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Shell microstructures of Cambrianmolluscs replicated by phosphateBruce Runnegar aa Department of Geology & Geophysics , University of NewEngland , Armidale, N.S.W., 2351, AustraliaPublished online: 27 Nov 2008.

To cite this article: Bruce Runnegar (1985) Shell microstructures of Cambrian molluscs replicatedby phosphate, Alcheringa: An Australasian Journal of Palaeontology, 9:4, 245-257, DOI:10.1080/03115518508618971

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Shell microstructures of replicated by phosphate BRUCE RUNNEGAR

Cambrian molluscs

RUNNEGAR, B., 1985:08:26. Shell microstructures of Cambrian molluscs replicated by phosphate. Alcheringa 9, 245-257. ISSN 0311-5518. The original microstructure of the aragonitic and calcite shells of Cambrian molluscs

is frequently visible at high magnifications on the surfaces of phosphatic internal moulds. More rarely, the nature of the original microstructure may be determined by examining the internal surfaces of phosphate layers which coated the shells, or by examining the phosphate fillings of the tunnels made by endolithic algae within the shells. Because the original aragonitic microstructures of molluscan shells are almost invariably destroyed by recrystallisation in rocks older than the Carboniferous, the discovery of replicated shell microstructures has provided a new understanding of the evolution of the molluscan shell. Most of the common molluscan microstructures - - spherulitic prismatic aragonite, tangentially arranged fibrous aragonite, crossed-lamellar aragonite, nacre and foliated calcite - - had probably appeared by at least the beginning of the Middle Cambrian, and most are found in representatives of the Class Monoplacophora. A simple biochemical model is used to explain the convergent evolution of nacreous and foliated structures from prismatic ones.

Bruce Runnegar, Department of Geology & Geophysics, University of New England, Armidale N.S.W. 235l, Australia; received 30 July 1984.

BECAUSE aragonite is thermodynamically unstable at low pressures (Brown et al., 1962), the original microstructure of aragonitic skeletons is rarely preserved in rocks older than the Mesozoic (Hall & Kennedy, 1967). Dry aragonite may survive almost indefinitely (Brown et al., 1962) and has done so in several bituminous shales of Carboniferous age (Stehli, 1956; Hallam & O'Hara, 1962; Hall & Kennedy, 1967; Yochelson et al., 1967; Brand, 1981), where organic matter in the sediment seems to have protected the aragonite from the catalytic action of underground water (Kennedy & Hall, 1967). Such deposits are unknown from the Early Palaeozoic, and as a result, only one or two examples of Early Palaeozoic molluscs with preserved microstructure have been reported (Erben et al., 1968a; Mutvei, 1983). However, it has recently become clear that secondary apatite fillings and coats may replicate the shell microstructure of Cambrian molluscs (Runnegar & Jell, 1976, 1980; MacKinnon, 1982; Runnegar & Bentley, 1983; Runnegar, 1983). I now report the occurrence of replicated aragonitic shell prisms, foliated

0311/5518/85/020245-13 $3.00 © AAP

calcite, nacre, crossed-lamellar aragonite and fibrous aragonite in 14 genera of Early and Middle Cambrian molluscs from Australia, New Zealand, France, China and the United States.

Specimens were etched from limestones with dilute acetic acid, picked by hand, and examined with a JEOL JSM 35 scanning electron microscope after being coated with gold. The replicated microstructure is most commonly visible on internal moulds of the shells at magnifications of xl00-x4000. Negative prints of the SEM negatives or normal prints from electronically inverted 'negatives' show the microstructure as it would have appeared on the inner surfaces of the shells (Figs 3A, B, D, E; 5E).

The difference between a positive image of a replicated surface and an electronically inverted 'negative' image is shown in Fig. 3B- C. The height of the steps between adjacent folia in these images is < 100 nm. Thus the apatite, at some sites, has achieved the casting quality of a transmission electron microscope replica (for example, compare Fig. 3A with TEM replicas illustrated by Taylor e t al., (1969)). It is expected that the study of phosphate replicas will prove to be a powerful

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tool for the analysis of the skeletal biominerals of other extinct animal groups, and that this kind of preservation will be found to be widespread in Cambrian and post-Cambrian rocks.

