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AMERICAN MUSEUM

NovitatesPUBLISHED BY THE AMERICAN MUSEUM OF NATURAL HISTORYCENTRAL PARK WEST AT 79TH STREET, NEW YORK, N.Y. 10024Number 2875, pp. 1-1 1, figs. 1-10 April 23, 1987

Iteration of Ligament Structures inPteriomorphian Bivalves

NORMAN D. NEWELL1 AND DONALD W. BOYD2

ABSTRACT

Pteriomorphia, the largest subclass of the Bi-valvia, includes such highly successful extantgroups as the marine mussels, pearl shells, oysters,and scallops. Shells ofdifferent groups display sur-prisingly diverse structural types of ligaments (di-agnostic traces ofwhich are recognizable in fossilsas old as the Early Cambrian) and the ligamentshave figured prominently in pteriomorph classi-fications.Within the context ofgeneral morphology, sev-

en or eight ligament grades have been used to char-acterize families of pteriomorphs. But the same,or very similar, ligaments appear repeatedly asevolutionary novelties in separate taxonomicgroups without evident phyletic origins or markedadaptive significance. Even their phylogenetic po-larity is frequently in doubt.

For example, one complex structure (duplivin-cular) is replaced geochronologically by a different,and simpler structure (alivincular) in families ofthe Arcacea, Anomiacea, Aviculopectinacea, andPteriacea. "Transitional" (new term), multivin-cular, and duplivincular ligaments all occur in thesuperfamily Pteriacea. Even where the geologicalsuccession of ligaments might be interpreted asindicating an ancestral-descendant relationship,the different grades of ligaments do not form gra-dational morphoclines.So this review is intended only to call attention

to taxonomic and phylogenetic difficulties in eval-uating ligament grades; we are withholding con-clusions about classification until later.

INTRODUCTIONThe bivalve subclass Pteriomorphia is a

major cluster of 46 families, according to theTreatise on Invertebrate Paleontology (New-ell, 1969). Its great diversity may well be an

' Curator Emeritus, American Museum of Natural History.2 Research Associate, American Museum of Natural History; Professor of Geology, University of Wyoming,

Laramie, Wyoming.

Copyright © American Museum of Natural History 1987 ISSN 0003-0082 / Price $1.65

AMERICAN MUSEUM NOVITATES

A B

Fig. 1. Cross sections showing the two liga-ment layers: lamellar (black) and fibrous (ruled).Hinge axis is indicated by cross. Stipple patternrepresents shell. A. Primitive; B. parivincular.(From Trueman, 1 951)

artifact of its classificatory history. As so de-fined it spans the entire Phanerozoic andcomprises about one-fourth ofall the familiesofbivalve molluscs, including such successfulgroups as the arks, scallops, pearl shells, oys-ters, and marine mussels. They includemonomyarian, anisomyarian, and isomyari-an forms with diverse ligament structures.

Early attempts at classification stressed sin-gle anatomical characters, such as gill types,that leave no indication on the shells (e.g.,Isofilibranchia), or the reduction or absenceof the anterior adductor muscle repeated inseparate branches. But these arrangementshave fallen into disfavor with increasingknowledge ofthe fossil record ofthe Bivalvia(Newell, 1965, 1969).The shell characters that delimit many of

the conventional family groups have been

conservative and stable for tens to hundredsofmillions ofyears, giving credibility to muchof the accepted family arrangement. Thestratigraphic sequence of fossils, together withdevelopment, skeletal morphology, anato-my, and life modes of living representatives,all contribute to ideas about phylogeny andclassification. There remain, however, manydifferences, partly semantic, as to limits andnames of higher categories (e.g., compareNewell, 1969, and Waller, 1978). These arebeing smoothed out with time.One of the most interesting characteristics

of this and some other groups of Bivalvia isa tendency toward evolutionary repetition ofshell characters, which is probably attribut-able to the limited ways in which structuralmodifications have been genetically con-strained. Diverse structural types of thepteriomorph ligament provide examples ofparallel repetition of a morphological char-acter in separate taxonomic branches.As with other bivalves, the ligament con-

sists of two parts. One of these is an upper,elongate, lamellar band usually under tensionwhen the valves are closed. The other is anunderlying fibrous layer subject to compres-sion when the valves are closed (fig. 1). Thelatter is stiffened by microscopic needles ofaragonite arranged at right angles to the innergrowing surface where both components aresecreted by the mantle.

