POSTILLA
| PEABODY MUSEUM
| YALE UNIVERSITY

NUMBER 171 30 MARCH 1977

TAXONOMY AND ECOLOGY OF THE PHILO-
BRYIDAE AND LIMOPSIDAE (MOLLUSCA:
PELECYPODA)

MICHAEL J. S. TEVESZ

POSTILLA

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CONTENTS

ASTPORMEISULES © save cA it be eR. OMe. tera eS
ist} Ob PPablestas Seer i ee ee ete os petit Pa eee 3}
TADSTRA GT Rare Rees tocar eet cake uence ecg ly et eee EOS el 4
LAL ILOCUC HOM cant Ci ta aes soe ace Cenk Th ARE et
Abbreviutions snd Definitions. «0.020... anette 7

2. Generic Revisions of the Families Philobryidae and Limopsidae .......... 8
REVISION OL the Genera ot the Philobryidacs, smutie men. 8
Morphological Definition ................... debe eic, AOE Peer 8

Walid Generar ister o. daca ie? Be eel eel ae) AC ee ot 9
GOnUSPAAGONGrCa ARGUS sean, ae, RL ee ee wD

CFENUMS PAU PONTE fire eek dati ee eR I

(Genus: GOSA sn. Ne pie eet ae nk Dena a 1D)

Gents: CFOS Ans eevee reek ne oo ee perenne 15

(GOTUS ISLIP ON COGAN Loci ac ean beatae tee aea ac a eer te oka ah)

CFCS PIALTIODSULD: open er ae a a, ee ee

MGV STIS IST SSL CCM fel c, erseihene chek seep tena nme MEL OO iiNet 18
(SCRUSINCOCOICIN: (ihac en eee a ee OO)

(GENUS SP HTOOT VCs. sah ake ror ee fre, Heenan ety et et LO

Revision Om the Genera, Of the witdOpsidae nists eS
NIOE PAOLO RIC AIM CHNTEION wrty rere coemrctin te. hr core iis ae

VAI: CET eres tee reo or ee eet ee act Mente nen eaten re Ps)

GENUS SETPICCONUGS nee en ee re soe el eee DD)

CT CIUS LAUT OD SIS 98 eeccses Fee tine Ungerer, hears oe ate ey,
IRGASSIDTIN CIS Wa eect ite een me rege ten St RMT ey han etd ND,
SiPECOLOCY Gaal, fete et peas ey eas Gene Teen te Pe get, ee en DO)
INCACE TIES AIG VICTNO USI. Merten din terete etm cretace Retiree)
ECOG SY Cie PEnOUMVICAC ii: Wea ilr a heg ree eee Oe)

EPA Dita emer reece arte Mi fetes tit. one Eo)

The POSTION citue IOCOMUOLLON <c...3rc1csok Meee a oe)
Becdin sandy CLS Ams wince, casero nie exes scrapie tepae ria ceeo oul
dikefe]KojenrAronndnterel byhan(oyok dele me ites letra re ee uot ori dees! Riaeaes 77

FEA Ditatan & Patera ayiivese tea Pape benoit eR air cai eee ici etre: Ree, aunt SZ

Dene sPOsition ald: WOCOMOMON s. °c ceth eoe nk ne ee
Beédin gvand Cleans ome trct.< ittwtcgh tacos Gant en ante eames ae
BUNCHOIMS cand MPU CHUOMS. tp even ict: doce were ee ee ae ee ey

4. Higher Taxonomy
Oadalhats eA AUanhnnyntscs. oneynes meinen ene tive tess cima te aera mr
Evolution . PASS ini te OL, cohen tian erates
Limopsid Origins Heater Eo siege ate CeIn cea osc ie Sa
PHHObE VIG, OUI SIMS i. co.cc. uae. Maes RE ale Maa Mista con teteul es «
Mechanisms ........ Jed MOTHS, BOL ASP. AARESELE pee oh Pike
Other Limopsid Offshoots. YAN hen ARSRMETERG A ER td AAA MO
Acknowledgments .......... Sig
Appendix A, Geological and ‘Geographical Ranges of Valid Cute aoe
Appendix B, Descriptions of Junior Subjective Synonyms
Philobryidae
Limopsidae
Ts iteTeet HOS UE Ie, PN LORE oie se each ae cay AMMO N, WEEE A op oH oe

FIGURES

i-? Morphologicaleteatures of the puilobryidishell co) ae ee 5

2. Morphological teatures:of the limopsid "shell" 6
3. Adacnarca nitens Pelseneer

Ae “AU PORTIA PATVULAEP OWL er aes coe RAAT Cane TT Sh Ter te :

Set COSATCOSTALGs CBELMEALG Oe eter te eee ee eee

OW CLALIS PLO CTC SSG ACUI CY rns eth rere tee eee ha ee sctne Ri yma tr ae

7. Limarca angustifrons Tate

See LMMOPSULG PUMWUOASTMED): toc ere cr mre tae tee one

OPT INSSAICE UOT O;USCOPSUMNULY Stitt te pene ie ste
MOT IN COCR CIC 4QNPULELGE SOW CHOY: hoc. ae vane iio te ere eee eee a, 21
TPL MODY Ge serosa (GAG DeMtCh): tiers canoe ete eos ee eee 22,
lie Wigaiment plu vamMOUS tle HHOOLYC: 11 neti AMC ee. te cea ee . 24
LS.” A VIpleCOMG: VALINGLA. CIDA) 2. eesti eee tra hee ee 26
Ar umopsis qurua UBNOCCHT) %...0..0 nee taser Me Selah te Se oan 28
19) Philobrya is Wie Bosiaen 2.0. EEO) dele SONS agp
lon dihalanteandecxlalanteatcas, 20.8 ttc ket ere er eee nr Eta eey Serr
LP ee OLLODEYIdmciigty, UaCts wir. niet: sch one iae ieee eae Cee
LS Ue OPSISCULe ePOSLMOD area. e On te eT ee cee Sree, ener, GS)
TOL TTODSIS* CLUALY ULACIS =r. art ern cat wee eee eee Te A rene. 34
20. Summary of arcoid phylogeny ............. att oes Duets ober gear erviv ene ise |,
21. Limopsis and limopsid-like grammatodonts we hs. Si. Beets Morac ant Set Od
22. Philobryid and limopsid similar in overall shell features” iia gn: 43
23 CAUCOIG denticlospen pete aed.
24, Limopsid and elycymerid ‘similar i in , overall shell features EERE Ni Hen 48
25. Mesozoic limopsids, cucullaeids, and glycymerids —a comparison 50
QGva LWSSGLCG PiGtG, MLC CLC y interes eccere rrp eh es Eee oa 23
Dede el dk deyesstes ipidonger Wels ketal ley) WasgeRiearna aap Soe ayn sisi d., iAetaceeh ace th: Acie) 5)
2S o PMLODIY O TRALCLGILGG™ (SUCIMDEL rss gt tn eee te ane ee, 7,
DOWNLOD Sta I AELOJIER SIS SILL, 00h ae hubsinCites ee ete ee nc otee ey O)

TABLES

1. Recent dispositions of philobryid generic and subgeneric names ........ 8

2. Recent dispositions of limopsid generic and subgeneric names ............ a)

TAXONOMY AND ECOLOGY OF THE PHILOBRYIDAE AND
LIMOPSIDAE (MOLLUSCA: PELECYPODA)

MICHAEL J. S. TEVESZ

Department of Geological Sciences
The Cleveland State University
Cleveland, Ohio 44115

(Received 23 March 1975)

ABSTRACT

The Philobryidae (Eocene-Recent; order Arcoida) are redefined to include
nine genera: Adacnarca, Aupouria, Cosa, Cratis, Limarca, Limopsilla, Lis-
sarca, Neocardia, and Philobrya. Philobryids are generally small (2-10 mm),
mytiliform, and have a ligament pit that lies between two series of interlock-
ing denticles. Philobryids are most common and widely distributed in the
Southern Hemisphere. They are epibyssate but also are efficient epifaunal
crawlers.

The Limopsidae (Jurassic-Recent; order Arcoida) are redefined to in-
clude two genera: Empleconia and Limopsis. Limopsids are small (10-60
mm), ovate, and have a ligament pit that lies in a dorsal area above numer-
ous teeth. Limopsids are endobyssate and are cosmopolitan at shelf depths.

Unlike the gill of other arcoids, the philobryid gill is composed of short,
stubby filaments and has few ciliary junctures. These features of the philobryid
gill represent adaptations for cleansing, strengthening the gill, and viviparity.
Some philobryids have an anterior inhalant area. The presence of this fea-
ture in taxonomically remote pelecypod groups and in groups of Recent,
highly specialized pelecypods shows that, contrary to prevalent opinion, its
presence does not necessarily indicate primitiveness or taxonomic affinity.

Morphological, distributional, and temporal evidence suggests that limop-
sids probably arose from grammatodonts and in turn gave rise to glycymerids
and philobryids. Neoteny may have played a role in philobryid and limop-
sid evolution.

POSTILLA 171: 64 p. 30 March 1977

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY 5

1. INTRODUCTION

The pelecypod order Arcoida (Middle Ordovician-Recent) has a rich and
continuous fossil history and is abundant and widespread in modern oceans.
Most arcoids have a sturdy rhomboidal shell, numerous transverse hinge
teeth, and an external ligament attached to a broad area above the dentition
(dorsal area). Living arcoids have filibranchiate gills, a ventrally cleft foot,
and are covered with a black or brown hairy periostracum. Although the
taxonomy and ecology of six of the eight arcoid families are fairly well
understood, two families, the Philobryidae and Limopsidae, are still poorly
known.

The Philobryidae (Eocene-Recent) are small (2-15 mm), mytiliform
pelecypods that are unique among arcoids in having in the adult form a
ligament pit that lies between two rows of interlocking transverse denticles
(Fig. 1). Unlike most arcoids, philobryids have an internal ligament, and

a

Fic. 1. Morphological features of the philobryid shell. a = prodissoconch,
b = ligament pit, c = denticles, d = tooth, e = anterior adductor muscle scar,
f = posterior adductor muscle scar, g = crenulation.

POSTILLA 171

some genera lack teeth. Philobryids are epibyssate, exclusively marine, and
occur from the intertidal zone to depths of over 1000 meters. There are 8
extant genera and more than 50 extant species. Most of these are found off
Australia, New Zealand, and Antarctica. Other localities include South
Africa, southern South America, Baja California (Mexico), the Caribbean,
the Mediterranean, and scattered islands in the South Atlantic Ocean. Philo-
bryids have a patchy distribution within their geographic range, and only
about 10 species are known to be common. One extinct genus and about
12 extinct species have been described. The scanty fossil record of the
family is confined to Australia and New Zealand.

The Limopsidae (Middle Jurassic-Recent) are small (10-55 mm), ovate,
and unique among arcoids in having both a radially arrayed ligament in a pit
and a smooth dorsal area (Fig. 2). Limopsids are endobyssate and are com-
mon at shelf depths throughout the world. There are 2 known extant genera
and over 40 extant species. More than 30 extinct species have been de-
scribed. The earliest limopsids are reported from Europe. Post-Jurassic fossil
limopsids are found throughout the world.

FIG, 2. Morphological features of the limopsid shell. a = dorsal area, b = lig-
ament pit, c = tooth, d = anterior adductor muscle scar, e = posterior ad-
ductor muscle scar.

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY te

Except for species descriptions, the philobryid and limopsid literature is
small. For philobryids, Bernard (1897), Thiele (1923), Dell (1964), Nicol
(1966), Cox et al. (1969), and Bergmans (1970) briefly deal with aspects of
supraspecific taxonomy. Pelseneer (1903), Clasing (1918), and Burne (1920)
describe and diagram the anatomy of five different species. Cox et al. (1969)
briefly summarize limopsid supraspecific taxonomy, and Dell (1964) com-
ments on their variability and taxonomic problems. Nicol (1967) points out
differences between the hard parts of Limopsis and Glycymeris. Pelseneer
(1888; 1911) and Burne (1920) describe the soft parts of four different
Limopsis species. No one has yet published observations on living limopsids
or philobryids, nor looked at either family as a whole.

