ligame,nt have onlyrarely been considered important. In our experience,however, these aÍe very uniform within a species andquite constant in phylogenetic development. Combittittgexternal orname,nt and ligame,nt area orname,nt leads toa good basis for (sub)ge,neric divisions of glycymeridids.

Genus Glycymeris da Costa,1778Subgenus Chevronia n. subgen.

Type species - Arca obovata Lamarck, 1819. Accord-ing to Jansse,n (1979), Glycymeris obovata obovata (seePl. 1, Fig. l) has ofte,n been misinterpreted, and is inneed of nome,nclatural stability. We here designate aslectotype of G. o. obovata the specimen illustrated in

Fawe (1918, pl. 34, fi5.230a-c). This valve has a labelin Lamarck's handwriting associated.Derivatio nominis - In allusion to chewons on theligament area.Diagnosis - Glycymeridids with subcircular, subequi-lateral shells, external ornament almost obsolete to fine,and when present consisting mainly of radial elements,occasionally with distinct, distant, radial grooves; liga-me,nt area with well-developed, regular chewon-shapedligamental grooves.Other species The following (sub)species (seeMayer-Eymar, 1868; Báldi, 1962; Maestrati & Lozouet,1996) we have studied may be attributed to the new sub-genus:

G. (Ch.) rtchteli (Deshayes, 1852)G. (Ch.) latiradiata latiradiata (Sandberger in

Giimbel, l86l)G. (Ch) l. obovatoides Báld|1962G. (Ch.) l. subfichteliBáld\1962G. (Ch.) obovata baldii Glibert & van de Poel, 1965G (Ch.) poustagnacensis Maestrati & lozouet,

1996G. (Ch.) saucatsensis (Mayer-EymÍu, 1868) (non

Cossmann & Peyrot, l9l4)G. (Ch.) subterebratularis (d'Orbigny, 1852)G. (Ch.) turonica (Mayer-Eymar, 1868)

We have not (yet) seen any Recent species.Remarks Other glyrymeridid shells with well-developed ligament grooves have been referred to thegenera Melminaea kedale, 1930 and Tucetona Iredale,1931. However, both of these show, in contrast withChevronia, a pronounced external orname,nt.

Glycymens (s. lat.) cor (Larnarck, 1805), which hasalso bee,n described as G. violascens (Lamarck, 1819),G. nummaria (Linné, 1758) and G. insubrica (Brocchi,1814), also has ligament grooves, but these are irregu-lar, often intercalating under the beaks and start theirdevelopment in varying stages of growth. In juveniles,

the ligame,nt area is always smooth. In addition, juve,nile

shells show a marked conce,ntric ornament. The subge,n-eric classification of this species and its close relatives,will have to be revised.

Glycymeris (Chevronia) obovata ringelei n. subsp.Pl. 1, Fig. 2

1878-81 Pectunculus glycymeris L., var. transversa -

Nyst, p. 166 Qtartim, non Wmd), pl. 17, fig. 8b?,€ , 9 .

?1950 Glycymeris (Gl.) pilosa (Linné, 1767) -Heeringp.33, pl. 8, f igs 9, 10.