7,--

b

O lOmmI a

c

@ + f

0 1 2mma I~~~~~~~~~~~~~

Fig. 2. Transitional ligament illustrated by the arcoid Lunarca ovalis. Recent, coastal New Jersey.Serial sections showing median split of fibrous and lamellar layers and added secondary lamellar bandbelow the beaks. Note that the split begins well in front ofthe posterior end ofthe hinge. Lamellar layersin black; fibrous layer lined; mantle stippled. Based on several specimens.

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1987 NEWELL AND BOYD: LIGAMENT STRUCTURES IN PTERIOMORPHIA

LIGAMENT GRADES

AUVINCULAR-EX

DUPLVINCULAR-AMPHIDETIC

AXI'

DUPLVINCULAR-OPISTHODETIC

AXIS AXIS

>2 ......

(TERNAL ( ALVINCULAR-INTERNAL

AXIS ,.'N I

MULT

IS *

TRANSITIONAL

AXIS * , 9SAXIS

AXIS- I -S ~~~~~~~~PARMVNCULAR

PRIMITIVE

Fig. 3. Ligament grades with possible but uncertain paths of change, some of which could haveresulted from neotenous reversal. The phyletic polarity is usually uncertain. S, secondary lamellar band.The alivincular and duplivincular figures at the top are amphidetic; the others are opisthodetic, black,lamellar, ruled, fibrous.

The fibrous layer is weak to tensionalstresses and in some forms tends to split asinterumbonal growth spreads the two valvesapart. Hence, the functional part of the fi-brous layer lies below the hinge axis. In thissituation expansion carries the obsolete partofthe ligament above the hinge axis. In sometaxa the fracture is repaired by a secondaryplug, or band, of lamellar conchiolin whichmaintains the juncture of the two valves (fig.2).The ligament usually lies behind the beaks,

the opisthodetic condition. Or it may extendboth in front of, and behind, the beaks whereit is said to be amphidetic (fig. 3). It seemsthat the amphidetic hinge appeared late inbivalve history (mid-Paleozoic) and there-fore was probably "derived" from the moreancient and evidently more primitive opis-thodetic condition.A stable trait in some families is the pos-

session of flat or slightly concave cardinalareas (fig. 4B) to which the ligament is at-tached. These result from the inward anddownward migration of the hinge axis asgrowth expands the valves between the um-

Fig. 4. Hinge axis in relation to internal andexternal ligament. A. Internal ligament withouthinge plates or cardinal areas; B. external ligamentwith cardinal areas and hinge plate; UC indicatesumbonal cavity. Stipple area represents shell; blackcircle, hinge axis.

3

.\ XAXIS

VINCULAR


7I

A

B C

D h".,,

Fig. 5. Growth-line traces of transitional ligament in several families of Pteriomorphia. The scalebars represent 5 mm. A. Pterioid (pergamidiid), Eurydesmaplayfordi Dickins. Permian, eastern Australia,Austral. Bur. Mineral. Resources no. 2232. B. Cyrtodont, Cypricardinia sp., Permian, West Texas, Am.Mus. Nat. Hist. no. 42886. C. Parallelodont, probably a n. gen., n. sp. Transitional ligament limited toposterior part of cardinal area, duplivincular on anterior part. Permian, West Texas, Am. Mus. Nat.Hist. no. 42887. D. Arcoid, Lunarca ovalis. Recent, Georgia. Am. Mus. Nat. Hist. no. M28579. Arrowpoints to groove of the secondary lamellar band.

bones. The cardinal areas are thus externaland they converge downward from the dorsalmargins to the hinge line at about 15 up to450.