Most of the shells studied in thin section were totally recrystallised and their original microstructure had been destroyed. However, some specimens from the Parara Limestone of South Australia appear to show traces of the original microstructure within the recrystallised carbonate (Fig. 2). A third source of information is provided by phosphatic fillings of tunnels made within the shells by boring endolithic algae (Fig. 7; Runnegar, 1985). The algae have preferentially exposed the crystallites of the shell as they constructed their tunnels, and the crystallites were replicated by apatite which filled the tunnels (Fig. 7C, D, G). This kind of preservation is important because it provides information about shell layers which display few topographic features on internal moulds. Finally, there appears to have been some replacement of the original microstructures by phosphate (Fig. 5C, F). This mode of preservation is uncommon.

The best preserved phosphatic replicas so far discovered are from the early Middle Cambrian Currant Bush Limestone of western Queensland (Runnegar & Jell, 1976) and the Parara Limstone, Yorke Peninsula, South Australia (Runnegar & Bentley, 1983). However, replicated microstructures have been observed on specimens from almost every site sampled, including those which have been moderately deformed (e.g., St Genies de Varensal, via Lamalon les Bains, France (Runnegar, 1983) and the Tasman Formation of New Zealand (MacKinnon, 1982). The oldest specimens with replicated microstructure are from the uppermost

(Meischucun) part of the Dengying Dolomite, western Hubei, China (Yfi, 1979) and the Tommotian of the Siberian Platform (Rozanov et al., 1969). Both horizons are approximately coeval and are of earliest Cambrian age.

Prismatic microstructures The proximal ends of polygonal shell prisms about 10-20 #m in diameter (Fig. 1E) are replicated on phosphatic internal moulds of many taxa. The replicated prisms either cover all or much of the shell interiors (e.g., Heraultipegma varensalense (Cobbold); Runnegar, 1983, fig. 9) or are confined to particular parts of the moulds. In Mellopegma georginensis Runnegar & Jell, Anabarella n. sp. and Yuwenia bentleyi (Fig. 7F; Runnegar, 1983, fig. 4), the replicated prisms are restricted to the apertural regions of the shells; in Latouchella? n. sp. (Fig. 1A-E) the ends of the prisms are visible on ridges on the moulds, but the intervening comarginal grooves are smooth and featureless at high magnifications (Fig. 1D). It can be seen from thin sections (Fig. 1 B-C) that the grooves and ridges are not reflected on the shell exterior. Thus the prismatic layer was confined to the outer part of the shell and the whole shell was apparently strengthened by periodic comarginal thickenings of another microstructural type. This interpretation is confirmed by the photomicrographs of thin sections shown in Fig. 2A-B. The outer layer prisms do not continue through the recrystallised zones of thickening which are presumed to have been composed of nacreous aragonite.

Prismatic structures have been observed on internal moulds of the monoplacophorans Obtusoconus rostriptutea (Qian) and Bemella? mirabilis Yii from the Dengying Dolomite (YiJ,

Fig. L Prismatic microstructures. A, phosphatic internal mould of Latouchella? n. sp., Early Cambrian, UNEL1761, base of Parara Limestone, Horse Gully, Ardrossan, Yorke Peninsula, South Australia, x42. B-C, unoriented thin- sections of the walls of two specimens of Latouchella? n. sp., UNEL1761, showing the corrugated internal surface and smooth outer surface of the shell; the dark layer on the exterior of the shell in C is a phosphate coat. D-E, enlargements of A (x 160, x 500); note that the replicas of the ends of the prisms are confined to the ridges on the internal mould. F-G, posterodorsal and anterodorsal edges of the adductor muscle scar of Pseudomyona queenslandica showing the smooth surface of the adductor scar, the replicated myostracal prisms at the edges of the scars, and the edges of the replicated calcite folia (Fig. 3A) away from the scars. F, Australian Museum (AM), Sydney, AMF61831, x450. G, Australian National University, Canberra, ANU29072, x 550. Both specimens from the Middle Cambrian Currant Bush Limestone, western Queensland.