1957 Glycymeris glycymeris Linné sp. 1758 formevariabilis Sowerby, 1824 Glibert, p. 16(partim).

1962 Glycymeris varíabílis deshayesi (Mayer, 1868) -

van Regteren Altena et al., p. 14, pl. 5, fig. l8a

1965 Glycymeris (s.s.) glycymerís variabilis (Sowerby,1824) - Glibert & van de Poel, p.83 @artim).

1974 Glycymeris glycymeris kattendijkensis Ringelé(MS name).

1979 Glycymeris glycymeris pilosatumida (Bucquoy,Dautzenberg & Dollfus, 1891) Geys &Marquet, pl. 14, frg.4.

Diagnosis - Glycymeridid with a thick and inflated,subcircular, orthogyrate, equilateral shell, with externalornament consisting of very fine radial threads and oÊten some distant radial grooves, with well-developedhinge teeth, an equilateral ligament area with distinct,rather coarse chewons and adductor muscle scars ofabout equal height and width, in contact with hingeplate.Locus typicus - Fifth harbour dock, Antwerp (Bel-gium).Stratum typicum - base Kattendijk Formation, Plio-cene.Derivatio norninis - Named after A. Ringelé, who wasthe first to recognise it as a distinct taxon.Types - Holotype is RGM 394 099 (a bivalved speci-men) from Antwerp (5th harbour dock, 12 April l97l),base Kattendijk Formation (ex van Nieulande Colln);paratypes are four bivalved specimens (van NieulandeColln, no. 108) from the type locality and level, and fourbivalved specimens (van Nieulande Colln, no. 110) fromKallo (ftunnel construction pit).Other material - Kallo (Beveren tunnel constructionpit, Kattendijk Formation), 5 bivalved specimens, Ivalve (van Nieulande Colln, nos 106, 107); Antwerp(connection 5th harbour dock to Amerika dock), 14valves, 2 bivalved specimens (RGM 494 115); Kallo(Bevere,n tunnel construction pit, base Oorderen Mem-ber, Lillo Formation), 4 bivalved specime,ns, 5 valves(van Nieulande Colln, nos ll2, 113); Kallo (tunnelworks, base Oorderen Member), 9 bivalved specimens(van Nieulande Colln, no. 109); Antwerp (Kaai 27,?Luchtbal Member), 1 valve (van Nieulande Colln, no.90, leg. A. Flaandrikman); Ellewoutsdijk/Baarland(Western Scheldt, dredged, derived Pliocene), 47 valves(Moerdijk Colln); Borsele (de Kaloot, province of Zee-land, beach material, derived Plioce,ne), 2 valves (vanNieulande Colln, no. 102); Sutton (Suffolk, England,Coralline Crag, exP. Cambridge Colln/L. van der Slik),I worn valve (cf. Glycymeris o. ringelei, RGM 394l l6).Description (lnlotype) Height, width and semi-diameter of right valve are 54.2, 58.2 and 18.5 mm,respectively; of left valve 54.2, 58.3 and 18.7 mm, re-spectively. Thick and firm, inflated, almost orthoglnate,near-equilateral, broadly oval, posterior side slightlylonger, transition from upper margins into venfral mar-gin somewhat shorter rounded; umbones protruding.Shell surface almost smooth, with very fine, somewhatirregular, radial threads; with a few distant radial

) -

grooves, in this respect reminiscent of G. latiradiata.Hinge well developed with rather long teeth, continuingunder ligament area (13 anterior, 12 posterior). Liga-me,nt area near equilateral, rather high, with incisedchewons (9 anterior, 8 posterior). Adductor musclescars large, b,road and callous, almost touching thehinge plate; pallial line parallel to the ventral shell mar-gin, clear. lnner margin with well-developed cre,nula-tions.Remarks - The present taxon has bee,n considered bymany authors to be related to G. pilosa Linné, 1758and/or G. glycymeris Linrré, 1758. Both these specieshave well-developed external orname,nt and smooth,inequilateral ligament areas, and are thus assignable toGlycymeris s. sfr., and are not closely related to G. obo-vata ringelei, which differs from G. o. baldii from theAntwerp Member (Berchem Formation, Mioce,ne) by itssmaller, thicker, relatively higher and more inflatedshell with a better developed hinge with relativelylonger hinge teeth.

The new subspecies differs in some respects from itsOligocene and Miocene relatives (see Dollfus &Dautzenberg, 1909); we follow Glibert & van de Poel(1965) in their treatment of Oligocene and Mioce,nerepresentatives of G. obovata and rank it as a new sub-species. Glycymeris o. ringelei may be distinguishedfrom G. variabilis in that it is more equilateral and or-thogyrate, has a less acce,ntuated radial ornament, abetter developed hinge and its anterior adductor musclescar is somewhat less slender and (almost) touches thehinge plate. Most specimens of G. variabilis are decid-edly less inflated than those of G. o. ringelei.

Glycymeris (Chevronia) variabilis(J. de C. Sowerby, 1824)Pls 2, 3; Pl. 4,Figs 1,2

*1824 Pectunculus variabilis J. de C. Sowerby, p. 111,pl.47l (lower and central figures).

1845 Pectunculus variabilis - Nyst, p.249, pl. 20, fig.la b.

l85l Pectunculus glycymeris L. - Woo{ p. 66, pl. 9,fig. lg.

l85l Pectunculus glycymeris L. var. subobliquus Wood,p.66, pl. 9, fig. lh, i.

1878-81 Pectunculus glycymeris, L. vat. transversa S.Wood - Nyst, p. 166 Qtartim), pl. 17, fig. 8a, c, f.

1879 Pectunculus glycymeris - Wood, p.43, pl. 6, fig.5a.

1879 Pectunculus glycymeris vaÍ. nummarius - Wood,p.43, pl. 6, f ig. 5b.

1937 Glycymeris glycymeris variabilis (J. Sow.) - vanRegteren AJtenq p. 55, pl. 3, fig. 50.

1950 Glycymeris glycymeris variabilrs (Sowerby, 1824)- Heering p. 30 (partim), pl.8, figs l-6.

1957 Glycymeris glycymeris Linné sp. 1758 formevariabilis Sowerby, 1824 Glibert, p. 16(partim).

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1962 Glycymeris glycymeris variabilis - van RegterenAltena et a1.,p.55 Qtartim),pl.4, fig.50.