Here we consider eight well-defined types,or grades, of ligaments. They may be desig-nated primitive, transitional, parivincular,opisthodetic-duplivincular, amphidetic-du-

4 NO. 2875


plivincular, external-alivincular, internal-alivincular, and multivincular (fig. 3). In con-ventional usage many of the pteriomorphfamilies display only one grade of ligamentbut there are some with more than one lig-ament type. Ambiguity thus introduced sug-gests a need to reevaluate the taxonomic sig-nificance of the several ligament grades in ageneral morphological and historical context.Our purpose here is to report some obser-

vations and ideas resulting from our con-tinuing studies of bivalves that lived in theseas around the time of the great Late Perm-ian and Early Triassic extinction crisis. It isa cursory review in which we postpone a con-sideration of pteriomorphian systematics.We are indebted to E. R. Trueman and

Thomas R. Waller for reading our manu-script and making useful suggestions.

THE PRIMITIvE LIGAMENT

The most ancient bivalves, Fordilla andPojetaia ofthe Lower Cambrian, had the sim-plest ligament remindful of modern Mytilusor Malletia (Runnegar and Bentley, 1983)(figs. IA, 3, primitive). The ligament in Myt-ilus, however, is reinforced below by shell"pseudonymphs."Runnegar and Bentley and some others

(e.g., Morris, 1979) referred to this ligamentas parivincular (figs. 1B, 3), but we prefer toreserve that term in the conventional sensefor the more advanced C-spring ligament ofthe heterodonts, unionids, and trigoniaceans(fig. 1B). The primitive ligament has also beencalled primary by Owen et al. (1953).

In the primitive ligament, lamellar and fi-brous layers form an elongate ribbon betweenthe dorsal margins of the valves (Trueman,1950). The lamellar part extends farther to-ward the rear than the fibrous part in an over-lapping relationship that keeps them in con-tact with the mantle so that both increase inlength as the shell grows. As the ligamentsplits anteriorly the gap in the fibrous part isreinforced by a secondary band of lamellarconchiolin.

THE TRANSITIONAL LIGAMENT

The transitional ligament may be opis-thodetic or amphidetic. It occurs in severalpteriomorphian groups, e.g., pergamidiids,

B

A

C

Fig. 6. Sections ofmodern arcacean ligaments.A. Transitional grade in Lunarca ovalis (=pexata)Say. Dot indicates location of hinge axis. Fibrouslayer (ruptured), below; lamellar above. B, C. Du-plivincular ligament of Scapharca transversa.Three-dimensional view (above) shows locationofsection (below). Two bands oflamellar ligamentare separated by split and inoperable fibrous layers(after Newell, 1942). Stipple pattern representsmantle; black, the shell.

cyrtodonts, arcids, and parallelodonts (fig. 5).A living example is the arcid genus Lunarcain which the structure is that of a juvenileduplivincular ligament possessing a singledorsal band rather than multiple lamellarbands (fig. SD). It may have been a simplerprecursor of the duplivincular ligament (seebelow), but it could also conceivably be pae-dogenetic, derived from that structure byneoteny. The phyletic polarity is uncertainand we suspect that both conditions may ex-ist.The transitional ligament is usually at-

tached to the outer areas ofhinge plates (figs.4B, 5, 6A). In addition to the single lamellarband at the dorsal margin of each cardinalarea a secondary band is inserted below thebeaks near the hinge line where the fibrouslayer splits (figs. 2e, f, 5D). This seems tocompensate for the loss of function of thesplit portion of the fibrous band in the inter-umbonal area.

THE DUPLIVINCULAR LIGAMENT

The duplivincular ligament (Newell, 1942,p. 29) is composed of a series of alternatingbands of lamellar and fibrous conchiolin ex-tending obliquely along each cardinal area

5


B

CZ- aD~~~~~~~~~~Aw~

Fig. 7. Duplivincular ligament in diverse pteriomorph families. Scale bars represent 5 mm. A. AnAnomiacean, Permanomia texana Newell and Boyd, with opisthodetic-duplivincular hinge. Permian,West Texas, Am. Mus. Nat. Hist. no. 28937. B. Arcid, Scapharca transversa (Say) with amphidetic-duplivincular ligament area. Recent, Florida, Am. Mus. Nat. Hist. no. M28708. C. Cyrtodont, Vanux-emia gibbosa Ulrich, opisthodetic-duplivincular ligament. Middle Ordovician, Tennessee, U.S. Natl.Mus. no. 46942. The very fine ligament grooves simulate growth lines but, unlike growth lines, the last