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Fig. 2. Thin-sections (cross-polarised light) from the base of the Param Limestone, UNELI761, Horse Gully, Ardrossan, Yorke Peninsula, South Australia showing possible relict prismatic structures in the recrystallised shells of Latouchella? n. sp. (A-B, compare Fig. IA-E) and Pojetaia runnegari (C-D). The arrows in A point to the edge of a comarginal thickening; the arrows in C and D indicate identical positions in the sections; A is an enlargement of B and D is an enlargement of C.

1979), Latouchella sp. from the Tommotian of Siberia (Rozanov et. al., 1969), Yochelcionella cyrano Runnegar & Pojeta and Mellopegma georginensis from the Currant Bush Limestone (Runnegar & Jell, 1976), and Anabarella n. sp. and Latouchella? n. sp. from the Parara Limestone (Fig. 1; Runnegar, 1983). In Anabarella, Mellopegma, and the riberiid rostroconch Heraultipegma varensalense (Runnegar, 1983), another shell layer overlaps the prismatic layer away from the shell margins. In Anabarella and Mellopegma this layer was nacreous but in Heraultipegma it appears to have consisted of inclined prisms (Runnegar, 1983). In the gastropod Yuwenia bentleyi the outer prismatic layer (Fig. 7F) was overlapped by an inner layer of crossed- lamellar aragonite.

The ends of the replicated shell prisms bear a close resemblance to the spherulitic aragonite prisms of the outer shell layer of living monoplacophorans (McLean, 1979) and bivalves such as Neotrigonia (Taylor et aL,

1969). They are therefore assumed to have been aragonitic and spherulitic but no microstructural or chemical evidence has yet been found to support this hypothesis.

The Early Cambrian bivalve Pojetaia runnegari (Jell) appears to have had a shell which was formed of a single layer of inclined spherulitic aragonite prisms (Runnegar & Bentley, 1983). Electron microprobe analyses of the magnesium content of the recrystallised shell support the conclusion that the shells were aragonitic rather than calcitic, as does the presence of aragonitic microstructures in the presumed descendants of Pojetaia (Runnegar & Bentley, 1983).

One specimen of Pojetaia from the Parara Limestone appears to have the outlines of the inclined prisms preserved in the recrystallised shell (Fig. 2C-D).

A third kind of prismatic structure is found near and within the adductor muscle insertion area of the bivalved monoplacophoran Pseudomyona queenslandica (Runnegar &

Fig. 3. Layered microstructures. A, electronically inverted image of the replicated foliated calcite of Psuedomyona queenslandica; AMF61831, x 2700. B-C, replicated foliated calcite of P queenslandica showing the effect of electronic inversion of the image (b) compared with a normal image of the same area, AMF61832, x 2700. D-E, electronically inverted images of replicated nacre tablets on the internal mould of Mellopegrna georginensis; AMF61822, Currant Bush Limestone. D x 5000, E x 3000. F, surface of the nacreous layer of a specimen of a living species of the bivalve Pandora for comparison with D and E; note the well developed screw dislocational growth of the nacre, x 700.

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Fig. 4. Histogram of measurements of the apparent interfacial angle at the growing tips of calcite folia of Pseudomyona queenslandica. The mean value is indicated by ×.

Jell) (Fig. IF-G; Runnegar, 1983). By analogy with the myostracal layers of living molluscs (Taylor et al., 1969), this layer is considered to have been composed of short spherulitic prisms of aragonite. It appears to be the only aragonitic structure within the otherwise calcitic shell (Runnegar, 1983).

Layered microstructures Replicated tablets of nacre have been observed on in te rna l moulds o f Mellopegma georginensis (Fig. 3D, E), Anabarella n. sp. (Runnegar, 1983) and an undescribed riberiid r o s t roc onc h f rom the Cur ran t Bush Limestone. In each case the nacreous layer covers the bulk of the interior of the shell. It is likely that many more Cambrian molluscs had nacreous interiors; the small number of convincing occurrences of replicated nacre probably results from the fact that exceptional preservation is required to make the outlines of the nacre tablets visible on phosphatic steinkerns.