1965 Glycyneris (s.s.) glycymeris variabilis (Sowerby,1824) - Glibert & van de Poel, p. 83.

1974 Glycymeris glycymeris variabilis (Sowerby, 1824)- Ringelé (MS), p.96 (partim), pl. 4, frg.5a, b.

1979 Glycyneris glycymerís variabilrs (Sowerby, 1824)- Geys & Marquet, pl. 15, fig. l.

1995 Glycymeris (Glycymerk) variabilis (J. de C.Sowerby, 1824) - Moerdijk & van Nieulande, pl.4, frg. l.

Original diagnosis - 'Obliquely suborbicular, ratherconvex, finely striated longitudinally, becoming smoothor sulcated by wear; teeth of the hinge and lines uponthe area of the ligame,nt, numerous; beaks shcirt nearly,close' (J. de C. Sowerby, 1824, p. 111).Rqised diagnosis - Moderately large, inequilateral,obliquely oval glycymeridid with a rather flat shell, anear-smooth external ornament with very fine, irregularradial threads, quite short hinge teeth, a ligament areawith distinct chewons and with the slender adductormuscle scars at some distance from the hinge plate.Types - The original type series consists of threespecime,ns; here we designate lectotype the specimen inJ. de C. Sowerby's pl. 471 (bottom figure) (BMNH43188a) (see Pl. 2, Fig. 1), since it most clearly repre-sents the well-known, quite flat and inequilateral form.The right valve illustrated in the ce,ntre of this plate isheavily eroded and worn, rather inflated and moreequilateral than the lectotlpe. However, it may be re-ferred to the present species as well, and is designatedparalectotype (BMNH 43188b) (see Pl. 2, Fig. 2). Thespecime,n in pl. 471, top figure, may be assigned to G.radiolyrata forma pseudodesluyesi n. forma (see below;Pl. 5, Fig. 3).Loctts typicw - Suffolk (no additional locality dataavailable).Stratum typicum - Red Crag (no additional strati-graphic details available).Description (ectotype) - Height, width and se,mi-diameter are 61 .9,64.9 and 17.6 mm, respectively; rightvalve, óliquely oval, inequilateral, the posterior sidelonger and drawn out ve,ntro'posteriorly, anteriorly andventrally regularly rounded, dorsoposteriorly straight(slightly damaged anteriorly above midline), umbo notvery promine,nt, faintly opisthogyrate, shell rather flatand moderately thick shelled. External ornament con-sisting of very fine, irregular radial threads, because ofwear only visible near ventral border; growth lines ir-regular, distinct, particularly posteriorly. As a result of'weaÍ, umbonal region is decorticated; four conce,ntricgrowth zones are visible in this region, mossed by about40 radial sulcae also promine,nt because of wear, run-ning to about shell midline.

Below the beak is a bnoadly triangular ligament area,slightly inequilateral, anterior part somewhat longer,and almost reaching the inside border of the hinge plate;

ornamented with incised ligament grooves in a regularrooÊshaped pattern, anteriorly with 8, posteriorly with 7grooves. Hinge not especially well developed hingeteth relatively short, 8 in number anteriorly as well asposteriorly. Below either e,nd of hinge plate, callus ofadductor muscle scars missing, as is inner shell layerwhich would have preserved the pallial line. Inner shellmargin cre,nulate, 35 crenulations visible.Material studied - Antwerp area (6th harbour dockconstruction pit, Oorderen Member), 14 valves (RGM393 819, leg./don. M. van den Bosch); connection 5thharbour dock to Amerika dock, ?Luchtbal Member, 2specimens (RGM 394 ll8), 3 valves (van NieulandeColln, no. 111); Sturyenberg, 1 bivalved specime,n, 6valves (MTG Colln); Kallo (sea sluice construction pit,Oordere,n Member), 3 valves (Vervoenen Colln, no. F251 B); Kallo (V/aesland tunnel construction pit, Oor-dere,n Member, level with Angulus benedeni), I valve(van Nieulande Colln, no. 17); Kallo @everen tunnelconstruction pit, Oorderen Member, level with A. bene-deni), I valve (Moerdijk & van Nieulande, 1995, pl. 4,fig. 1; RGM 393 826); Mitt-St llubert, borehole mate-rial, Pliocene,24 valves (very inequilateral and thick-shelled form, van Nieulande Colln, no. 137); Reek(province of Noord-Brabant), auger drill 41 (45F148),18.00-31.80 mbs, Pliocene, c. 290 valves CIuv., I frag-ment of an adult specimen, with very coarse radial lirae,NITG Colln, MOL 683); Nijmegen (province of Gel-derland), 'Midde,n-Pliocee,n', 85 valves, 21 fragments(I.IITG Colln, MOL 686); Borsele-Baarland, dredged,reworked from Pliocene strata, 28 valves (MoerdijkColln); Baarland, 5 valves (very inequilateral andthick-shelled form, Moerdijk Colln); Veere-Domburg,washed ashore, reworked from Pliocene strata, 23 valves(Moerdijk Colln); Borsele-de Kaloot, 7 valves (Moer-dijk Colln); Walton-on-the-Naze (England, Red Crag,RGM 394 ll9, ex F.J. Janssen Colln), I I valves.Remarks - A borehole at Reek has yielded numerousjuvenile valves with a remarkably coarse radial orna-ment, from sediments of Pliocene age. In reworkeddregded material from the Western Scheldt a very in-equilateral, oblique and thick-shelled forma of G. vari-abilis is known. From borehole Mill-St. Hubert, numer-ous similar specime,ns are available, but their adductorscaÍs are markedly broader and almost touch the hingeplate, as in G. obovata ringelei.