6 NO. 2875


(figs. 6B, C, 7). Each lamellar band is in-serted in a narrow groove that extends frombelow the beaks to the hinge margin. Thebands resemble replications of the dorsal la-mellar band of the transitional ligament.The duplivincular ligament is commonly

opisthodetic in juveniles and most adults butalso it may be symmetrically amphidetic orbecome amphidetic during growth, as inScapharca transversa (fig. 7B). The groovesin the amphidetic varieties form an invertedchevron pattern with the apex under thebeaks. The duplivincular ligament character-izes several families in the Cyrtodontacea,Pteriacea, Pectinacea, Anomiacea, and Ar-cacea.

Recognition ofthe duplivincular conditionin extinct taxa poses no problem where theligament area exhibits coarse grooves that in-tersect the hinge margin at a high angle (fig.7A, B, D, and E). At the other end of theduplivincular spectrum, however, the angleof intersection may be so low that all but theone or two last-formed grooves extend allalong the length of the hinge and appear tobe parallel with its ventral margin (fig. 7C,F). This has led Dickins (1983), to concludethat the relationship of grooves to margin isnot significant. We, however, believe thatconfusion arises only where the grooves arevery fine and the preservation of fossils isimperfect.

Discrimination between certain duplivin-cular, transitional (fig. 5), and elongate ali-vincular (fig. 8A, B) ligament areas can bedifficult. Pojeta (1978, p. 236) encountered aproblem in dealing with Ordovician cyrto-dontids. Noting that the parallel grooves onhis shells apparently did not intersect thehinge margin, he concluded that the ligamentwas not duplivincular. He also rejected thegrowth-line explanation because the grooveson the ligament area seemed to lack lateralcontinuity with growth lines on adjacent parts

of the shell. As an alternative, he suggestedthat the grooves represent successive inser-tions ofa single, ventrally migrating ligamentband. If so, the situation would be unlike anyliving bivalve known to us.

THE ALIVINCULAR LIGAMENT

This ligament is characteristically amphi-detic, rarely opisthodetic. The compressionalpart is a- more or less triangular pad, the re-silium, segregated in a triangular depression,the resilifer, between anterior and posteriorareas of lamellar ligament extending alongthe length of the hinge axis (fig. 8A, B, D, E).Some have elongated resilifers in which the

base ofthe triangle greatly exceeds its height.Rarely, the resilifer is very broad, asymmet-ric, and marked with longitudinal groovesthat suggest seasonal growth interruptions (fig.8A).The alivincular ligament may be attached

to the cardinal areas of hinge plates, as withthe Aviculopectinidae and Pteriidae. This iscalled the external alivincular ligament (figs.3, 4B, 8A, B, E).

Or, the alivincular ligament may be essen-tially internal, in which case true cardinalareas are lacking (figs. 3, 4A, 8D). The inter-nal resilium of the Pectinidae, for example,differs from the external resilium of otherPteriomorphia. The central part ofthe formerlacks aragonite needles (fig. 9B) and the cal-cified lateral portions are rigid and of uncer-tain function (Newell, 1937; Trueman, 1953;Waller, 1976). They may be evolutionary rel-icts of the fibrous resilium.The curious Upper Paleozoic aviculopec-

tinacean genus Euchondria (Euchondriidae)has an external alivincular ligament. The adulthinge retains juvenile pseudotaxodont teeth(fig. 1OA) similar to those ofmany immaturepteriomorphs (fig. lOB). Some modern pec-tinids also retain these neotenous structures

formed grooves are progressively shorter and they intersect the hinge margin. D. Cyrtodont with opis-thodetic-duplivincular ligament grooves, Ptychodesma knappianum Hall and Whitfield. Middle Devo-nian, New York State, Am. Mus. Nat. Hist. no. 361888. E. Myalinid, Septimyalina perattenuata (Meekand Hayden). Ligament opisthodetic-duplivincular. Upper Pennsylvanian, Kansas, Am. Mus. Nat. Hist.no. 42888. F. Myalinid, Selenimyalina meliniformis Newell. Ligament opisthodetic-duplivincular. Thenumerous ligament grooves are comparatively fine. Upper Pennsylvanian, Kansas, Am. Mus. Nat. Hist.no. 42889.