The purpose of this study is to revise the supraspecific taxonomy of both
families and elucidate their autecology. First, the generic taxonomy of both
families is revised. This revision consists of providing morphological defini-
tions of both families and redescribing and discussing all valid genera in
each family. Second, the life habits of these two families are described and
analyzed. Much of the information in this second part comes from ob-
serving live individuals from both families. Third, information from the
first two parts of this study is used to complete the supraspecific taxonomic
revision. Criteria for morphologically characterizing the Arcoida are pro-
posed and philobryids and limopsids are compared to these criteria. Then, the
evolutionary history of both families is traced.

Abbreviations and Definitions

USNM United States National Museum of Natural History, Washing-

ton, D.C.
BM British Museum (Natural History), London
AM Australian Museum, Sydney
SAM South Australian Museum, Adelaide
M. Monotypy
O.D. Original Designation
S.D. Subsequent Designation

In the description of the type species for each genus, the dimensions cited
refer to an individual used in the description that the author considers to
be an average-size adult for the particular species. Width is measured dorso-
ventrally. Length is measured anteroposteriorly.

Except where otherwise noted, the description of the type species for
each valid genus is based on Recent material.

“Range of Morphological Variation” refers to the range for the genus as
a whole, not just the type species.

POSTILLA 171
2. GENERIC REVISIONS OF THE FAMILIES PHILOBRYIDAE AND LIMOPSIDAE

A redescription and discussion of each valid philobryid and limopsid genus
is presented. A valid genus is here considered to be a form separated from
all others by multiple, nongradational morphological differences. Because of
their more conservative nature, hinge features are subjectively weighted
more highly than features such as shell shape, color, and the nature of the
inner margins.

Revision of the Genera of the Philobryidae

1) Morphological Definition

A member of the Philobryidae is here considered to be any bivalve that,
as an adult, has both of the following features: 1) a ligament pit that lies
between two rows of numerous, interlocking transverse denticles and 2) a
filibranch gill with a posteriorly directed, ventral ciliary tract. The available
generic and subgeneric names for the Philobryidae and their dispositions
by recent workers are shown in Table 1. The valid philobryid genera sorted
from these names are described and discussed below. Their geological and
geographic ranges are given in Appendix A, and redescriptions of the
junior subjective synonyms are given in Appendix B.

TABLE 1. Recent dispositions of philobryid generic and subgeneric names.

Dell (1964)

Valid genera. Philobrya, Adacnarca, Lissarca, Verticipronus, Neoconcha
(= Neocardia).

Synonyms of Philobrya. Philippiella, Stempellaria, Stempellia (also name pre-
viously occupied).

Vokes (1967)

Nomenclaturally valid names listed under the heading “Philobryidae.” Adacnarca,
Cosa, Cosatova, Cratis, Denticosa, Hochstetteria, Hochstetterina, Limarca,
Limopsilla, Lissarcula, Micromytilus, Notomytilus, Philippiella, Philobrya,
Stempelleria.

Cox et al. (1969)

Valid genera. Philobrya, Aupouria, Cosa, Cratis, Limarca.

Synonyms of Philobrya. Philippiella, Stempellaria, Stempellia (also name pre-
viously occupied).

Subgenera of Philobrya. Philobrya, Adacnarca, Neocardia, Micromytilus, Hoch-
stetterina, Hochstetteria, Notomytilus.

Bergmans (1970)

Valid genera. Lissarca, Cratis.

Synonym of Lissarca: Austrosarepta;

of Cratis: Denticosa.

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY 9

2) Valid Genera

Genus ADACNARCA
AUTHOR. Pelseneer, 1903, p. 24.

TYPE SPECIES. (M.) Adacnarca nitens Pelseneer, 1903, p. 24.

Described here from Pelseneer’s (1903) original description and illustrations;
also from examination of several nontype individuals from the USNM
(Fig. 3).

Ligament pit short and triangular. Anterior series of denticles equal to 1/3 xX
length and 1X width of posterior series. No teeth. Outer surface with numer-
ous, fine, radial lines. Shape ovoid; slightly anteriorly reduced. Color white.
This species seldom exceeds a length of 5 mm or a width of 6 mm. Although
Pelseneer (1903) mentions that this species has an anterior adductor muscle,
no other worker (including myself) has noticed this feature in A. nitens.

JUNIOR SYNONYM. Hochstetterina Thiele, 1934, p. 796.

TYPE SPECIES. (O.D.) Hochstetteria crenella Vélain, 1877, p. 131.

The following material was examined: 1) the original descriptions and
illustrations of A. nitens and H. crenella and 2) several dozen of nontype
A. nitens from the USMN. This comparison shows that these two species
differ principally in shell shape and external ornamentation. A. nitens has
a prominent anterior, is highly inflated, and is radially lined externally.
H. crenella has a reduced but lobate anterior, is moderately inflated, and is
concentrically lined externally.

For the Pelecypoda, Stanley (1970) shows that shell shape and external
ornamentation are closely tied to life habits. Therefore, shell outline and
external ornamentation alone are here considered insufficient reasons for
separating Adacnarca, and Hochstetterina as distinct genera.

RANGE OF MORPHOLOGICAL VARIATION. Shell shape: submytiliform to ovate;
strongly inflated to flat. External ornamentation: radial only to concentric
only. Inner margins: prominently crenulated to smooth. Anterior denticle
series: 1/3 X—1 X length of posterior series.

DIAGNOSTIC FEATURES. Adacnarca is edentulous and has a small, triangular
ligamental pit.

POSTILLA 171

FIG. 3. Adacnarca nitens Pelseneer. USNM 613015; from Antarctica. A) Right
valve, interior. B) Right valve, exterior. Scale = 1 mm.

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY 11

FIG. 4. Aupouria parvula Powell. Paratype; BM; from Three Kings Island, New
Zealand. A) Right valve, interior. B) Right valve, exterior. Scale = 0.5 mm.

POSTILLA 171

Genus AUPOURIA
AUTHOR. Powell, 1937, p. 164.

TYPE SPECIES. (O.D.) Aupouria parvula Powell, 1937, p. 164.

Described here from paratypes collected off Three Kings Island, New
Zealand; located in the BM (Fig. 4).

Ligament pit shaped like an inverted U and located directly under umbo.
Anterior and posterior series of denticles about the same length and width.
Two prominent anterior teeth. Adductor muscle scars round, prominent
and about the same size. Inner margins smooth. External surface with faint
concentric markings. Prodissoconch prominently domed. Shape ovoid. Color
white. Length = 2.2 mm. Width = 2.1 mm.

JUNIOR SYNONYMS. No other generic names have been proposed for forms
resembling the type species.

RANGE OF MORPHOLOGICAL VARIATION. Too few available individuals to de-
termine.

DIAGNOSTIC FEATURES. Aupouria is the only philobryid that has anterior but
no posterior teeth.

Genus COSA
AUTHOR. Finlay, 1926, p. 449.

TYPE SPECIES. (O.D.) Hochstetteria costata Bernard, 1896a, p. 194.

Described here from syntypes, USNM 107763; collected from Stewart Island,
New Zealand (Fig. 5).

Ligament pit short and triangular. Anterior series of denticles equal to
1/3X length and 1X width of posterior series. No teeth. Inner margins
crenulated. Outer surface with about eleven raised radial ridges overlaid by
a concentric pattern. Prodissoconch with prominent raised rim. Color light
tannish. Mytiliform; greatly reduced anteriorly. Length = 2.5 mm. Width
45,07 Ty

JUNIOR SYNONYMS. No other generic names have been proposed for forms
resembling the type species.

RANGE OF MORPHOLOGICAL VARIATION. Shell shape: mytiliform to ovate.
External ornamentation: radial to cancellate to concentric. Inner margins:
crenulated to smooth. Color: white to tan.

DIAGNOSTIC FEATURES. Cosa is the only edentulous philobryid with all of the
following features: highly prominent denticles, a prominent prodissoconch
and a small, triangular ligament pit.

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY 13

FIG. 5. Cosa costata (Bernard). Syntype; USNM 107763; from Stewart Island,
New Zealand. A) Left valve, interior. B) Left valve, exterior. Scale = 1 mm.

Genus CRATIS
AUTHOR. Hedley, 1915, p. 698.
TYPE SPECIES. (O.D.) Cratis progressa Hedley, 1915, p. 698.

Described here from examination of syntypes; collected dead from 100 fms.
off Port Macquarie, New South Wales, Australia; AM c37760 (Fig. 6).

Ligament pit strongly triangular and located directly under umbo. An-
terior and posterior series of denticles about the same length and width.
Two to three perpendicular anterior teeth; three oblique to horizontal posterior
teeth. Inner margins crenulated; most strongly so posteroventrally. External
surface with prominent cancellate ornamentation; beaded. Prodissoconch
with a broad rim and prominent central boss. Shell shape submytiliform;
margins rounded. Color white. Length = 3.2 mm. Width = 4 mm.

JUNIOR SYNONYMS.
Denticosa Iredale, 1930, p. 385.

TYPE SPECIES. (O.D.) Philobrya cuboides Verco, 1907, p. 223.
Cosatova Iredale, 1939, p. 304.

TYPE SPECIES. (O.D.) Philobrya recapitula Hedley, 1906, p. 471.

POSTILLA 171

FIG. 6. Cratis progressa Hedley. Syntype; AM c37760; from 100 fms., off Port
Macquarie, New South Wales, Australia. 4) Right valve, interior. B) Right valve,
exterior. Scale = 1 mm.

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY 15

Iredale’s descriptions of Denticosa (1930) and Cosatova (1939) fail to show
how these supposedly distinct genera differ from Cratis. To discover these dif-
ferences, several individuals of all three type species were compared. The
examined holotype of P. cuboides is in the SAM; the examined syntypes of
P. recapitula are in AM c19390.*The principal differences among the type
species of Cratis, Cosatova, and Denticosa are in tooth number and the
shape of the anterior margin. P. cuboides has 1 to 2 anterior teeth and a
rounded anterior margin. P. recapitula has 1 anterior tooth and a straight
anterior margin. C. progressa has 2 to 3 anterior teeth and rounded anterior
margin. Both of these features are quite variable intraspecifically within the
Philobryidae. For example, some Philobrya magellanica (USNM 110735)
have a deeply concave anterior margin while others in the same lot have
a nearly straight anterior margin. Many other species in this family show
a similar but less pronounced variation in the shape of this feature. The
reason for this variation may be related to the epibyssate life habit of philo-
bryids. Since their byssus emerges anteriorly, the anterior margin of the
shell is in prolonged contact with the substratum. As in the byssate Mytilus
and Anomia, shell shape may conform to the shape of the substratum upon
which it rests. Tooth number can vary from valve to valve in these three
species. For example, the left valve of most C. progressa has three anterior
teeth, where the right valve has only two anterior teeth. In other dentate
philobryids, tooth number may vary intraspecifically by as many as four per
series (Nicol, 1966).

Since the principal differences among these type species are few and are
manifest in features that are quite variable intraspecifically in philobryids,
Cosatova and Denticosa are here considered generically indistinguishable
from Cratis.

RANGE OF MORPHOLOGICAL VARIATION. Shell shape: anterior round and
prominent to anterior straight and highly reduced. Dentition: 1-3 anterior
teeth; 1-3 posterior teeth. Prodissoconch: flat to knobbed to prominently
spiked. Denticles: anterior series 1X to 1/4 length of posterior series.

DIAGNOSTIC FEATURES. Cratis is the only philobryid that has all of the fol-
lowing features: a triangular ligament pit, anterior and posterior teeth, and
a beaded external surface.

Genus LIMARCA
AUTHOR. Tate, 1886, p. 135.
TYPE SPECIES. (M.) Limarca angustifrons Tate, 1886, p. 135.
Described here from examination of the holotype; from Eocene deposits
(borehole) near Adelaide, South Australia. SAM T1002 (Fig. 7).