Some specimens of G. variabilis are more inflated,often with a somewhat coarser radial ornament than intypical examples. The anterior adductor muscle scar isless slender as well. However, these shells are inequilat-eral and oblique as in tlpical G. variabilis, with a nar-row hinge and short teeth, with similar characters of theligame,nt area and with the anterior adductor musclescar at some distance from the hinge plate. In this re-spect, they differ from G. obovata ringelei, and seem torepresent an early form of G. variabilis, possibly ofstratigraphical signifi cance.

Material of this inflated form of G. variabilis exam-ined is from the following localities: Sudbourn HaII(Coralline Crag), 5 valves (I.IITG Colln, MOL 69\;Gedgrave (Coralline Crag), 1 valve (RGM 39a LL7);Ikllo (sea sluice construction pit, base Oorderen Mem-ber, probably reworked from older strata), 2 valves(Vervoenen Colln, no. F 251 B); Bergen op Zoom(borehole Vreederust, 498,1173, 81.30-95.70 mbs), 1valve (Heering, 1950, pl. 8, figs 13, t4,20,NITG Colln,MOL 675); lVestern Scheldt (province of Zeeland,dredged, reworked from Plioce'ne sfrata; Terneuzen(Put van Terneuze,n, ZZ8 (Schot), I valve and I dam-aged valve (KZGW Colln, no. 3082 GM 98-03); EIle-woutsdijlq I valve (KZGW Colln, no. 648b, leg. C.Brakman); Baarland, 5 valves (Moerdfk Colln);Schaar van de Spijkerptaat, I valve.Discussion - Whe,n we had just started our studies ofPliocene glycyneridids from the North Sea Basin, weassumed G. variabill,s to be the direct descendant ofwhat we now refer to as G. obovata ringelei. Shells ofG. (Ch.) turonica (Pl. 4, Fig. 3) from the Miocene andPlioce,ne (Redonian) of the Nantes area (France), how-ever, also have much affinity with G. variabilis. Gly-cymeris turonica is a common species from the Miocene(Helvetian) in the Tours area (France). Generally, buterïoneously, this species is lumped with G. bimaculatadeshayesi or \Mith G. texta (Dujardin, 1837). TheHelvetian form is quite small, with an almost equilat-eral, inflated shell, a very fine, mainly radial externalornament and many distinct chewons on the ligame'lrtaÍea. Specime,ns from the Redonian of the Nantes aÍeaaÍe as a rule larger and clearly inequilateral; these werediscussed by Latuiat-Rage (1981) and referred to as'Glycymeris (Glycymeris) glycymeris (Linné, 1758)sous-especes pilosa (Linné, 1758) et variabflÍs (Sow-erby, 1824)'. The Redonian shells have many charactersin common with G. variabilis, especially with the moreinflated forms: clearly opisthoglrate, inflated, inequilat-eral and oblique shell, a moderately developed hingeand position of the adductor muscle scaÍs. Juve'nilespecimens of G. turonica and G. variabilis are verysimilar; the external ornament in the former is similarto that of the more finely ornarne,nted shells of the latter.In addition, G. turonica and G. variabilis have the con-centric colour bands in common. The number of liga-me,nt chewons in G. turonica is as a rule much higher.Howeve,r, in a lot from St-Julien-de-Concelles (see Pl. 4,Fig. 3), the number of ligament chewons is lower, as insome specime,ns of G. variabilis. The specimen illus-trated in Lauriat-Rage (1981, Pl. 4, fig. 1), from Le Pi-geon Blanc, also has fewer ligament chewons thanspecimens collected at e.g. Sceaux d'Anjou. Judgingfrom shell featrnes, it seems plausible that G. turonicaand G. variabilis are related.

At present, we are unable to decide whether G. vari-abilis is directly related either to G. obovata ringelei wto G. turonica. For the time being, it seems best to con-

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sider G. obovata, G. turonica and G. variabilis distincttaxa.

Genus and subgenus Glycymeris da Costa, 1778

Type species - Arca glycymeris Linné, 1758, by mono-q/py.Diagnosis - 'subcircular, subequilateral, with smallumbones; teeth relatively short, transverse, becomingobsolescent medially; surface smooth or costate' (Newellin Moore, 1969, p. N267).Revised diagnosis - Subcircular, subequilateral; teethrelatively short, transverse, effacing medially; externalornament fine to coarse, consisting of radial as well asconce,ntric elements, all Recent species with a haryperiostracum, ffid a (near-)smooth ligame'nt area.Remarks - The subgenus is redefined here to distin-guish it from Chevronia (see above).