The calcitic analogue of nacre, foliated calcite, is well preserved on internal moulds of the Middle Cambrian taxa Pseudomyona queenslandica, Tuarangia paparua (MacKinnon) and Eotebenna pontifex Runnegar & Jell (MacKinnon , 1982;

Runnegar, 1983). This material is considered to have been calcitic rather than aragonitic for the following reasons. First, the replicas show that the microstructure was composed of imbricated thin layers which, unlike nacre, grew in one direction only over localised areas. Second, SEM surveys showed no evidence either of screw-dislocational growth which is characteristic of sheet nacre, or the ends of crystal stacks which are characteristic of the other common form of nacre. And third, the mean value of 168 measurements of the interfacial angles at the growing tips of the folia (Fig. 3A) is 107.8 o (Fig. 4). This is larger than the expected value of 102 ° for type 1 foliated calcite (Runnegar, 1984), but it lies far from the values expected in nacre ([llO]//ilO ] 64 °/100]//010} 90 ° [ll0]//li0] 116 °, [110]/[010] 122°; Fig. 4; Wada, 1972; Mutvei, 1977). The larger-than-anticipated mean value probably results from viewing the angles at directions less than 90 o to the shell surface. For example, the 8 o tilt required to make a stereopair of this structure resulted in observed difference of 3 ° in the interfacial angle.

Fibrous microstructures The shells of at least three species of the asymmetric monoplacophoran Pelagiella had shells composed of tangential layers of fibrous

Fig. 5. A, phosphatic internal mould and partially replaced shell of Pelagiella subangulata, Early Cambrian, Parara Limestone, Curramulka, Yorke Peninsula, South Australia, x43. B-C, enlargements of A to show the fibrous nature of the outer shell layer (partially obscured by a thin phosphate coat); note the growth lines (white arrow) and the spiral ornament (black arrow). B, x 160, C x 1400. D, part of phosphatic internal mould of P subangulata showing the radial orientation of the fibres of the inner shell layer, same loc. as A, x 50. E, electronically inverted image of the spirally arranged crystals of the inner shell layer of Pelagiella deltoides, Middle Cambrian, Currant Bush Limestone; the shell aperture is in the direction of the bottom of the photograph, x 3300. F, two views of bundles of fibres in the phosphatised outer layers of shells of P subangulata from the Parara Limestone, Ardrossan; the bundles of fibres occur on the periphery of the whorl and are perpendicular to the growth lines, x800 (left) and x700 (right).

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Fig. 6. Four views at different magnifications of a single hyolith cone from the Middle Cambrian Currant Bush Limestone, western Queensland. The irregular transverse ridges are interpreted as the boundaries of first-order crossed- lamellae, and the fine striations between the ridges (D) are considered to be replicas of the third-order lamellae, A ×130, B x200, C x1300, D x3700.

crystals which are presumed to have been c o m p o s e d o f a r agon i t e . In Pelagiella subangulata (Tate) from the Parara Limestone (Fig. 5A-D, F), the inner shell layer was composed of fibres or bundles of fibres (Fig. 5F) which were al igned in a radial direct ion with respect to the protoconch (Fig. 5D). By contrast, the outer layer of the same species had the fibres or bundles of fibres arranged more or less perpendicular to the growth lines (Fig. 5A-B). The spiral o r n a m e n t of P. subangulata crosses the fibres of the outer layer obl iquely (Fig. 5B).

In Pelagiella deltoides Runnegar & Jell from the Cur ran t Bush Limestone, the inner shell layer was composed of flattened, elongated crystallites ar ranged in a spiral fashion (Fig. 7E). The na ture of the outer layer of this species in unknown . A third species of Pelagiella from the Middle C a m b r i a n of the

United States is preserved as phosphatic coats of the outer surface of the shell. The inner surfaces of these coats (not illustrated) are almost identical to the fibres of the outer layer of P. subangulata (Fig. 5B-C). In P subangulata the fibres of the outer shell layer appear to have been replaced by phosphate (Fig. 5C, F).