Ligament pit straight and very long, extending posteriorly. Anterior series
of denticles equal to 1X width and 1/4X length of posterior series. Three

POSTILLA 171

f1G. 7. Limarca angustifrons Tate. Drawn from holotype, SAM T 1002; from
Adelaide. A) Right valve, interior. B) Left valve, exterior. Scale = 1 mm.

oblique anterior and four oblique posterior teeth. Inner margins crenulated.
Outer surface with prominent cancellate ornamentation; beaded. Shape
submytiliform. Prodissoconch with prominent central boss. Length = 5.5
mm. Width = 5.5 mm.

JUNIOR SYNONYMS. No other generic names have been proposed for forms
resembling the type species.

RANGE OF MORPHOLOGICAL VARIATION. Too few available individuals to de-
termine this.

DIAGNOSTIC FEATURES. Limarca is the only dentate philobryid with a long,
straight ligament pit.

Genus LIMOPSILLA
AUTHOR. Thiele, 1923, p. 289.
TYPE SPECIES. (O.D.) Limopsis pumilio Smith, 1904, p. 43.

Described here from syntypes collected from Cape Colony, South Africa;
located in the BM (Fig. 8).

Ligament pit large, triangular, and located directly under umbo. Anterior
and posterior series of denticles about the same length and width. Right
valve with three anterior and four posterior teeth. Left valve with three
anterior and five posterior teeth. Posterior adductor muscle scar slightly
larger than anterior adductor muscle scar; both scars prominent and ovoid.

PHILOBRYIDAE, LIMOPSIDAE ‘ONOMY, ECOLOGY 47,

FIG. 8. Limopsilla pumilio (Smith). Syntype; BM; from South Africa. A) Deft
valve, interior. B) Left valve, exterior. Scale = 0.5 mm.

POSTILLA 171

Inner surface with radial lines; inner margins smooth. Outer surface with
concentric ridges. Faint saclike depression on central part of dorsal margin.
Shape subtriangular; margins rounded. Color white. Length = 3.0 mm.
Width = 3.3 mm.

JUNIOR SYNONYMS. No other generic names have been proposed for forms
resembling the type species.

RANGE OF MORPHOLOGICAL VARIATION. Too few available individuals to de-
termine.

DIAGNOSTIC FEATURES. Limopsilla is the only philobryid with both a saclike
depression on the dorsal margin and a broad hinge area.

Genus LISSARCA
AUTHOR. Smith, 1877, p. 185.

TYPE SPECIES. (M.) Arca (Lissarca) rubrofusca Smith, 1877, p. 185.

Described here from syntypes collected from Kerguelen Island in the Indian
Ocean, and located in the BM (Fig. 9).

Ligament pit triangular and located near umbo. Anterior series of den-
ticles equal to 1X width and 3/4 length of posterior series. 4 anterior
teeth; 4 posterior teeth. Inner margins prominently crenulated. Outer sur-
face with fine concentric lines. Prodissoconch flat, smooth. Shape subellip-
tical. Color reddish. Length = 4 mm. Width = 2.5 mm.

JUNIOR SYNONYMS.
Austrosarepta Hedley, 1899, p. 430.

TYPE SPECIES. (M.) Austrosarepta picta Hedley 1899, p. 430.
Lissarcula Thiele, 1923, p. 290.

TYPE SPECIES. (M.) Lissarcula australis Thiele 1923, p. 290.
Notolimopsis Maxwell, 1969, p. 167.

TYPE SPECIES. (O.D.) Notolimopsis pulchra Maxwell 1969, p. 167.

After examining the syntypes of Lissarca rubrofusca, the syntypes of Aus-
trosarepta picta (AM _ c6378), Thiele’s (1923) description of Lissarcula
australis, and Maxwell’s (1969) description and illustrations of Notolimop-
sis pulchra, it is clear that the main difference between these forms is tooth
number. L. rubrofusca has 4 anterior and 4 posterior teeth; A. picta has 2
anterior and 3 posterior teeth; L. australis has 2 anterior and 2 posterior
teeth; N. pulchra has 4 to 6 teeth in each series.

Tooth number variation has already been discussed for some philobryids,
and at least one Lissarca species shows great variation in tooth number.
Examination of the many Lissarca notorcadensis in the USNM confirms

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY 19

FIG. 9. Lissarca rubrofusca Smith. Syntype; BM; from Kerguelen Island. 4) Right
valve, interior. B) Right valve, exterior. Scale = 1 mm.

comments made by Nicol (1966) concerning the same material. He says
that there may be as many as six teeth on either side (of the umbo) or as
few as two. The present writer has observed that the larger individuals of
this species usually have the most teeth. This large intraspecific variation
in tooth number in certain species of Lissarca argues against a separation of
Lissarca, Austrosarepta, Lissarcula, and Notolimopsis based on small differ-
ences in tooth number.

20 POSTILLA 171

RANGE OF MORPHOLOGICAL VARIATION. Shell shape: subrhomboidal to subel-
liptical. Tooth number: 2 to 6 anterior teeth; 2 to 6 posterior teeth. Outer
surface: smooth to prominent radial folds. Color: red to white.

DIAGNOSTIC FEATURES. Lissarca is the only philobryid with anterior and
posterior teeth, and is always longer than it is wide.

Genus NEOCARDIA
AUTHOR. Sowerby, 1892, p. 63.

TYPE SPECIES. (M.) Neocardia angulata Sowerby, 1892, p. 63.

Described here from paratypes collected from South Africa and located in
the BM (Fig. 10).

Ligament pit long and narrow, extending well posterior of the umbo.
Anterior series of denticles about 1x width and 1/2 length of posterior
series; both series tapering. 2 posterior teeth. Inner margins smooth.
Outer surface uniformly covered with concentric ridges; prominent radial
ridges posteriorly only. Prodissoconch with raised edges. Shape submytili-
form. Color white. Length = 4 mm. Width = 4.1 mm.

JUNIOR SYNONYMS. No other generic names have been proposed for forms
resembling the type species.

RANGE OF MORPHOLOGICAL VARIATION. External surface: smooth; with con-
centric lines only, varying to forms with cancellate ornamentation; with
prominent radial ridges.

DIAGNOSTIC FEATURES, Neocardia is the only philobryid with posterior teeth
only.

Genus PHILOBRYA
AUTHOR. Carpenter, 1872, Index p. 21.

TYPE SPECIES. (M.) Bryophila setosa Carpenter, 1864, p. 314.

Described here from syntypes, USNM 16187; collected from Cape St. Lucas,
Mexico (Fig. 11).

Ligament pit narrow, elongate, and extending well posterior of the umbo;
does not descend across the hinge. Anterior series of denticles equal to
1 1/2 width and 1/4X length of posterior series. No teeth. Inner margins
smooth. External surface with faint concentric lines. Prodissoconch slightly
domed. Shape mytiliform. Color tannish to white. Length = 2.5 mm.
Width = 3 mm.

JUNIOR SYNONYMS.
Hochstetteria Vélain, 1877, p. 129.

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY 21

FIG. 10. Neocardia angulata Sowerby. Paratype; BM South Africa. A) Right
valve, interior. B) Right valve, exterior. Scale = 1 mm.

POSTILLA 171

FIG. 11. Philobrya setosa (Carpenter). Syntype; USNM 16187; from Cape St.
Lucas, Baja California, Mexico. 4) Left valve, interior. B) Left valve, exterior.
Scalé = 0:5 im,

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY 23

TYPE SPECIES. (S.D.-Kobelt, 1881) Hochstetteria aviculoides Vélain, 1877,
p. 130.
Philippiella Pfeffer, 1886, p. 119.

TYPE SPECIES. (S.D.-Dell, 1964) Philippiella quadrata Pfeffer, 1886, p. 119.
Notomytilus Hedley, 1916, p. 20.

TYPE SPECIES. (O.D.) Philippiella rubra Hedley, 1940a, p. 207.
Stempelleria Clasing, 1918, p. 22.

TYPE SPECIES. (M.) Avicula magellancia Stempell, 1899, p. 230.
Micromytilus Cotton, 1931, p. 335.

TYPE SPECIES. (O.D.) Myrinia crenatulifera Tate, 1892, p. 131.

The original descriptions and illustrations of the five species listed above,
along with the syntypes of N. rubra. (AM c17723) and holotype of M. cre-
natulifera (SAM D13051) were compared to syntypes of P. setosa. The
principal difference among these forms is the length and shape of the liga-
ment pit. The pit of N. rubra descends sharply across the dorsal margin; the
pit of P. setosa parallels the dorsal margin; the ligament pit is relatively long
in P. setosa and relatively short in M. crenatulifera. The length and shape
of the pit in the other species falls between these extremes.

Figure 12 shows that, for philobryids, a continuum of ligament pit lengths
and curvatures exists between these extremes. Thus there are no natural dis-
continuities in these two aspects of this feature that could be used for sepa-
rating genera.

RANGE OF MORPHOLOGICAL VARIATION. Shell shape: mytiliform to ovoid; um-
bos hooked to straight. Anterior denticle series: 1/3xX-1/6X length of pos-
terior series to 1X-1. 1/2 width of posterior series. Inner margins:
smooth to crenulated. Ligament pit: straight to strongly curved; relatively
short to relatively long.

DIAGNOSTIC FEATURES. Philobrya is the only edentulous philobryid with a
nontriangular, somewhat elongate, ligament pit.

Revision of the Genera of the Limopsidae

1) Morphological Definition

A member of the Limopsidae is here considered to be any bivalve that
has the following features:

a) ligamental pit;

b) radially arrayed, alternating fibrillar and lamellar ligamental material;

c) small, mostly internal dorsal area that is smooth or that has oblique

striations.

The available generic names for this family and their dispositions by re-

cent writers are given in Table 2.

CURVED

INTERMEDIATE P rubra
P sublaevis

STRAIGHT, LONG
P sefosa

INTERMEDIATE SHORT
Pmodiolina P crenatulifera

Fic. 12. Ligament pit variations in Philobrya. (After Bernard, 1897; Hedley, 1904a.) Scale = 2 mm.

LLL VITILSOd

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY 25

TABLE 2. Recent dispositions of limopsid generic and subgeneric names.

Vokes (1967)

Nomenclaturally valid names listed under the heading “Limopsidae.” Aspalima,
Aupouria, Austrosarepta, Circlimopa, Cnisma, Cosmetopsis, Cyrenolimopsis,
Empleconia, Felicia, Glycilima, Limopsis, Limopsista, Lissarca, Loringella,
Lunopsis, Nipponolimopsis, Oblimopa, Ovalarca, Pectunculina, Phrynelima,
Senectidens, Vasconella, Versipella, Vetoarca.

Cox etal. (1969)

Valid genera. Empleconia, Limopsis, Hoferia, Lissarca, Nipponolimopsis, ?
Nucunella, Pichleria,? Vasconella.

Synonyms of Limopsis (Limopsis). Trigonocoelia (obj.), Lunopsis, Cnisma,
Limopsilla, ? Lissarcula, Loringella, Phrynelima, Glycilima.

Synonyms of Limopsis (Pectunculina). Cosmetopsis, Aspalima, Senectidens,
Versipella, Limopsita, Circlimopa, Oblimopa.

Synonyms of Hoferia. Bittnerella, Elegantarca, Arcoptera.

Synonym of Lissarca. Austrosarepta.

Synonym of Vasconella. Boussacia.

Subgenera of Limopsis. Pectunculina, Limopsis.

2) Valid Genera

Genus EMPLECONIA
AUTHOR. Dall, 1908, p. 393.

TYPE SPECIES. Limopsis vaginata Dall, 1891, p. 190.

Described here from examination of paratypes, USNM 122547, from the
Bering Sea (Fig. 13).

Ligament pit approximately one-half of dorsal area. About 11 anterior
and 7 posterior teeth. Posterior margin prominently infolded; forms declev-
ity between closed valves. Adductor muscle scars ovoid and prominent. Inner
margins smooth. External surface with numerous concentric lines. Shape
subelliptical; anteriorly reduced; margins rounded. Covered with a brown,
hairy periostracum. Length = 30 mm. Width = 28 mm.