Glycymeris (G.) radiolYrataMoerdijk & van Nieulande, 1995

Pls 5,6

Discussion - When we first described G. radiolyrata, afew valves of a form obviously related to it, but larger(up to 90 mm) and more inflated we,re before us. Nowthat we have seen additional material, the followingfeatures of this form may be added: a relatively thickershell, with a better developed hinge, the adductor mus-cle scar rests on an internal buttress. The majority ofvalves may be subdivided ftrther into two categories,viz. aprincipally equilateral form with fine and regularshell ornament, and an inequilateral form with verycoarse and irregular orname,nt. Both e,nds of the contin-uum are clearly distinct, but some specime,ns are inter-mediate, suggesting therr to be related. Below, we willrefer to these forms as forma pseudodeshayesi n. forma,in view of their having similar sfrong shell inflation andfine reticulate orname,nt as G. bimaculata deshayesi(Mayer-Eymar, 1868) [Miocene (Helvetian) of Touraineand Redonian (Mio-Plioce'ne) of western France], and asforma exaggerata n. forma, since characters of external66ameÍrt are exaggerated in comparison to the typicalform.

Formapseudodeshayesi (Pl. 5, Figs 1-3) ditrers fromthe typical form in attaining larger dimensions, in beingmore inflated, in having a better developed hinge and inthe position of the adductor muscle ssars, close to thehinge plate. It may be distinguished from forma exag-gerata (see below), as well as from the typical form, bythe finer and more regular, reticulate shell orname'lrt.

Forma exaggerata (Pl. 6, Figs 1-3) also differs fromthe typical form in being more inflated, in having abetter developed hinge and in having the adductor mus-cle scars close to the hinge plate. It is distinguished

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from both forma pseudodeshayesi and the tlpical formin having very coarse, reticulate shell ornament.

From boreholes at Ouwerkerk and Bergen op Zoom(the Netherlands), we have only seen f pseudodeshay-esÍ, based on excellently preserved material. The levelwhich yielded the specimens at Ouwerkerk is anequivalent of the Luchtbal Member (Middle Pliocene).From the same borehole, samples from some metresabove the level with this inflated and finely orname,ntedform have yielded the typical form. No specime,ns of f.exaggerata have been found in their originalstratigraphical context in any of the outcrops/ex-cavations studied. This complicates a satisfactory inter-pretation of the material. The only articulated specimenoriginates most probably from the upper part of theKattendijk Formation, judging from sediment fill. Thesedistribution patterns indicate that these formae may beof stratigraphical value. The bulk of the material fromBelgian outcrops, as far as could be determined, origi-nates from the base of the Oordere,n Member (LilloFormation), where the typical form of G. radiolyrataalso occurs. Specimens showing features intermediatebetween f. pseudodeshayesi, f. exaggerata and the tlpi-cal form do occur in our material, albeit rarely. Typicalfeatures of f. pseudodeshayesi in particular would sug-gest that G. radiolyrata evolved from G. bimaculata s.lat. Previously illustrated material that may be.attributedto f. pseudodeshayesi includes:

1824 Pectunculus variabilis J. de C. Sowerby pl. 471,top figure.

1950 Glycymeris spec. la Heering p.34, pl. 7, figs 15,16,19,20.

1950 Glycymeris (Gl.) pílosa (Linné) var. tumidaB.D.D. - Heering,p.34, pl. 8, figs ll,12.

Specimens illustrated by Wood (1851, pl. 9, fig. 1d),under the name Pectunculus glycymeris L. (pl. 9, fig.1d), and Wood (1879, pl. 6, frg. 4a) as P. pilosus var.insubricus Brocchi, could also belong here.

Specimens recorded previously in the literature that maybe referred to f. exaggerata include:

1957 Glycymeris glycymerís Linné sp. 1758, forme Ía-nidaB.D.D., l89l - Glibert, pl. 1, fig. 4.

1992 Glycymeris cf. bimaculata (Poli, 1795) - Moer-dïk & van Nieulande in Moerdijk et al., p. 18Qtartim), pl. 4, bottom right-hand figure.

Material of f. pseudodeshayesi examined for the presentpaper is as follows:

Ouwerkerk (borehole 42H19-4(A40), I 14.50-1 15.75mbs, Luchtbal Member, 3 valves, 1 fragme,nt (RGM 394l2l, leg. Deltadienst); Bergen op Zwm (Vreederust,borehole 4981173,81.30-95.70 mbs), I valve (Heering,