7


Fig. 8. Diverse pectinoid ligaments. Scale bars represent 5 mm. A. Deltopectinid, Deltopecten sp.Interpreted by comparison with B, below, as external-amphidetic. The very elongate resilifer is markedby regular, probably seasonal, growth lines. Permian, South Kennedy, Queensland, Australia, Am. Mus.Nat. Hist. no. 42890. B. Strebloptenid, Streblopteria sp., similar to fig. 8A but lacking conspicuousgrowth lines. Permian, West Texas, Am. Mus. Nat. Hist. no. 42891. C. Pterinopectinid with an am-phidetic-duplivincular ligament. Pterinopectinella sp. Permian, West Texas, U.S. Natl. Mus. no. 388871.D. Entoliid, Entolium sp. Illustrating internal-alivincular ligament. Permian, Texas, U.S. Natl. Mus. no.388870. E. Aviculopectinid, Etheripecten vanvleeti (Beede). External-alivincular ligament. Permian,Texas, Am. Mus. Nat. Hist. no. 42893.

that continue to be functional well into ma- cussed Euchondria he mistook the gaps be-turity (fig. lOC). In 1937 when Newell dis- tween denticles for multivincular resilifers.

NO. 28758


A

Fig. 9. Internal alivincular ligament ofChlam-ys islandica Miiller, modern pectinid. A. Trans-verse section of lamellar band near posterior endof the hinge (stipple area represents mantle). B.Transverse section at hinge midlength of uncal-cified compressional ligament (center), flanked byrelict nonfunctional fibrous ligament pads. (Crossrepresents hinge axis. Black indicates shell. AfterNewell, 1937.)

THE MULTIVINCULAR LIGAMENT

This ligament possesses multiple resilifersof the external alivincular type and it is usu-ally opisthodetic. Among living bivalves (fig.3) the multivincular ligament is characteristicof the Isognomonidae (Pteriacea). It is alsothe mode ofseveral extinct families includingthe very successful Mesozoic Inoceramidaeand some aberrant oysters which have beengiven the generic name Pernostrea (Stenzel,1971, p. 974). The Noetiidae (Arcacea) pos-sess superficially similar multiple resilia butthey are unique in being formed of lamellarrather than fibrous bands.

Sequential development of the multivin-cular ligament in an individual suggests thatit could have been derived phyletically fromthe external-alivincular ligament by serialrepetition. However, as well as we can nowascertain the stratigraphic succession does not

Fig. 10. Pseudotaxodont hinges characteristicof the early development of some pteriomorphi-ans. Scale bars represent 0.5 mm. A. Neotenoushinge in Euchondria levicula Newell, a small adultaviculopectinacean with an external ligament.Middle Pennsylvanian near St. Louis, Mo., Univ.Kansas no. 58805; B. external ligament injuvenilePteria staminensis, Recent, after Bernard (1896);C. internal ligament of Pecten magellanicus, Re-cent, after Jackson (1890).

support this hypothesis. Morris (1979, fig. 8)thought that the multivincular ligament mightbe derived from the duplivincular structurebut we do not know of any evidence for this.

In the Permian Period the multivincularligament suddenly appeared in a populationofthe Australian ambonychiid genus Atomo-desma (Aphanaia) which had a very different(transitional) ligament (Browne and Newell,1966; Kauffman and Runnegar, 1975; Newelland Boyd, 1978; Dickins, 1983). The Perm-ian multivincular variety was given the ge-neric name Permoceramus by Waterhouse(1970). Later Muromtseva (1979) found thesame association of Permoceramus andAtomodesma in northeastern Siberia. Exceptfor the ligament, the shells of the two generaare virtually identical and they are very likethe Mesozoic Inoceramidae.Everyone who has studied the geological

occurrences of Permoceramus has regardedit as marking the intrapopulation origin ofthe multivincular pteriacean family Inoce-ramidae which otherwise is not known priorto the Jurassic a hundred million years later.