The ul trastructures seen in Pelagiella are reminiscent of crossed-lamellar aragonite (MacClintock, 1967; Taylor et al., 1969), but the fibres are not steeply inclined to the inner sur face of the shell a nd there is no organisa t ion of bundles of fibres into first- order lamellae.

Crossed- lamel la r s tructures which are presumed to have been aragonit ic have been described in Ordovician and Permian hyoliths (Runnegar et al., 1975). In the Permian shell, the first-order lamellae were aligned parallel

Fig. 7. A, phosphatic internal mould of the Early Cambrian gastropod Yuwenia bentleyi, AMF61819, x 35. B, another specimen in the same orientation as A showing obscure comarginal ridges (arrows), considered to mark the outcrop of first-order crossed-lamellae, and adhering phosphate casts of tunnels made by endolithic algae, × 190. C, enlargement of specimen B to show that the ridges on the internal mould are reflected in the walls of the algal borings (arrows show the trend of the ridges), x900. E, sub-circular algal borings on an internal mould of Y. bentteyi showing that the direction (large arrow) of the striations on the borings (small arrow) is independent of the direction of the tunnel, x330. F, the obscure pitted structure on the right side of this specimen of Y. bentleyi is interpreted as replicas of the ends of shell prisms of the outer shell layer (arrow marks position of edge of shell aperture), x300. G and D, enlargements of the algal borings shown in E; replicated fibres (third-order crossed-lamellae) are orientated in two directions in G (arrows); the photographs are taken perpendicular to the trend of the large arrow in E. G x 1800, D × 2500. All specimens are from the base of the Parara Limestone, UNLI761, Horse Gully, Ardrossan, Yorke Peninsula, South Australia.

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to the cone aperture in the inner shell layer and perpendicular to it in the outer shell layer.

Phosphatic internal moulds of hyolith cones from the Currant Bush Limestone have irregular comarginal ridges and grooves which are interpreted as the boundaries of the first- order lamellae (Fig. 6A-C). Fine striations which occur between the ridges are considered to be the replicas of the third-order lamellae (the fundamental fibres of crossed-lamellar structure).

A possibly similar series of obscure comarginal ridges and grooves is visible on an internal mould of the Early Cambrian gastropod Yuwenia bentleyi (Fig. 7B). These structures are replaced by a pitted structure near the shell aperture; this is thought to represent the ends of the prisms of the outer shell layer (Fig. 7F). Algal borings, now filled with phosphate, show that the fibres of the inner shell layer are inclined in two directions (Fig. 7G) as is typical of crossed-lamellar aragonite. It is therefore concluded that the shell of Yuwenia was composed of an outer aragonitic prismatic layer and an inner layer of low-angle crossed-lamellar aragonite.

Evolution of the molluscan shell micostructures When molluscs began to form continuous calcareous exoskeletons in the latest Precambrian or earliest Cambrian, they used the orthorhombic polymorph of calcium carbonate, aragonite, as the construction material (Carter, 1980; Runnegar & Bentley, 1983). This may have been because the carbonate was deposited extracellularly in an environment likely to be contaminated by sea water (Digby, 1968). It would therefore have been simpler to manufacture aragonite which is readily precipitated from sea water (Towe & Malone, 1970; Folk, 1974) because of its lower standard free energy of formation (Bathurst, 1971) than to use calcite which may require the use of ATP to exclude Mg from the lattice (Duckworth, 1976).

The first shell-bearing molluscs were undoubtedly very small (c. 1 ram; Runnegar, 1983), and their shells seem to have consisted of a single layer of spherulitic aragonitic

prisms beneath the organic periostracum. This kind of microstructure survives in the outer shell layer of living primitive molluscs such as Neopilina and Vema (McLean, 1979), in the myostracal layers of all molluscs (Taylor et al., 1969), and in the ligament of all bivalves (Runnegar, 1983). It is also found as the only or outer shell layer in primitive Early Cambrian molluscs (Fig. 1), in the Early Cambrian bivalve Pojetaia (Runnegar & Bentley, 1983), and in the earliest Cambrian rostroconch Heraultipegma (Runnegar, 1983).