JUNIOR SYNONYMS. No other generic names have been proposed for forms re-
sembling the type species.

RANGE OF MORPHOLOGICAL VARIATION. Tooth number: 5 to 12 anterior and
posterior teeth.

DIAGNOSTIC FEATURES. This is the only limopsid with a deeply infolded pos-
terior margin.

POSTILLA 171

FIG. 13. Empleconia vaginata (Dall). Paratype; USNM 122547; from 351 fms.,
Bering Sea. A) Right valve, interior. B) Right valve, exterior. Scale = 4 mm.

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY 27

Genus LIMOPSIS
AUTHOR. Sasso, 1827, p. 476.

TYPE SPECIES. (O.D.) Arca aurita Brocchi, 1814.

Described here from Brocchi’s (1814) original figures and nontype material,
USNM 62848. Brocchi’s material is from the Pliocene of Italy (Fig. 14).

Ligamental pit approximately 1/3 of dorsal area; dorsal smooth. Den-
tition consisting of about 15 small, transverse teeth arranged in an arcuate
series below dorsal area. Anterior adductor muscle scar about 3/4%X area of
posterior scar. Both scars ovoid and prominent. Inner margins smooth. Ex-
terior with numerous concentric lines. Shape ovoid; anteriorly reduced.
Length = 12.5 mm. Width = 13 mm.

JUNIOR SYNONYMS. There are about seventeen available generic or subgeneric
names based on species which resemble L. aurita. This proliferation of names
for Limopsis-like forms stems from the following two related causes: 1) no
one has outlined the range of morphologic variation in Limopsis and 2) few
authors proposing these names bothered to compare their prospective genus
with Limopsis aurita.

To see which, if any, of these Limopsis-like forms represent valid genera,
the type species for each name was examined and briefly described (Appen-
dix B). Inspection of these descriptions shows that all these forms possess
the same principal morphological features. Thus, there are no major morpho-
logical discontinuities that can be used to separate these forms as discrete
genera. Moreover, all these forms share the same principal features with
Limopsis aurita. These other names are therefore considered to be junior
subjective synonyms of Limopsis.

RANGE OF MORPHOLOGICAL VARIATION. Ligament pit: covers 1/6 to 3/4 of
dorsal area. Tooth number: 7 to 28. Shell shape: subelliptical to subcircular to
Subtriangular; equilateral to anteriorly reduced. Inner margins: smooth to
crenulated. External surface sculpture: cancellate to concentric to radial.

DIAGNOSTIC FEATURES. Limopsis is the only limopsid with a nonfolded pos-
terior margin.

Reassignments
The genera mentioned below often are associated with the Philobryidae or
Limopsidae (e.g. Cox et al., 1969; Vokes, 1967). These genera fail to satisfy
the previously given morphological definitions of either family and are here
reassigned.

Hoferia Bittner, 1894, p. 190 (Triassic).
Pichleria Bittner, 1894, p. 189 (Triassic).

POSTILLA 171

FIG. 14. Limopsis aurita (Brocchi). USNM 62848. A) Right valve, interior. B)
Right valve, exterior. Scale = 2 mm.

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY 29

These forms are placed in the parallelodontid subfamily Grammatodon-
tinae (Arcoida) because they have a broad, external dorsal area, moderately
large, oblique to parallel teeth, and are subquadrate. Pichleria has a feature
on the dorsal area that possibly represents a ligament pit, yet is grammato-
dont-like in all other shell features. Its similarities to the limopsids are dis-
cussed later, and it is likely that this is a transitional form between limopsids
and grammatodonts.

Vasconella Boussac, 1911, p. 42 (Eocene).
Deltaodon Barnard, 1962, p. 249 (Recent).

Because of its posteriorly elongate, external ligament, Deltaodon prob-
ably belongs to the Arcidae (Arcoida). Vasconella possibly lacks a liga-
mental pit and certainly lacks a limopsid-like dorsal area. Since it resembles
Deltaodon, it too is assigned to the Arcidae.

Vetoarca Stephenson, 1947, p. 165 (Cretaceous).
Nucunella d’Orbigny, 1850, p. 66 (Tertiary).
Ovalarca Woodring, 1925, p. 52 (Miocene).

Vetoarca apparently lacks a dorsal area. It closely resembles certain mem-
bers of the Noetiidae in shell shape and dentition, as does Ovalarca. Nucun-
ella is problematic but probably also belongs to the Noetiidae (Arcoida).

Verticipronus Hedley, 1904b, p. 88 (Recent).

The external ligament and absence of denticles exclude this form from
the Philobryidae. Dentition and shell shape are vaguely reminiscent of the
Cyamiidae (Veneroida).

3. EcoLocy

Materials and Methods

Life habits of philobryids and limopsids were observed in the field and also
Studied in the laboratory with the animals placed on their native substrata
in sea water regulated to the temperature at which they were collected. Par-
ticle transport in the mantle cavity was studied by removing one valve and
Corresponding mantle lobe.

Ecology of the Philobryidae

1) Habitat

Living specimens of Philobrya modiolus, P. crenatulifera, and Lissarca rhom-
boidalis were collected by the author at depths of 0 to 1 m in Pirates Bay
at Eaglehawk Neck, Tasmania, during March, 1973. Pirates Bay is a small,
Narrow bay with a broad entrance located on the east side of the Neck.

30 POSTILLA 171

There are no apparent restrictions to either water circulation within the bay
or to water exchange with the adjacent Tasman Sea. Moreover, no large
bodies of freshwater empty into the bay. Thus, water conditions within
Pirates Bay are probably normal marine.

P. modiolus was found on both a wave-swept sandstone platform in the
northern part of the bay and on a protected boulder-strewn beach in the
northeast corner of the bay. P. crenatulifera and L. rhomboidalis were
found only on the boulder-strewn beach. The philobryids are part of an
intertidal, rocky shore biotic assemblage that includes littorinids, patelloids,
siphonariids, thaids, mytilids, cirrepids, actiniarians, asteroids, and a variety
of algae. The intertidal zonation of Pirates Bay is described by Guiler
(1952).

2) Life Position and Locomotion

All three philobryid species live attached by a byssus to either brown or
red algae. The animals are usually attached to the lower one-fourth of the
plant and are oriented with the sagittal plane normal to the attachment sur-
face and the posterior margin directed upward (Fig. 15). The byssus em-
erges anteriorly and the anterior and anterodorsal margin are in contact
with the plant.

FIG. 15. Philobrya in life position.

In the laboratory, when detached and laid on either valve, all three species
performed the following movements:
1) Valves open slowly about 15 to 20°; posterior tips of gills are pro-
truded; water exchange between the mantle cavity and medium begins.
2) Foot is protruded anteriorly, then elongates anteriorly and posteriorly,
forming a prominent toe and heel respectively. Extended toe-heel length
approximately equals shell length.
3) Foot swings back and forth in sagittal plane; then descends downward
until heel or toe adheres to substratum.

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY 31

4) Foot pulls against bottom until shell rotates to a vertical position.
Through this movement, the sagittal plane becomes normal to the sub-
stratum and the shell comes to rest on the anterior margin.
5) Foot reextends, reattaches, and pulls shell along to the attachment site.
This movement is repeated several times.
The time interval for steps 1 to 5 may be as great as 20 minutes. After step
5 is completed, the animal may anchor itself by secreting several byssal
threads.

3) Feeding and Cleansing

The mantle margins of the three species are unfused and water carrying
food particles enters the mantle cavity both anteriorly and ventrally (Fig.
16). Particles accepted as food travel to the mouth by way of ciliary tracts
on the gills and palps (Fig. 17). There is one major oralward tract on the
gill. It is located subaxially and receives particles directly from the anterior
and ventral inhalant currents and forms numerous ascending, parallel tracts
that occur on the sides of filaments.

ens 4 \
sto at,

FIG. 16. Inhalant and exhalant areas. Left: Philobrya modiolus. Center: P. crena-
tulifera. Right: Lissarca rhomboidalis.

Particles rejected by the gill travel posteriorly along a tract located on
the ventral edge of each demibranch. This tract receives particles from de-
scending tracts and the palps, and it deposits these particles in a rotating,
mucus-bound ball of particles located near the posteroventral tip of the gill.
This ball is periodically expelled from the mantle cavity by rapidly closing
the valves. The gill also has at least three ways of cleansing itself of un-
wanted particles without the aid of cilia. It may slough off particles by con-
tracting violently, spread its filaments and allow particles to fall through, or
it may push particles away with its posteroventral tip.

POSTILLA 171

Fic. 17. Philobryid ciliary tracts. Lissarca is figured. a = visceral mass,
b = palps, c = foot, d = byssus, e = gill, f = posterior adductor muscle.
Axrows show directions of ciliary tracts. Scale = 1 mm.

The philobryid gill is remarkable because it is composed of short, stubby
filaments connected by a few strong interfilamentary junctures. Contrast-
ingly, the gills of other arcoids, including limopsids, are composed of long,
slender filaments interconnected by numerous, relatively weak ciliary junc-
tures. Philobryids are viviparous and the young are frequently seen resting
on the gills.

Particles rejected by the gills and palps are ultimately dumped onto the
mantle where they are carried posteroventrally by cilia and expelled by
rapid valve closure.

Ecology of the Limopsidae

1) Habitat
Five Limopsis loringi were dredged during 15 February to 3 March 1973, off
Malabar and Port Jackson Heads, New South Wales, at depths of 63-70 m
on a gravelly sand substratum. The water temperature and dissolved oxygen

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY 33

content on the bottom were 15.5°C and 6.4 mg/I to 6.8 mg/I, respectively.
Associated live molluscs include the pelecypods Placimen, Neotrigonia, and
Talabrica and the gastropods Chicoreus, Gazameda, and Conus.

Three L. soboles were dredged during 1 to 30 April 1973, between Mag-
netic Island and Keeper Reef, Australia (lee side of the Great Barrier Reef),
at depths of 42 to 48 m on a gravelly carbonate substratum. Associated live
molluscs include the pelecypods Amusium, Glycymeris, and Pratulum and
the gastropods Murex, Conus; and Colus. Both Limopsis species were always
found associated with a gravelly hash composed of broken sabellid (poly-
chaete) tubes. :

2) Life Position and Locomotion
In the laboratory, both species performed the following sequence of move-
ments:
1) Valves open 5 to 10°.
2) Foot emerges ventrally and slowly moves in the sagittal plane.
3) .Foot rotates downward and buries its ventral edge in the sediment.
4) Anteroventral part of the shell is pulled into the sediment by periodic
thrusting and retracting of foot.
5) Burrowing activity ceases when the shell is almost completely buried
in the sediment. In the attained life position, the shell lies downward with
the posterior margin barely protruding above the substratum (Fig. 18).
6) Three to four byssal threads are secreted from a duct in the anterior
part of the ventral cleft of the foot.

Members of this species are slow burrowers and usually take over 45
minutes to complete steps | to 6.

en
hn
Os

yy)

Whe :

oN J

FIG. 18. Limopsis life position. Arrows show inhalant and exhalant areas.
Scale = 5 mm.

34 POSTILLA 171

3) Feeding and Cleansing

Limopsis has discrete, posteriorly located inhalant and exhalant apertures
which are both formed by local pressing together of the unfused mantle
margins. As is the case with the philobryids, particles are drawn in with
water by the inhalant current and are sorted by the gills and palps (Fig. 19).

There are two major kinds of oralward tracts on the gill. One is located on
the dorsal outer surface of each demibranch and the other is located beneath
the gill axis. Particles reach these tracts directly from the inhalant current
and from numerous parallel, ascending tracts on each surface of the demi-
branchs.

FIG. 19. Limopsis ciliary tracts. a = gill, b5 = anterior adductor muscle,
c = foot, d = posterior adductor muscle, e = visceral mass, f = byssus.
Arrows show directions of ciliary tracts. Scale = 5 mm.