9


Why was Permoceramus so short-lived ascompared with the later inocerami?

Because of this exceptional Permian oc-currence Kauffman and Runnegar (1975) havesuggested that the multivincular family Ino-ceramidae may logically be extended to in-clude not only Permoceramus, but also theassociated Atomodesma with its transitionalligament.Could Permoceramus simply be an unsuc-

cessful variant that was repeated again in theJurassic as Inoceramus? Ifthey are not closelyrelated this would be a most remarkable ex-ample of nearly identical iteration becausePermoceramus is morphologically an ino-ceramid in all known details. Or did Per-moceramus retire to an unknown haven dur-ing the Permo-Triassic environmental crisisuntil its chance to spread throughout the worldas Inoceramus in the later Mesozoic?

ADAPTIVE SIGNIFICANCE OFLIGAMENT PATTERNS

The adaptive significance of ligament typeis not clear. In a series ofexperiments in 1950-1969, Trueman studied the relative efficiencyof several kinds of ligaments in living bi-valves. This was done by measuring stressdifferences between opening and closing mo-ments of the valves as follows (Trueman,1964).

Ratio of Ligament Resistance toMuscle Adduction (in Grams)

Arcacea (duplivincular) 83/105, 79%Mytilacea (primitive) 660/780, 84%Ostreacea (alivincular-external) 370/425, 87%Pectinacea (alivincular-internal) 160/167, 96%

Evidently there is some relationship be-tween ligamental resistance and mode of lifeof the various species, but it is not marked.Shallow burrowers among living pterio-morphs (e.g., arcoids, fig. 2) have relativelyweak ligaments which are aided in openingthe valves by a strong foot (Thomas, 1978),while swimmers have relatively stronger andmore efficient ligaments. Among these, thePectinidae have internal alivincular liga-ments but the Limidae, with external alivin-cular ligaments, are also swimmers.

Contrasted with the more convex arca-ceans, the Paleozoic Pterinopectinidae, with

duplivincular ligaments and probably a small,weak foot, had thin, flattened shells shapedvery much like living scallops. By analogythey were probably active members of theepifauna.The Mytilidae, Ambonychiidae, Myalini-

dae, and Inoceramidae probably were alladapted to similar modes of life-i.e., sed-entary and byssate. Apart from the fact thatthey were all opisthodetic, they had differentligament styles-what we are calling primi-tive, transitional, duplivincular, and multi-vincular, respectively. There is little, if any,correlation in these families between shellform and ligamental grade. The ligamentsalone do not tell us about life modes.

CONCLUSIONAlthough the several distinctive kinds of

pteriomorphian ligament patterns have beengiven high priority in taxonomic discrimi-nation and some appear to have been stablefor long geological intervals, they are notthemselves reliable guides to relationships athigh taxonomic levels. The various ligamentgrades are basically the same in constructionand function but they differ in patterns thathave uncertain interrelationships. The highfrequency of convergence of ligament gradesin separate taxonomic categories indicates aneed for a thorough revision of morphologyand taxonomy ofthe subclass beginning pref-erably at low taxonomic levels.

LITERATURE CITED

Bernard, Felix1895-97. Sur la development et la morphologie

de la coquille chez les lamellibranches.Soc. Geol. France, Bull. ser. 3, 24 (1896):54-82, 412-449.

Browne, I. A., and Norman D. Newell1966. The genus Aphanaia Koninck, 1877-

Permian representative of the Inoce-ramidae. Am. Mus. Novitates, 2252:1-10.

Dickins, J. M.1983. Posidoniella, Atomodesma, the origin of

the Eurydesmidae, and the develop-ment of the pelecypod ligament. Bur.Mineral Res. Australia, Bull., 217:59-65.

10 NO. 2875


Jackson, R. T.1890. Phylogeny of the pelecypoda-The Av-

iculidae and their allies. Boston Soc. Nat.Hist., Mem., 4:277-400.