1950, pL.7, figs 15, 16, 19,20, NITG Colln, MOL 690)and 3 fragments (NITG Colln, MOL 688); Bergen opZoom (borehole Waterleiding 3, 49F,130, 87.6A-94.50mbs), 1 valve (Heering, 1950, pl. 8, figs 11, 12, NITGColln, MOL 689); Bergen op Zoom (waterleiding Ber-gen op Zoom 3, 87-94 (?mbs), 1 valve, RGM 4326; SíJansteen (borehole 55N24, borehole A), 8.60-9.40mbs), 1 fragment (MTG Colln, MOL 69t); Kallo(Beveren tunnel construction pit, base Oorderen Mem-ber, 3 valves, van Nieulande Colln, nos 93,93a, ll4);Antwerp (Boudewijnsluis excavation, base OorderenMember, I valve (Marquet Colln); Kallo (sea sluiceconstruction pit, ?base Oordere,n Mernber, 8 valves,Vervoene,n Colln, no. F 717 Aa;1 valve, van NieulandeColln, no. 101; Kallo (Beveren tunnel construction pit,Oorderen Member, level with Atrina(?), I valve, vanNieulande Colln, no.94; Antwerp (construction pit 5thharbour dock, level unknown, from sediment dump), 1valve (RGM 2657); Antwerp (Oorderen polder, Plio-cene, level unknown),2 valves (RGM 56908); Antwerp(Ford plant north of Noordkasteel, level unknown, fromsediment dump), I valve (RGM 394 097); Antwerp(5th harbour dock, level unknown, from sedimentdump),6 valves (RGM 100 394, 100 396-397, 100 a01);Antwerp (level unknown), 1 valve and 1 specime,n(RGM 56 896, 494 098). Dredged, reworked from Plio-cene strata is material from Baarland (l valve, Moer-drjk Colln), Ellewoutsdijk/Baarland (4 valves, vanNieulande Colln, no. 103; Vervoe,ne,n Colln, no.F 717.

From the Red Crag is 1 valve (J. de C. Sowerby,1824, pL. 471, top figure, BMNH 43188c) (see Pl. 5,Fig. 3 here).

Material of f. acaggerata examined for the present paperincludes:

Antwerp? (original label stating, 'Pectunculw pílosusuit het Diestien, in het Scaldisien gevonden; gere-manieerd (gevuld met Diestien, ged. met Ditrupa)' (:'Pectunculus pilosus from the Diestian, found in theScaldisian; reworked (filled with Diestian, part withDitrupa'), 1 bivalved specimen (NITG Colln); Kallo(Vrasenedok excavation, base Oorderen Member,probably reworked from older strata), 1 valve (MarquetColln); Kallo (Beveren tunnel construction pit, baseOorderen Mernber, probably reworked from olderstrata), 1 valve (van Nieulande Colln, no. 93b); Verre-broek (Liefl<enshoekf,unnel construction pit, OorderenMember, level with Atrina(?), probably reworked fromolder strata), I valve (van Nieulande Colln, no. 100, leg.G. Garding); Kallo (sea sluice construction pit, ?baseOorderen Member, probably reworked from olderstrata), 3 valves (Vervoenen Colln, no. F 717 A, A2);dredged, reworked from Plioce,ne strata are specimensfrom Ellewoutsdijk, I valve (Moerdijk Colln); Baar-

land, 1 valve (Moerdijk Colln); Ellewoutsdijld Baar-land, 1 valve (van Nieulande Colln, no, 105b); washedashore, reworked from Pliocene strata: Nieuwesluis, Ivalve (Simons Colln).

Additional data - As far as the stratigraphical dishi-bution of the typical form is concerned, it should benoted that it has also been found in the Ouwerkerkborehole (province of Zeeland), at a level which isequivalent to the Luchtbal Member, as well as in thebasal Oorderen Member during excavation of the Verre-b'roekdok, which levels have yielded well-preservedmaterial. Data are as follows: Kallo (Verrebroekdok,base Oorderen Member, - 17.10-17.50 m, 1 valve(Moerdijk Colln); Ouwerkerk (borehole 42H-19-4(A40), 106-107 mbs, Luchtbal Member, 3 valves, 2fragments (RGM 394 l20,leg. Deltadienst).

ACKNOWLEDGEMENTS

We wish to thank the following people for assistance invarious ways: G. Garding, H. Gunst, A. Janse, P. Jeffery(BMNH), V/. Krull, A. Krull-Kalkman, R. Marquet, T.Meijer (NITG-TNO), R. Pouwer O[TG-TNO), R.Rijken, G. Simons, M. Vervoenen, J. van der Voort andF. Wesselingh (NNM-Naturalis).

REFERENCES

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Dollfus, G. & Ph. Dautzenberg 1909. Conchyliologie duMiocène moyen du Bassin de la loire, l. Pélécypodes. -Mém. Soc. géol. Fr. (Paléont.),27(5'1: l-500,46 pls.

Fawe, J., 1918. Catalogue illustré de la Collection Lamarck,Conchiftres dimyaires fossiles, Première partie. Genève(Georg & Cie), I 17 pls.

Geys, J.F. & R. Marquet,1979. Veldatlas voor de cenozoischefossielen van België, l. Neogeen. Rotterdam (Backhuys),125 pp.,45 pls.