Particles accepted by the gill pass directly to the distal portion of the
palps, the inner surface of which contains approximately twelve ridges.
From here accepted particles travel to a proximal, smooth portion and,
from there, to the mouth. Almost all particles reaching the smooth portion
reach the mouth.

Particles rejected by the gills travel posteriorly down a tract which runs
along the ventral edge of each demibranch. This tract receives particles
mostly from a series of parallel, descending tracts. The ventral tract deposits
its particles near the posteroventral margin where they are periodically ex-
pelled by rapid valve closure.

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY 35

Particles rejected by the ridged portion of the palps often travel to the
sixth ridge (counting from the gill), change direction by 90°, then move to
the edge of the palp and drop off onto the mantle or foot. Most particles
falling onto the mantle are carried posteriorly and ventrally, and are even-
tually expelled. Most of the particles reach the mantle by way of the gill or
foot, but some come there directly from the inhalant current.

The posterior part of the foot has a. few weak ciliary tracts. These run
posteriorly, and dump particles onto the mantle. Most of the particles reach
the foot by way of the gills.

Functions and Implications

Philobryids have the following four features that are either uncommon or
absent in most bivalve groups:

1) a ventral, posteriorly directed, gill ciliary tract;

2) a gill with short, stubby filaments and few interfilamentary junctures;

3) an anterior inhalant area;

4) the ability for efficient epifaunal crawling.

The gill tract developed in philobryids and other arcoids (including limop-
sids) probably in response to a need for an additional mode of mantle
cavity cleansing.

Most pelecypods have a basic set of ciliary rejectory tracts that aid in
cleansing the mantle cavity. This set usually includes ventrally directed gill
tracts, laterally directed palp tracts and posteriorly directed foot and mantle
tracts. The tracts alone are probably not sufficient for rapid, thorough cleans-
ing because almost all pelecypods have developed one or more of the fol-
lowing additional means of cleansing the mantle cavity:

1) Partially sealed mantle cavity.

This serves as a cleansing function by closing off avenues of particle en-

trance to the mantle cavity. It is characteristic of many protobranchiate

and eulamellibranchiate bivalves.

2) Rapid flushing system.

a) Bivalves with an extensively fused mantle cavity can build up
relatively high water pressures within the cavity. When released, the
rapid flow helps clean out the mantle cavity.

b) Bivalves with an enlarged, centralized posterior adductor and a
localized, springy internal ligament can clap their valves together
rapidly and repeatedly. This action produces strong cleansing cur-
rents (Yonge, 1936) and is characteristic of many pterioids and other
monomyarian bivalves.

Elevated inhalant current or permanent inhalant aperture, or both.

a) Elevation helps keep the current away from unwanted particles
from the substratum. Elevation may be accomplished through sipho-
nal formation or living with the posterior margin well above the sub-
stratum (e.g., Atrina).

6)

wm

POSTILLA 171

b) A permanent inhalant aperture localizes the inhalant current and
diminishes the area through which unwanted particles could enter.
These apertures are formed by mantle fusion and are characteristic
of most eulamellibranchs and protobranchs.
4) Preintake particle selectors.
The palp probiscides of the protobranchs serve this function (Yonge,
1939). Marginal tentacles on the mantle and siphons do not necessarily
serve this function (personal observation).

Philobryids and other arcoids probably did not develop any of these addi-
tional cleansing features because they lack the necessary morphological ma-
tériel. For example, arcoids are probably incapable of developing cleansing
features that necessitate mantle fusion [1, 2a, 3a (siphons), 3b] because of
the generally assumed pumping inefficiency of the filibranch gill. To main-
tain water flow in a partially enclosed volume (= mantle cavity after fusion),
it is necessary to have an efficient pumping device.

Although some filibranchs are capable of forming a localized inhalant
area by pressing together parts of the mantle (Yonge, 1955), this is prob-
ably not very efficient in excluding unwanted particles because the apertures
formed are only temporary. This inefficiency may be inferred by noting that
in many filibranchs, water enters the mantle cavity in a variety of places
(Yonge, 1953). Feature 2b would be difficult to develop in arcoids because
they have an external, nonspringy ligament, and feature 4 is ruled out be-
cause arcoids are filter feeders, and proboscides are only useful to deposit
feeders. Feature 3a would be difficult to implement because many arcoids
have inhalant areas at opposite ends of the shell. Elevating one would lower
the other.

Thus, in order to develop an additional means of cleansing, it seems a
reasonable path left to arcoids was modifying existing ciliary mechan-
isms. One way of doing this would be to reverse the direction of the ventral
tract. In most bivalves (and presumably in the ancestors of the arcoids),
this tract runs anteriorly and deposits particles near the mouth. Reversing
this tract so it runs posteriorly would effectively add to the extant cleansing
mechanism because it would deposit unwanted particles near the postero-
ventral margin where they could easily be expelled through rapid valve
closure.

Reversing this tract was probably simply effected by the arcoids. Purchon
(1956) reports that the ventral gill tract in Anadara granosa runs anteriorly.
Lim (1966) reports in a later study of the same species that the tract runs
posteriorly, as expected. Assuming neither of these workers is in error in
observing this obvious feature, this means that some arcoids have the ability,
by either volition or mutation, to at least temporarily change the direction
of this tract. Since the tract is reversible it probably would have been rela-
tively easy for arcoids to change its average direction from anterior to pos-
terior, and thus provide themselves with an additional means of mantle
cavity cleansing.

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY 37

The structure of the philobryid gill can be interpreted as an adaptation
for strengthening the gill filaments and viviparity. In contrast to the philo-
bryid gill, the ordinary filibranch gill is comparatively fragile. For example,
sand-sized abrasive particles that become wedged between the demibranchs of
the ordinary gill are sometimes capable of breaking the ciliary junctures and
tearing off pieces of filaments (personal observation). By having shorter,
thicker filaments, the philobryid gill is less prone to tearing. Also, reducing
the number of junctures and reinforcing existing junctures makes the prob-
ability of ciliary rupture less likely. This tougher structure with fewer junc-
tures permits pushing and spreading movements which would certainly
damage the ordinary gill.

A strongly constructed gill would be advantageous to philobryids because
they are viviparous. There are often 10 to 25 young, brooded to a relatively
large size (sometimes 1/15 adult shell length), and each one has at least
two sharp corners that could possibly damage the ordinary filibranch gill.
In addition to cleansing, the separating and pushing movements that the
gill structure allows could also be used for getting the young off the gills
and out of the shell.

The presence of an anterior inhalant area in philobryids is remarkable be-
cause Yonge (1939) considers an anterior inhalant area to be a primitive
feature in pelecypods. Also, recent writers (e.g. Allen, 1968) have inter-
preted the presence of this feature in certain living pelecypods as indicating
both primitiveness and taxonomic affinity.

It is here contended that, while pelecypods may have passed through an
evolutionary grade where the inhalant current was anterior, the presence of
such a current in any particular pelecypod does not necessarily indicate
primitiveness or taxonomic affinity. The taxonomic distribution of this fea-
ture indicates this.

Members of the following groups are known to have an anterior inhalant
area: Nuculidae, Solemyidae (Yonge, 1939), Lucinacea (Allen, 1958), Ery-
cinacea (Popham, 1940), Crassatellacea, Carditidae (Allen, 1968), Arcidae,
Glycymeridae (Atkins, 1936), and Philobryidae. Also, Stasek (1963) men-
tions that this feature is present in the early postlarval stage of several
pelecypods. Because this feature is present in taxonomically remote groups
(e.g. Solemyidae and Crassatellacea), it is not necessarily an indicator of
taxonomic affinity. The absence of this feature in post-Paleozoic ancestral
groups and its presence in relatively recently evolved descendant groups (e.g.
limopsids-philobryids) suggests that it represents an adaptation developed
within the descendant group. Therefore, the feature is not necessarily an
indication of descent from some primitive ancestor and a pelecypod that
has it is not necessarily primitive.

Besides philobryids, only members of a few pelecypod groups such as the
Mytilidae, Limidae (Young, 1953), Anomiidae (Yonge, 1957), and Ery-
cinacea (Popham, 1940) crawl effectively epifaunally as adults. These
bivalves usually crawl either by anchoring the foot to the substratum and
drawing the shell up to the anchorage site by contracting the pedal muscles
or by pedal locomotory waves.

POSTILLA 177

Philobryids, anomiids, and erycinaceans are relatively rapid crawlers be-
cause the bond between the foot and substratum (often suction) can be
easily and rapidly broken after the shell draws up to the anchorage site.
Thus crawling is easy and members of these groups are very active and can
crawl for extended periods of time.

Crawling is more laborious for limids, mytilids, and other larger bivalves
because each step often requires making and breaking a byssal bond with the
substratum. Possibly for this reason, these forms are usually not active
crawlers and spend most their time byssally attached to the substratum.

The most active epifaunal crawlers are often small. Enigmonia aenig-
matica, the mobile anomiid, seldom gets longer or wider than 4 cm.
Erycinaceans and philobryids usually do not exceed 1.5 cm and are usually
much smaller. Small size is probably advantageous for this kind of crawling,
because as the animal gets larger the area available for anchorage (the surface
of the foot) increases by a power of two while shell volume increases by a
power of three (Thompson, 1942). Therefore, as the animal becomes larger,
the shell often becomes too bulky and heavy for the foot to effectively move
it epifaunally.

4, HIGHER TAXONOMY

Ordinal Affinities

While all recent workers place the Limopsidae in the Order Arcoida, the
ordinal affinities of the Philobryidae are still controversial. Although most
workers consider philobryids to be arcoids, Nicol (1966) asserts that they
are mytilaceans and supports this contention by pointing out that both
philobryids and mytilaceans have similar hinge ligaments, shell shapes, and
anterior adductor musculature.

However, this evidence is not conclusive. In the first place, the ligament
evidence is incorrect. All philobryids have an internal ligament, while most
mytilaceans have an external ligament. Secondly, the evidence regarding
shell shape and adductor musculature is ambiguous since many other non-
mytilacean groups (e.g. Ambonychiidae, Myalinidae, Pteriidae) also have
anteriorly reduced shells and reduced anterior adductors. In many cases,
anterior reduction of the shell has been interpreted as being an adaptation
for epifaunal stability that occurs in taxonomically diverse groups (Stanley,
1970). In addition, reduction of the anterior adductor often occurs along
with anterior reduction of the shell.

There are other striking differences between philobryids and mytilaceans.
For example, the inner shell layer in philobryids is crossed-lamellar while
it is nacreous in mytilids; the philobryid foot is broad and pointed at both

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY 39

ends, while the mytilacean foot is finger-shaped; the ventral-most gill ciliary
tract is posteriorly directed in philobryids and anteriorly directed in my-
tilaceans. Also unlike most mytilaceans, several philobryids have true hinge
teeth.

This writer feels that both limopsids and philobryids belong to the Order
Arcoida because members of both families possess the following combina-
tion of features which, it seems, are found together only in arcoids: 1) a
dentition with numerous anterior and posterior teeth; 2) a ligament con-
sisting of alternating sheets of fibrillar and lamellar material; 3) a broad foot
with a prominent ventral groove; and 4) filibranchiate gills with a posteriorly
directed ventral ciliary tract.

This list of features comes from examining scores of individuals from all
arcoid families except the Philobryidae and Limopsidae and listing com-
mon features not found in most other bivalves. Features 1 and 2 are or were
once present in all examined arcoids. Features 3 and 4 are present in all
examined living arcoids. All these features are present together in many mem-
bers of the Philobryidae and all members of the Limopsidae. No nonarcoid
bivalves possess more than two of these features.

Although these features set philobryids and limopsids apart from all non-
arcoid bivalves, two other nonarcoid groups, the limids and nucinellids, are
frequently confused with philobryids. The dentition and. adductor muscula-
ture of nucinellids are superficially similar to the same features in philo-
bryids. Nevertheless, the corresponding features occupy a posterior position
in philobryids and an anterior position in nucinellids. Moreover, Allen and
Sanders (1969) show that living nucinellids have protobranchiate gills.