Kauffman, E. G., and Bruce Runnegar1975. Atomodesma (Bivalvia) and the Perm-

ian species of the United States. J. Pa-leontol., 49:23-51.

Morris, N. J.1979. On the origin of the Bivalvia. In M. R.

House (ed.), The origins of major in-vertebrate groups. New York: AcademicPress, pp. 381-413.

Muromtseva, V. A.1979. Representative of the Inoceramidae in

the Upper Permian deposits ofthe Ver-khoyan'e region. Upper Paleozoic andMesozoic: Islands and coast of the Arc-tic Ocean USSR. NIIGA, Geol. Minis-try, USSR:34-37.

Newell, Norman D.1937 (1938). Late Paleozoic pelecypods: Pec-

tinacea. Kansas Geol. Surv. Publ., 10(1):1-123.

1942. Late Paleozoic pelecypods: Mytilacea.Kansas Geol. Surv. Publ., 10(2): 1-115.

1965. Classification ofthe Bivalvia. Am. Mus.Novitates, 2206:1-25.

1969. Classification of Bivalvia. In R. C.Moore (ed.), Treatise on invertebratepa-leontology, Part N, Mollusca 6, vol. 1,pp. 205-218. Lawrence: Geol. Soc. Am.and Univ. of Kansas.

Newell, Norman D., and Donald W. Boyd1978. A palaeontologist's view ofbivalve phy-

logeny. Phil. Trans. Roy. Soc. LondonB, 284:203-215.

Oliver, P. Graham1981. The functional morphology and evo-

lution of recent Limopsidae (Bivalvia,Arcoidea). Malacologia, 21:61-93.

Owen, G., E. R. Trueman, and C. M. Yorge1953. The ligament in the Lamellibranchia.

Nature, 171:73-75.Pojeta, John

1978. The origin and early taxonomic diver-sification of pelecypods. Phil. Trans.Roy. Soc. London. B 284:225-243.

Pojeta, John, and Bruce Runnegar1976. The paleontology of rostroconch mol-

lusks and the early history of the Phy-

lum Mollusca. U.S. Geol. Survey ProfJPap. 568:1-88.

Runnegar, Bruce, and Christopher Bentley1983. Ancestry, ecology and affinities of the

Australian Early Cambrian bivalve Po-jetaia runnegari Jell. J. Paleontol., 57:73-92.

Stenzel, H. B.1971. Oysters, In R. C. Moore (ed.), Treatise

on invertebrate paleontology, Part N,Mollusca 6, vol. 3, pp. 953-1224. Law-rence: Geol. Soc. Am. and Univ. ofKansas.

Thomas, R. D. K.1978. Limits to opportunism in the evolution

of the Arcoida (Bivalvia). Phil. Trans.Roy. Soc. London B, 284:335-344.

Trueman, E. R.1950. Observations on the ligament ofMytilus

edulis. Q. J. Microscop. Sci., 91:225-235.

1951. The structure, development, and oper-ation of the hinge ligament of Ostreaedulis. Q. J. Microscop. Sci., 92:129-140.

1953. The ligament ofPecten. Q. J. Microscop.Sci., 94:193-202.

1964. Adaptive morphology in paleoecologi-cal interpretation. In John Imbrie andNorman D. Newell (eds.), Approachesto paleoecology, pp. 45-70. New York:Wiley.

1969. Ligament. In R. C. Moore, (ed.), Trea-tise on invertebrate paleontology, PartN, Bivalvia, vol. 1, pp. 58-64. Geol. Soc.Am. and Univ. of Kansas.

Waller, T. R.1976. The development ofthe larval and early

postlarval shell of the bay scallop, Ar-gopecten irradians. Bull. Am. Malacol.Union for 1976:46.

1978. Morphology, morphoclines and a newclassification ofthe Pteriomorphia. Phil.Trans. Roy. Soc. London B, 284:345-364.

Waterhouse, J. B.1970. Permoceramus, a new inoceramid bi-

valve from the Permian of eastern Aus-tralia. N.Z. J. Geol. Geophys., 13:760-766.

I1I

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