Glibert, M., 1957. Pélécypodes du Diestien, du Scaldisien etdu Merxemien de la Belgique, première note. - Bull.Inst. r. Sci. nat. Belg., 33(9): 140, 1 pl.

Glibert, M. & L. van de Poel, 1965. Les Bivalvia fossiles duCénozoïique étranger des collections de I'Institut Royal desSciences Naturelles de Belgique, 1: Palaeotarodontida etEutuodontida. - Mém. Inst. r. Sci. nat. Belg., (2)77: l-tt2.

Heering, J., 1950. Pelecypoda (and Scaphopoda) of thePliocene and older-Plistocene [sÍc] deposits of theNetherlands. - Meded. Geol. Sticht., (C)a(l)9: l-225, 17pls.

Jansseno R, 1979. Revision der Bivalvia des Oberoligoziins(Chattium, Kasseler Meeressand). - Geol. Abh. Hessen,78 : l :118 ,4 p ls .

L^auriat-Rage, A., 1981. Les Bivalves du Rédonien (Pliocene

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Atlantique de France); signification stratigraphique etpaléobiogéographique. - Mém. Mus. natn Hist. nat., n.s.(C: Sci. Terre), 45:3-173,18 pls.

Maestrati, Ph. & P. Lazauet, 1996. Les Glycymerididae(Mollusca, Bivalvia) de I'Oligocène Supérieur (Chattien)du Bassin de I'Adour, France. - Ann. Paléont. (Vert.-Invert.), 82(l):3-25.

Marquet, R., 1995. Pliocene gastropod faunas from Kallo(Oost-Vlaanderen, Belgium). Part l. Introduction andArchaeogastropoda. - Contr. Tert. Quatern. Geol., 32(l-3): 53-85, 6 pls.

Mayer-Eymtr, C., 1868. Catalogue systématique et descriptifdes fossiles de terrains tertiaires qui se trouve au Muséefedéral de Ziirich, 3. Mollusques; Famille des Arcides,Ztxich,124 pp.

Meuter, F.J. de & P.G. Laga 1976. Lithostratigraphy andbiostratigraphy based on benthonic Foraminifera of theNeogene deposits of Northern Belgium. - Bull. Soc.belge GéoI., 85(4): 133-152, I pl.

Moerdijk, P.W. & F.A.D. van Nieulande, 1995. Glycymeris(Glycymeris) radiolyrata nov. sp. (Bivalvia, Glycy-merididae) from the Pliocene of the North Sea Basin. -

Contr. Tert. Quatern. Geol., 32(1-3):3-17, 4 pls.Moerdijk, P.W., R. Pouwer, R. Rijken & F.A.D. van

Nieulande, 1992. Fossiele schelpen van Zeeuwse strandenen stromen, deel l. - Werkgr. Geol. Kon. Zeeuwsch Gen.Wetensch., Publ. 2: l-38, l0 pls.

Newell, N.D., 1969. Family Glycymerididae. 1n: Moore, R.C.(ed.). Treatise on Invertebrate Paleontolory, Part N(2),Mollusca 6: N267-N269. Geol. Soc. Am., Boulder/Univ.Kansas Press, Lawrence.

Nicol, D., 1956. Distribution of living glycymeridids with anew species from Bermuda. -Nautilus, 70:48-53.

Nyst, P.H., 1845. Description des coquilles et polypiersfossiles des terrains tertiaires de la Belgique. Bruxelles(Nyst), 675 + ii pp., 48 pls.

Nyst, P.H., 1878-1881. Conchyliologie des terrains tertiairesde la Belgique, 1: Terrain pliocene. - Ann. Mus. r. Hist.nat. Belg., 3: l-263 (1878); 28 pls (1881).

Regteren Altena, C.O. van, 1937. Bijdrage tot de kennis derfossiele, sub-fossiele en recente mollusken, die op deNederlandse stranden aanspoelen, en hunner verspreiding,Amsterdam (Universiteit van Amsterdam), 184 pp., 12pls.

Regteren Altena, C.O. van, A. Bloklander & L.P. Pouderoyen,1962. De fossiele schelpen van de Nederlandse strandenen zeegaten. Tweede serie, l. - BasteriU26:5-16.

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Sowerby, J. de C., 1823-1829. The Mineral Conchologr ofGreat Britain, or coloured figures and descriptions of thoseremains of testaceous animals or shells which have beenpreserved at various times and depths in the earth, 5.Iondon (Ardine/Richard Taylor), 160 pp., pls 408-503.

Wood, S.V., 1851-1860. A monograph of the Crag Mollusc4or, description of shells from the Middle and UpperTertiairies of the east of England,2. Bivalves. - Monogr.Palaeontogr. Soc. Lond., l-150, pls l-12 (1851); 15l-216,pls 13-20 (1853); 217-342, pls 2l-31 (1857), 1-2 (1860).

Wood, S.V., 1872-1874. Supplement to the monograph of the

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Crag Mollusca, or, description of shells from the Middleand Upper Teniairies of the east of England, 3. Univalvesand Bivalves. - Monogr. Palaeontogr. Soc. Iond., 3l +99 pp., pls 1-7 (1872);99431, pls 8-21 (1874).