Although small limids resemble philobryids in shell shape and some hinge
features, there are two obvious ways in which small limids may be distin-
guished from philobryids. First, there are seldom more than 10 denticles on
the limid hinge while there are seldom fewer than 30 on the philobryid
hinge. Secondly, the soft parts of limids are characterized by numerous, long
tentacles on the mantle margin.

There is no danger of confusing limopsids with any nonarcoid group.

Evolution

Various views of arcoid phylogeny are illustrated in Figure 20. How do
philobryids and limopsids fit into the picture?

1) Limopsid Origins

Three Jurassic species are the oldest limopsids known to the writer. The
oldest of these is Limopsis minimus (Sowerby) [= L. oolithica (Buvignier);
= Pectunculus minimus Buvignier], which is reported from the Middle Jur-
assic (Bathonian) of England and southern Europe (Sowerby, 1825; Giirich,
1934). According to Sowerby’s (1825) original description and figures, this
species has 4 to 5 anterior and posterior teeth, an anteroposteriorly elongate

POSTILLA 171

[irvvreiert
A
—E7 | PHILOBRYIDAE
NOETIIDAE Eoc.—Rec
Crét “Kec. ‘ :
A
Generally Assumed
GLY CYMERIDAE

Ls Cret..~Rec.

A if
Nicol, 1950
hn ae of THIS PAPER

ARCIDAE : 54 eo
Jur.-Rec. Maer ee Aza THIS PAPER
J Nee
CUCULLAEIDAE
Jur.-Rec.
Stanley, 1972
Stanley, 1972
LIMOPSIDAE
M. Jur.-Rec.
—- THIS PAPER
NS = ei a P
PARALLELODONTIDAE

Dev.-Cret.

f

Newell, 1954

CYRTODONTIDAE
M,/Ord.=Dev.

Fig 2 Summary of arcoid Phylogeny

Addendum

Tevesz, M.J.S. 1977. Taxonomy and ecology of the Philobryidae and Limopsidae

(Mollusca: Pelycypoda), Postilla Number 171, Peabody Museum of Natural
History, Yale University, New Haven, CT.

Page 40. Add the legend: Fig. 20. Summary of arcoid phylogeny.

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY 41

shell, and a smooth outer surface. Although Sowerby neither mentions nor
figures a ligament pit, Arkell (1929-36), after examining specimens and
comparing them to Recent limopsids, says that it has a ligament pit and
unequivocally considers it a true limopsid.

The other two species are from the Upper Jurassic. One is Limopsis cor-
allensis (Buvignier), which occurs in England and France (Arkell, 1929-
36). It has an anteroposteriorly elongate shell, ligament pit, about 14 teeth,
a concentrically lined outer surface, and a dorsal area with oblique stria-
tions. The other is Limopsis sp. Pickering, which, according to Arkell
(1929-1936), has an anteriorly reduced, dorsoventrally elongate shell and
is known only from the Oxfordian of England. Arkell thinks it is possible
that the individuals he previously called L. corallensis may belong to this
species.

The origins of the Limopsidae can probably be traced to the parallelodon-
tid subfamily Grammatodontinae (Triassic-Cretaceous; -Eocene? -Recent?)
because early limopsids look more like certain grammatodonts than any
other pre-Middle Jurassic arcoids.

Grammatodonts generally have a rounded, subquadrate shell that is slight-
ly longer than wide and their dentition consists of about 4 or 5 shorter,
oblique anterior teeth and about 4 or 5 longer, oblique posterior teeth. The
dorsal area is often grooved and the adductor muscle scars are about the
same size.

By comparison, two of the earliest limopsids, L. minimus and L. coral-
lensis, resemble grammatodonts because they have anteroposteriorly elon-
gate shells with rounded margins. In addition, L. corallensis has nearly
equal sized muscle scars and a striated dorsal area. Moreover, there are
two Triassic grammatodonts, Hoferia and Pichleria, which especially re-
semble early limopsids. The dentition of these grammatodonts, like the
limopsid dentition, consists of small, equally spaced teeth. Also, Hoferia
like early limopsids, has external concentric markings, is ovate, and is
known from Europe (Bittner, 1894). These similarities between limopsids
and grammatodonts are shown in Figure 21.

Stanley (1970) shows that modern free-burrowing arcids have a shell
length/height ratio of less than 1.35, while modern epibyssate forms have
the ratio greater than 1.35. Applying the same statistic to grammatodonts,
it is possible to infer that while many were probably epifaunal, some, in-
cluding Hoferia and Pichleria, like Limopsis, were probably infaunal.

No other pre-Middle Jurassic arcoids share nearly as many similarities
with the earliest limopsids, or even limopsids in general, as do these gramma-
todonts.

The main morphological difference between grammatodonts and limop-
sids is that all limopsids have a ligament pit and most have a smooth dorsal
area, whereas most grammotodonts lack a ligament pit and have oblique
grooves on the dorsal area. This difference chiefly means the grammatodont
ligament covered more of the dorsal area than the limopsid ligament. This

POSTILLA 171

FIG. 21. Limopsis and limopsid-like grammatodonts. 4) Hoferia; B) Limopsis;
C) Pichleria.

point may be illustrated by comparing the ligaments of both groups. The
limopsid ligament consists of a radially arrayed pattern of fibrillar and
lamellar material (Newell, 1938; personal observation) and it can be inferred
that the composition and arrangement of ligamental material in gram-
matodonts was similar, because the dorsal area of most well-preserved gram-
matodonts is marked with oblique grooves. This pattern, in modern arcoids,
reflects the composition and arrangement of ligamental material. The grooves
mark the insertion sites of lamellar material while the spaces mark the loca-
tion of fibrillar material. Oblique grooves are produced on the dorsal area
when this material is radially arrayed. Since the pattern is etched into the
shell, these marks are usually preserved long after the ligament disintegrates.
In limopsids, the ligament is usually entirely confined to the pit and is often
composed of comparatively few sheets of material. Judging from the
chevrons, the grammatodont ligament covered the entire dorsal area.

2) Philobryid Origins

Three lines of evidence suggest the ancestry of the Philobryidae may be
traced to the Limopsidae. The first is the overall morphological similarity
between members of the two families. This similarity can be most con-

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY 43

L

FIG. 22. Philobryid and limopsid similar in overall shell features. 4) Cratis pro-
gressa; B) Limopsis antillensis. Scale = 1 mm.

44 POSTILLA 171

vincingly illustrated by comparing Cratis progressa (Recent) with Limopsis
antillensis (Recent). Both these species have a dentition consisting of 2
to 4 anterior teeth and 3 to 6 posterior teeth, a similar anteriorly reduced
outline, and a cancellate external surface (Fig. 22). Moreover, Limarca
(Eocene), one of the first known philobryids, also shows these similarities
with Limopsis antillensis and other philobryids such as Lissarca and Limop-
silla resemble limopsids in general. Like limopsids, these genera have a
dentition consisting of small, transverse anterior and posterior teeth and all
have an anteriorly reduced shell. Moreover, adult limopsids and philobryids
have a ligament pit which contains radially arrayed ligamental material.
No other arcoids show as many similarities to philobryids as do limopsids.

The principal differences between limopsids and philobryids in no way
discredit arguments, based on overall similarity, that suggest their ancestor-
descendant relationship. The principal differences in external features are the
presence of denticles in philobryids and the presence of a dorsal area in
limopsids.

No other adult arcoids have denticles; moreover, only two adult arcoid
features even remotely resemble denticles. These are the transverse striations
on the dorsal area of certain noetiids and arcoid teeth in general. These
striations in noetiids are sites of ligament insertion, and, unlike denticles, do
not interlock. Ligamental insertion in philobryids takes place only within
the ligament pit and not on the surface where denticles are located. More-
over, ligamental insertion in philobryids does not produce transverse stria-
tions.

Teeth are the only other arcoid feature with which denticles could pos-
sibly be homologous. However, this relationship is unlikely, because arcoid
teeth are located below the ligament, while denticles are located lateral to the
ligament. Secondly, denticles are located well above and separate from the
teeth in philobryids.

Where do denticles come from? Bernard’s (1896b) illustrations of young
arcoids show that many possess denticles. These denticles apparently
serve as articulating aids prior to the development of teeth. Also, similar
denticlelike features are present in the early growth stages of several non-
arcoid pelecypods (Cox et al., 1969). This strongly suggests that philobryid
denticles represent juvenile features retained by the adult (neotenous fea-
tures). Since early limopsid growth stages have denticles, their presence in
adult philobryids does not obstruct the proposed phylogenetic relationship
of these two families.

Philobryid denticles are interlocking and hence serve an articulating func-
tion. Since they are located above the teeth, they still interlock when the
valves open widely (15 to 20°) and the teeth no longer interlock. In life,
philobryids often open the shell to this angle before crawling commences.
One reason for this wide opening may be to allow the foot freedom of move-
ment before and during locomotory activity. Thus, denticles can serve to
articulate the shell when the teeth cannot, and the presence of denticles may

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY 45

have allowed some philobryids to lose their regular dentition without suffer-
ing a loss of articulating ability.

The second principal difference in shell features, the presence of a dorsal
area in limopsids and its absence in philobryids, may be more apparent
than real. The surface containing denticles possibly represents a dorsal
area, since it is a discrete surface located above the dentition and adjacent
to the ligament pit. Unlike the dorsal area of most limopsids, it is parallel to
the sagittal plane and very small. Nevertheless, some limopsids, including
L. nipponica, also have a small dorsal area that is nearly parallel to the
sagittal plane.

The major differences in the soft parts of philobryids and limopsids are the
sturdy gill and highly reduced or absent anterior adductor muscle of the
former. The philobryid-like gill is unique within the Arcoida and therefore
represents a feature evolved entirely within the Philobryidae. As such, the
philobryid gill does not suggest relationships with nonlimopsid arcoids. As
pointed out earlier, a reduced or absent anterior adductor muscle character-
izes a wide variety of bivalves. It often occurs along with anterior reduction
of the shell, which can be an adaptation for epifaunal stability (Stanley,
1970).

Thus, this first line of evidence shows that there are several close simi-
larities between philobryids and limopsids and also shows that their prin-
cipal differences do not necessarily indicate other origins for philobryids.
Furthermore, it suggests that limopsids have all the morphological matériel
necessary to produce the philobryid form.

The second line of evidence comes from the spatial and temporal rela-
tionships of the two families. Limopsids, which first appear in the Jurassic,
well predate philobryids, which first appear in the Eocene. Moreover, limop-
sids occur in Eocene and pre-Eocene sediments in the Australia-New Zea-
land region, where philobryids first appear (Flemming, 1966). This sug-
gests that limopsids had the potential in space and time to give rise to the
philobryids.

The third line of evidence follows from analogous morphological-life habit
transitions in other groups. The types of changes necessary to produce a
philobryid-like form from a limopsid-like form have occurred previously
within several other pelecypod orders. These changes produce an anteriorly
flattened, alobate form from an anteriorly rounded, lobate form. Such
changes also accompany a transition from an endobyssate to epibyssate
life habit.

These types of changes and their significance were first brought to light
and discussed by Stanley (1972). He feels that such morphological and life
habit changes have occurred in groups such as the Pterioida, Modiomor-
phoida, and early Heterodonta. For evidence, he points out that some of the
earliest known members of these groups have some of the following fea-
tures: a reduced, lobate anterior, an elongate shell, a broad byssal sinus, and
the absence of appreciable ventral flattening. He also shows, through recent

46 POSTILLA 171

analogies, that these features occur in endobyssate pelecypods. Then, he
mentions that some of the forms evolved later in these groups have a
flattened, alobate anterior. Based on recent analogies and considerations of
physical stability, he suggests that these features are adaptive to an epibys-
sate life habit. This suggests that many epibyssate stocks arose from endo-
byssate stocks.