Wooq S.V., 1879. Seoond supplement to the monograph ofthe Crag Molluscq with descriptions of shells from theUpper TeÍtiairies of the east of England. Morrcgr.Palaeontogr. Soc. [ond., l-58, I pl.

Manuscript received 30 Jture 1999, revised version accepted30 May2000.

PLATE 1

Ê9l. Glyynak (Clwroniz) oMa obovsta ( ^n''d|*, l8l9), imedc of lefr valve, Dcnberg, Ahnetal (C€ÍmEry), OligomÊ,Cbctian, Kass€l€,r Me€Í€ssod (.4" Jmse Colln), x 2.25

l-rg 2. Glycyneris (Chetronia) obovam r'mgelet n t&p, RGM 394 099 (bolotlDe), A - €'ceric of lefr valve (x l.O, B - inerior ofright v.lve (x 1.5), C - interic oflct valvc (x 1,5); Antw€rp, 5th harbour dock (provincc ofAntwqp, Bclgiun), Plioccorc,bosc Kdtdlijk Fondion, er vm Niculandc Colln"

- t 2 -

PLATE 2

Fig. l. Glycymeris (Charcnia) variabilis (J. de C. Sowerby, 1824), BMNH 43188a (lectotpe), right valve, A - exterior, B - interiorviews, Sutrolk (England), Pliocene, Red Crag (previously illustrated by J. de C. Sowerby, 1824, pl. 471, bottom figne), x1.3.

Fig2. Glycymeris (Ch.) variaáÍfts, BMNH 43188b (paralectogpe), right valve, A - exterior, B - interior views, lWoodbridge

(Sutrollq England), Pliocene, Red Crag (previously illustrated by J. de C. Sowerby, 1824,pl.471, centra figure), x 1.4.Fig. 3. Glycyneris (Ch.) variabilis,left valve of a juvenile specimen, somewhat eroded x l0 (scale bar in mm). Borsele-de Kaloot

(province of Zeeland), beach material, derived Pliocene (Moerdijk Colln).

- 13 .

PLATE 2

- 1 4 -

PLATE 3

Fíg l, Glycyneris (Ch.) variabilis, riÉrt valyc, A - ext€rioÍ; B - interior views, Walton-on-th€-Naze (England), Pliocene Rcd Crag(v'an Nieulande Collq leg. J. van <hr Voot), x 2.

Frg 2. Glycyneris (Ch") variabilis, l€ft valve of incquildeml and thick-shelled form, A - intcrior, B - e)d€riq vi€rÀ's, Mill-St.Hubort, Langí$ootn (province ofNoord-Brabant, the Netherlancls), Pliocene, van Niculande Colla, no. 137), x2.5.

- 1 5 -

PLATE 3

?Ëry't""tJ

- Í d "

Fi$ lj

rryI

Fig;3. a lff-, lnnenior d ld

- 17 -

PLATE 4

- 1 8 -

PLATE 5

Fig, l. Gly.ynerk (G) radiolyrataf. psadodcshayesi n fcura, lÊft valvc, À - o<terio, B - intaiu vicws, borehole 42H194(A40),114.50-115.75 mbs, Plioc€ne, 'InclÍbal M€NnbcÍ', leg; Deltadi€ost, RGM 394 l2l, x 5 (photogehs by V/. Krull,Oostbqg).

Frg.2. GlfLyrnefis (G) radiolyrata í psadodeshry*i n fffma, dÉt val'c, Antwcrp, Boudcwijnsluis, basc Ocderen Member,lvÍuqua Collq x 1,6.

Fig,3. Glycryefis (G) radiolyrata í ryatdodesluyesi n frrna, BMNH 43188a, riglt valve, A - €rdcriq, B - intcrior views, x l,C -dctail of hingp pl*r, x 2, ?Suftlk (png!and), Red Crag (pr,sviously illusffied by ,. rb C. Sowcrby, 1821, pl. 471, topfgure).

- 1 9 -

PLATE 5

-20 -

PLATE 6

Fig. 1. Glycymeris (G.) radiolyrata f. uaggeraÍa n. formq A - interior, B - exterior views, x 1.15, C - detail of umbonal region,Kallq Beveren tunnel construction pit, base Oorderen Member (probably reworked from older strata), 10 April 1975, vanNieulande Colln, no. 93b.

Fig. 2. Glycymeris (G) radiolyrata f. uaggeraÍc n. forma, Kallo, sea sluice construction pit, ?base Oorderen Member (probablyreworked frorr older strata), Vervoenen Colln, no. F 717 M,x 1.75.

Fig.3. Glycyneris (G.) radiolyrata I uaggeraÍa n. forma, left valve, Kallo, Vrasenedok excavation, base Oorderen Member(probablyreworked from older strata), Marquet CollÍr, x 1.