A limopsid-philobryid transition would parallel this widespread, well-
documented type of morphological and life habit transition. Limopsids have
some of the features characteristic of the endobyssate grade of evolution.
These are a reduced but lobate anterior, the absence of ventral flattening,
and, in some cases, a broad byssal sinus. Moreover, limopsids are here ob-
served to be endobyssate. On the other hand, many philobryids show fea-
tures characteristic of the epibyssate grade of evolution, such as a flattened,
alobate anterior. Also, philobryids are here observed to be epibyssate.

Thus, this third line of evidence shows that the major morphological and
life habit changes that would occur in evolving a philobryid from a limop-
sid have occurred several times in other pelecypod lines. This evidence sug-
gests that a limopsid-philobryid transition would not be unusual, and
furthermore, would be feasible within the limits of their respective mor-
phologies and ways of life.

In short, philobryids look more like limopsids than any other arcoids.
Moreover, the morphological-life habit transition that would be required in
evolving a philobryid from a limopsid has several precedents. For these rea-
sons, limopsids are here considered the most probable ancestors of philo-
bryids.

3) Mechanisms
The importance of neoteny in pelecypod evolution has been recently stressed.
Yonge (1962) suggests the byssus in adult pelecypods is a neotenous fea-
ture and Stasek (1963) and Ockelman (1964) suggest neotenous origins for,
respectively, erycinaceans and the venerid Turtonia. Stanley (1972) sum-
marizes recent bivalve literature concerning neoteny and invokes neoteny as
a mechanism in major life habit transitions in the Pelecypoda.

Certain morphological features of philobryids and limopsids suggest
that neoteny was a factor in the evolution of both these families. Besides
denticles, Bernard (1896b) shows that the juveniles of several arcoid genera
(e.g. Arca, Glycymeris, Cucullaea) have a ligament pit that later disappears
in the adult (Fig. 23). Many erycinaceans and Turtonia are smal! (2 mm-
10 mm) and byssate. Philobryids have all these features characteristic of
juvenile and neotenous pelecypods. Limopsids have a byssus and ligament
pit. Moreover, the earliest known limopsid, Limopsis minimus, is a small
species (less than 10 mm long).

Since adult philobryids and limopsids have features of juvenile arcoids
and also share features with presumably neotenous groups, it is conceivable
that neoteny was a mechanism in the evolution of both these families.

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY 47

le

Juvenile G/ycymeris Juvenile Arca

Adult Cratis
FIG. 23. Arcoid denticles. (After Bernard, 1896b; Hedley, 1915.)

4) Other Limopsid Offshoots

Two pieces of evidence suggest limopsids may be considered as _ possible
ancestors of Glycymeridae (Lower Cretaceous-Recent). The first is the truly
striking similarities between limopsids and glycymerids, which are especially
evident in their dentitions and shell shapes. Members of both families
characteristically have numerous transverse, evenly spaced, arcuately ar-
rayed hinge teeth and ovoid, approximately equilateral shells (Fig. 24). The
shape and arrangement of the soft parts are very similar in Limopsis and
Glycymeris (Pelseneer, 1911), and their mantle, foot, and gill ciliary tracts
are almost identical (Atkins, 1936; personal observation).

The main difference is that the limopsid dorsal area is marked by a pit,
whereas the dorsal area in glycymerids is marked by chevron grooves. The
preceding discussion about the significance of this difference, applied to
grammatodonts, also applies here. Also, a ligament pit is present in juvenile
glycymerids (Bernard, 1896b).

The second piece of evidence for this relationship is Limopsis .corallensis,
a morphological intermediate between limopsids and glycymerids. Buvig-
nier’s (1842) figures of this species show that, like most glycymerids, L. cor-
allensis is nearly equilateral, anteroposteriorly elongate, and has two nearly
equal-sized adductor muscle scars. In addition, L. corallensis, like the first
undoubted glycymerid, Glycymeris marulensis (Leymerie), is known only
from Europe (Arkell, 1929-1937; Nicol, 1950). Although Cossmann (1924)
reports two Jurassic species of Limopsis (minimus and oblongus) that he
refers to the genus Pectunculus (now called Glycymeris), this writer thinks
both these forms are Limopsis minimus. Although they seem to lack a liga-
ment pit, this is probably a result of their poor preservation.

POSTILLA 171

FIG. 24, Limopsid and glycymerid similar in overall shell features. 4) Limopsis
sp.; B) Glycymeris pilosus.

Most importantly, however, the dorsal area of L. corallensis is apparently
marked both by a ligament pit and oblique grooves. Arkell (1929-1937)
also notes the presence of these grooves in L. corallensis when he mentions
that the cardinal area (= dorsal area) is “finely striated.” Thus, the hinge
of L. corallensis combines the characteristics of both limopsids and glycy-
merids. This is important because it suggests that not all limopsids lost the
oblique grooves of their grammatodont ancestors and hence did not have to
redevelop them in giving rise to glycymerids.

These ideas about limopsid-glycymerid relationships are contrary to those
of Nicol (1950), who has made the only extensive attempt so far to trace
glycymerid ancestry. Nicol believes that glycymerids arose from cucullaeids.
For his evidence, Nicol shows how the dentition, dorsal area, ligament, shell
shape, umbonal position, ornamentation, and adductor musculature of cer-
tain glycymerids resembles corresponding features in certain cucullaeids.

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY 49

He then relates that glycymerids are first known from the Lower Cretaceous
of France, and that there are late Jurassic and Lower Cretaceous cucullaeids
from roughly the same area. Because of this evidence, he “. . . is certain that
the ancestor of the Glycymeridae was a species belonging to the genus
Idonearca of the family Cucullaeidae.”

This evidence is not convincing because limopsids share most of these
morphological, spatial, and temporal similarities with glycymerids. Nicol’s
conclusions can therefore be challenged by showing several similarities be-
tween glycymerids and limopsids that are not common to glycymerids and
cucullaeids. This can be done first by using Nicol’s own words. On page 95,
he outlines the morphological changes necessary to derive a glycymerid from
a cucullaeid, using for his reference points Idonearca and Glycymerita.
He feels the following changes are among those necessary to derive the
latter from the former:

- 1. The outline changed from quadrate or subrhomboidal to sub-
quadrate with an arched dorsal margin and a more symmetrically
rounded outline. The posterior truncation and constriction became less
pronounced, and the convexity became less in Glycymerita than in the
ancestral cucullaeid.

2. With the rounding of the dorsal border the ligamental area de-
creased in length in Glycymerita, and the hinge plate became more
arched and weaker.

3. The central teeth changed little, although they appear to have be-
come shorter in Glycymerita. The most pronounced change occurred
in the modification of the side teeth. The ancestral cucullaeid had long
side teeth arranged almost one above the other, all of them parallel or
nearly parallel to the dorsal margin. Usually the distal side teeth of
Idonearca are slightly divergent ventrally. The side teeth in Glycymerita
became much shortened and arranged in a converging arc instead of
being arranged one above the other. This shortening of the side teeth
became necessary because the hinge plate became arched and weaker.

These very points show similarities which are common to glycymerids
and limopsids, but which are not common to glycymerids and cucullaeids,
because none of these changes is necessary if glycymerids arose from limop-
sids.

However, the best way of showing similarities which are common to
glycymerids and limopsids, but which are not common to glycymerids and
cucullaeids, is to compare the limopsid and cucullaeid forms that are thought
to be ancestral to the glycymerids. Using these antecedent forms minimizes
the possibility that the comparisons are biased by convergence.

‘L. corallensis, the writer’s choice, an Idonearca, Nicol’s choice, and a
Cretaceous glycymerid are illustrated in Figure 25. Nicol mentions that
members of a group of species such as J. gabrielis, I. forbesi, and I. moreana

POSTILLA 171

FIG. 25. Mesozoic limopsids, cucullaeids, and glycymerids—a comparison. A)
Idonearca forbesi; B) Glycymeris sublaevis; C) Limopsis corallensis.

could have given rise to the glycymerids. 7. forbesi is illustrated here. It
might be argued that this comparison is unfair because Nicol states that it
is the dentition of young rather than adult cucullaeids that resembles the
dentition of glycymerids. However, he offers no evidence to show that this
is true for Idonearca.

Whatever the case, Figure 25 shows that the glycymerid generally looks
more like Limopsis corallensis than Idonearca forbesi, especially in denti-
tion and shell shape.

Much more work needs to be done to firmly establish the ancestry of
the glycymerids, and the possibility of the group’s having arisen from
cucullaeids can by no means be ruled out. Rather, the point being made
here is that, based on what is known from the available literature, a
reasonable case may be made for limopsids as possible glycymerid
ancestors,

ACKNOWLEDGMENTS

The writer thanks the following people for contributing to this work.
Professors Willard D. Hartman, Donald C. Rhoads, and Karl M. Waage
of Yale University critically reviewed the manuscript. Drs. Joseph Rose-

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY 51

water and Thomas Waller of the U.S. National Museum of Natural History
and Dr. John Pojeta of the U.S. Geological Survey provided access to
mollusc material and contributed suggestions.

Drs. R. J. MacIntyre and L. Lehoczky of the University of New South
Wales, Kensington, Australia, and Dr. Winston Ponder of the Australian
Museum, Sydney, provided invaluable help and hospitality that led to suc-
cessfully obtaining live material for study. Dr. Ponder also provided access
to museum material and compilations from his “little book.”

The field work for this project was funded by National Science Founda-
tion Grant GB-35482 and Sigma Xi, the Scientific Research Society of
North America. Cleveland State University contributed additional support
towards publication.

This work was submitted in March, 1974, as a dissertation for the
degree of Doctor of Philosophy at Yale University. Professor A. Lee
McAlester, formerly of Yale University*, provided excellent supervision
of this: dissertation.

*Current address: Southern Methodist University.

POSTILLA 171

APPENDIX A
Geological and Geographical Ranges of Valid Genera
Fleming, 1966; Cox et al., 1969)

Geological Range Geographical Range
Philobryidae

Adacnarca Recent Antarctica

Aupouria Pliocene-Recent New Zealand

Cosa Pliocene-Recent Australia; New Zealand;
Caribbean; Western North
America

Cratis Miocene-Recent Australia; New Zealand;
Caribbean

Limarca Eocene South Australia

Limopsilla Recent South Africa

Lissarca Miocene-Recent Antarctica; Australia; New
Zealand

Neocardia Recent South Africa

Philobrya Miocene-Recent Western North America;
Central America; Antarcti-
ca; Southern South Ameri-
ca; South Pacific; South
Atlantic; Australia; New
Zealand; Mediterranean;
Indian Ocean

Limopsidae
Limopsis Middle Jurassic-Recent Cosmopolitan

Empleconia Recent North Pacific; Bering Sea

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY 53

APPENDIX B
Descriptions of junior subjective synonyms

I. Philobryidae
AUSTROSAREPTA Hedley, 1899, p. 430
TYPE SPECIES. (M.) Austrosarepta picta Hedley, 1899, p. 430.

Described here from examination of syntypes, AM c6378; collected from
Middle Harbor, Sydney; also nontype material, USNM 162148 (Fig. 26).

FIG. 26. Lissarca picta (Hedley). USNM 162148; from Sydney Harbor. Right
valve, interior. Scale = 0.5 mm.

Ligament pit short and triangular. Anterior and posterior series of den-
ticles about the same length and width. Two anterior and three posterior
teeth. Margins mostly smooth; sharp folds descending obliquely from the
umbo to the posterior margin. External surface concentrically lined. Color
reddish brown; mottled. Shape subelliptical. Length = 2.5 mm. Width =
2 mm.

COSATOVA Iredale, 1939, p. 304
TYPE SPECIES. (O.D.) Philobrya recapitula Hedley, 1906, p. 471.

Described here from examination of syntypes, AM c19390; collected from
17 to 20 fms., Masthead Island, Queensland, Australia.

54 POSTILLA 171

Ligament pit short and triangular. Anterior series of denticles about 1/2x
length and 1X width of posterior series. One anterior and three posterior
teeth. External surface with cancellate ornamentation; beaded. Portions of
the inner margins are crenulated. Prodissoconch with a central spike. Shape
mytiliform; anterior margin straight. Color white. Length = 2 mm. Width
== ep soninnls

DENTICOSA Iredale, 1930, p. 385
TYPE SPECIES. (O.D.) Philobrya cuboides Verco, 1907, p. 223.

Described here from examination of holotype from the SAM; collected
from the Backstairs Passage, Australia (20 fms.).

Ligament pit short and triangular. Anterior and posterior series of den-
ticles about the same length and width. 2 anterior and 3 posterior teeth.
Inner margins crenulated. External surface with cancellate ornamentation;
beaded; anterior reduced; shape submytiliform. Length = 2.2 mm. Width
SP) fesanany,

HOCHSTETTERIA Vélain, 1877, p. 129
TYPE SPECIES. (S.D., Kobelt, 1881) Hochstetteria aviculoides Vélain, 1877,
Did,

Described here from Vélain (1877). This species was first identified from
specimens collected from Ile Saint-Paul, Indian Ocean.

Ligament pit large and triangular. Anterior denticle series 1/3X length
and 1X width of posterior series. No teeth. Shape mytiliform. Inner mar-
gins crenulated. Length = 2 mm. Width = 3 mm.

HOCHSTETTERINA Thiele, 1934, p. 769.
TYPE SPECIES. (M.) Hochstetteria crenella Vélain, 1877, p. 131.

Described here from Vélain (1877). This species was first reported from
Ile Saint-Paul and Amsterdam, both in the Indian Ocean.

Ligament pit triangular. Anterior series of denticles 1/2X length and 1x
width of posterior series. No teeth. Inner margins crenulated. External sur-
face with concentric lines. Shape modioliform. Length = 1 mm. Width
(Wi natnay,

LISSARCULA Thiele, 1923, p. 290
TYPE SPECIES. (O.D.) Lissarcula australia Thiele, 1923, p. 290.

Described here from Thiele (1923). First identified from the Torres Strait
and Dirk Hartog Island, Australia.
Two anterior and two posterior teeth. Predominantly concentrically orna-

mented. Surface of inner margins uneven. Length = 2 mm. Width =
2.2m,

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY 55

MICROMYTILUS Cotton, 1931, p. 335
TYPE SPECIES. (O.D.) Myrina crenatulifera Tate, 1892, p. 131.

Described here from examination of holotype, SAM D31051; collected from
MacDonnell Bay, Australia, in shell sand.

Ligament pit rather short, not triangular; descends across hinge. Anterior
denticles 1/3 length and 1X width of posterior series. Both series taper-
ing. Posterior inner margins strongly crenulated. External surface with con-
centric markings. Color brownish red. Submytiliform. Length = 2 mm.
Width = 2 mm.

NOTOLIMOPSIS Maxwell, 1969, p. 167
TYPE SPECIES. (O.D.) Notolimopsis pulchra Maxwell, p. 167.

Described here from Maxwell (1969); holotype collected from the Lower
Miocene of New Zealand.

Ligament pit short and triangular. Anterior and posterior series of den-
ticles about the same length and width. Four to six anterior and posterior
teeth. Inner margins smooth. External ornamentation predominantly con-
centric. Length = 3.5 mm. Width = 3.1 mm.

NOTOMYTILUS Hedley, 1916, p. 20
TYPE SPECIES. (O.D.) Philippiella rubra Hedley, 1904a, p. 207.

Described here from examination of syntypes, AM c17723; collected from
Eaglehawk Neck, Tasmania, Australia, in shell sand; also nontype material,
USNM 348938 (Fig. 27).

FIG. 27. Philobrya rubra (Hedley). USNM 348938; from Victoria, Australia. Left
valve, interior. Scale = 1 mm.

POSTILLA 171

Ligament pit elongate posteriorly, sharply descending across hinge. An-
terior denticles 1/2 length and 1X width of posterior series; both series
tapering. No teeth. Inner margins prominently crenulated posteriorly. Ex-
ternal surface concentrically lined. Color reddish. Shape mytiliform; anterior
margin concave. Length = 2.9 mm. Width = 3.8 mm.

PHILIPPIELLA Pfeffer, 1886, p. 119
TYPE SPECIES. (S.D.; Dell, 1964) Philippiella quadrata Pfeffer, 1886, p. 119.

Described here from Pfeffer (1886) and Dell (1964). First identified from
South Georgia, South Atlantic Ocean.

Ligament pit elongate posteriorly. No teeth. Two adductor muscle scars.
Mytiliform. Prodissoconch apparently punctate.

STEMPELLERIA Clasing, 1918, p. 22
TYPE SPECIES. (M.) Avicula magellanica Stempell, 1899, p. 230.

Described here from Stempell (1899) and nontype material, USNM 110735;
first identified from southern South America (Fig. 28).

Ligament pit straight, extending posteriorly. Anterior denticle series 1/3 xX
length and 1X width of posterior series. Inner margins smooth. External
surface with radial and concentric markings; markings rather faint. No
teeth. Shape submytiliform. Periostracum. extending about 2 mm over edge
of shell. Length = 9 mm. Width = 9 mm.

II. Limopsidae
The forms described in this section, like Limopsis aurita, generally have the
following principal features:
1) a triangular ligament pit situated in a dorsal area.
2) two adductor muscle scars, usually with the anterior scar smaller than
the posterior scar.
3) numerous small, transverse teeth arrayed in an arcuate pattern under
the hinge.
4) an anteriorly reduced shell with rounded margins.
Additional morphological information is provided here for each name.

ASPALIMA Iredale, 1929, p. 160
TYPE SPECIES. (O.D.) Limopsis erectus Hedley and Petterd, 1906, p. 224.

Described here from examination of syntype, AM c32046; collected off
Sydney.

11 teeth, all in one series. Ligament pit comprises 1/4 of dorsal area. Shell
outline ovate. External ornamentation concentric.

CIRCLIMOPA Iredale, 1939, p. 243
TYPE SPECIES. (O.D.) Circlimopa woodwardi mutanda Iredale, 1939, p. 243.

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY 57

FIG, 28. Philobrya magellanica (Stempell). USNM 110735; from the Straits of
Magellan. A) Left valve, interior. B) Left valve, exterior. Scale = 1 mm.

58 POSTILLA 171

Described here from examination of syntypes, AM c60262; collected from
Low Isles, Queensland, Australia.

Approximately 24 teeth in a continuous series. Ligament pit comprises
about 1/5 of dorsal area. Ornamentation predominantly radial. Shape sub-
circular.

CNISMA Mayer, 1868, p. 27
TYPE SPECIES. (M.) nuculatus Lamarck, 1805, p. 217.

Described here from Lamarck (1805) and Deshayes (1824).

6 posterior and 4 anterior teeth. Ligament pit comprises about 1/5 of dor-
sal area. External surface with cancellate ornamentation; apparently beaded.
Shell greatly reduced anteriorly.

COSMETOPSIS Rovereto, 1898, p. 162
TYPE SPECIES. retifera Semper.

Described here from Rovereto (1898).
Concentric external ornamentation. Shape ovoid.

CYRENOLIMOPSIS, Habe 1953, p. 207
TYPE SPECIES. (O.D.) adamsiana Yokoyama, 1920, p. 175.

Described here from Habe (1953).
Shape subtriangular. Ornamentation concentric.

FELICIA Mabille and Rochebrune, 1889, p. 115
TYPE SPECIES. (O.D.) Felicia jousseaumi Mabille and Rochebrune, 1889,
joy IU),

Described here from Mabille and Rochebrune (1889) and from nontype
material from the USNM. (Fig. 29).

Nicol (1966) and other writers consider F. jousseaumi to be a junior sub-
jective synonym of Limopsis marionensis, and the present writer agrees. Al-
though Mabille and Rochebrune (1889) do not mention a ligament pit, it is
likely that the absence of this feature in their material is due to poor pres-
ervation. L. marionensis is the largest known living limopsis and this writer
has seen one individual of this species over 60 mm long. This species usually
has about 10 to 25 teeth, a ligament pit that extends over about 2/3 of the
dorsal area, and is subelliptical in shape.

GLYCILIMA Iredale, 1931, p. 204
TYPE SPECIES. (O.D.) Glycilima paradoxa Iredale, 1931, p. 204.

Described here from examination of lectotype, AM c62324, collected from
100 fms., 16 mi. east of Wollongong, New South Wales, Australia.

9 teeth in a single series. Ligament pit comprises most of dorsal area.
Ornamentation predominantly concentric.

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY 59

FIG. 29. Limopsis marionensis Smith. USNM 653091; from Antarctica. Right
valve, exterior. Scale = 10 mm.

LIMOPSISTA Finlay and Marwick, 1937, p. 24
TYPE SPECIES. (O.D.) Limopsis (Limopsista) microps Finlay and Marwick,
1937, p. 24.

Described here from Finlay and Marwick (1937). Holotype located in the
New Zealand Geological Survey Collection; from the Danian of. New Zea-
land.

Approximately 10 anterior and 10 posterior teeth. Ligament pit comprises
about 1/3 of dorsal area. Shape ovoid. Ornamenation concentric.

LORINGELLA Iredale, 1929, p. 160
TYPE SPECIES. (O.D.) Limopsis loringi Angas, 1873.

Described here from Angas (1873) and from AM c11329.

Approximately 17 teeth. Ligament pit about 1/3 of dorsal area. External
ornamentation concentric. Small individuals subcircular. Large individuals
always wider than long.

POSTILLA 171

LUNOPSIS d’Orbigny, 1850, p. 243
TYPE SPECIES. (O.D.) hoeninghausii Miller, 1847.

Described here from Miiller (1847).
Concentric external ornamentation, with radial elements as well. Ovate.

NIPPONOLIMOPSIS Habe, 1951, p. 45
TYPE SPECIES. (O.D.) Limopsis nipponica Yokoyama.

Described here from Habe (1951).

4 posterior lateral teeth and 3 perpendicular anterior teeth. Ligament pit
comprises about 1/4 of dorsal area. External surface with a cancellate pat-
tern; apparently beaded. Shell greatly reduced anteriorly.

OBLIMOPA Iredale, 1939, p. 242
TYPE SPECIES. (O.D.) Oblimopa macgillivrayi actaviva Iredale, 1939, p. 242.

Described here from examination of syntype, AM c60261; from the Low
Isles, Queensland, Australia.

Approximately 8 anterior and 12 posterior teeth. Ligament pit comprises
about 1/2 of dorsal area. Shell shape ovate.

PECTUNCULINA d’Orbigny, 1844, p. 182
TYPE SPECIES. (O.D.) Pectunculus scalaris Sowerby, 1825, p. 113.

Described here from Sowerby (1825).

27 teeth. Ligament pit comprises about 1/3 of dorsal area. Shell wider
than long, with rounded margins, and covered externally with about 25
prominent radial ridges.

PHRYNELIMA Iredale, 1929, p. 160
TYPE. SPECIES. (O.D.) Limopsis brazieri Angas, 1871, p. 21.

Described here from Angas (1871).
Shell shape ovate. External ornamentation concentric.

SENECTIDENS Iredale, 1931, p. 204
TYPE SPECIES, (O.D.) Senectidens dannevigi Iredale, 1931, p. 204.

Described here from examination of lectotype, AM E4832; collected from 30
fms. off Gabo Island, New South Wales, Australia.

Approximately 21 teeth, all in one series; teeth very faint under ligament
pit. Two anterior teeth are parallel rather than transverse. Ligament pit com-
prises about 1/2 of dorsal area. External ornamentation concentric.

VERSIPELLA Iredale, 1931, p. 203
TYPE SPECIES. (O.D.) Versipella soboles Iredale, 1931, p. 203.

PHILOBRYIDAE, LIMOPSIDAE: TAXONOMY, ECOLOGY 61

Described here from examination of syntypes, AM c24366; collected from
east of Sydney Heads, New South Wales, Australia.

Approximately 20 teeth, all in a single series. Ligament pit comprises
about 1/3 of dorsal area. Concentric external ornamentation. Shell outline
subelliptical.

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