Spherulitic aragonitic prisms are composed of radiating aggregates of fibrous crystals of aragonite having a diameter of 102-103 nm (Wise & Hay, 1968; Towe & Thompson, 1972). The component fibres are best seen in bivalve ligaments where they are more widely dispersed by intercrystalline matrix (Runnegar, 1983). They are normally pseudohexagonal in cross-section, may be twinned on Ill0/, and are elongated parallel to the c axis of aragonite. Their primary growth surface is therefore the basal pinacoid [001/.

It is almost certain that there is no substrate control on the formation of spherulitic prisms other than that imposed by the orientation and shape of the surface on which they begin to form. Such prisms are moulded by surface forces rather than by chemical bonds and they are found in both inorganic and unstructured biologic deposits (Bryan & Hill, 1941; Taylor, 1973; Runnegar, 1983). They begin as hemispherical radial aggregates (Flajs, 1977, pl. 1, fig. 1; Watabe, 1974, fig. 12; Wendt, 1975, fig. 2) which are nucleated at random on the substrate, and they grow competitively to achieve a uniform size by obeying the rules of soap bubble geometry (Almgren & Taylor, 1976).

The same rules allow calcite to form polygonal prisms, but these are rarely composite structures in mollusc shells (Taylor et al., 1969), because the normal habit of calcite results in crystals which tend to be prismatic or equidimensional rather than fibrous (Smallwood, 1977). Thus the difference between the composite spherulitic prisms of aragonite and the simple polygonal prisms of calcite is a consequence of the minera logy alone. Both of these microstructures may be regarded as primitive,

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and neither needs to be specified by a protein substrate (Taylor, 1973). They would begin to accrete automatically on the inner surface of the per ios t racum providing that the extrapallial fluid (the liquid lying between mantle and shell) contained sufficient calcium and carbonate ions.

Nacreous linings in prismatic shells had appeared by at least the middle Early Cambr ian (Runnegar , 1983). Because spherulitic prismatic aragonite plus nacre is found in the shells of primitive monoplacophorans, primitive bivalves and gastropods, and cephalopods (Erben et al., 1968a, b; Taylor et al., 1969; Erben & Krampitz, 1973), this pair of microstructures is generally considered to be the most primitive combination to be found in mollusc skeletons (but see Batten, 1982).

Spherulitic prisms of aragonite present the /001/ faces of innumerable fibres to the secreting surface of the mantle. The fundamenta l difference between these aragonite fibres and the flat tablets of nacre lies in the difference in habit, not in a difference in form. Both kinds of crystals are bounded by [010] + [110} and terminated by [001 / (Wind & Wise, 1976, fig. 13; Marsh & Sass, 1980; Wada, 1972), but in nacre, growth on [001 / is very slow whereas in the fibres it is very fast. It is very well known that crystal habit may be dramatically changed by a variety of factors including crystal poisons such as organic dyes, high supersaturation and pressure. For example, Franke et al. (1981) found that cerussite, the Pb analogue of aragonite, formed tall prismatic crystals at low pressures and nacre-like tablets at pressures of 5-10 Kb.

It seems likely that proteins with the repetitive amino acid sequences (Asp-Gly)n or (Asp-Ser)n may be involved in limiting growth on the [001/direction of aragonite (Weiner & Traub, 1980, 1984). Proteins of this kind have been isolated from the organic matrices of a variety of different molluscan and non- molluscan shell carbonates. They are believed to occur as/3-pleated sheets, to bind calcium, and to have inter-residue dimensions that would fit well with the arrays of Ca atoms in the surfaces of both nacre tablets and calcite folia (Weiner & Traub, 1980, 1984; Runnegar,

1984). It is therefore not difficult to envisage how layered structures such as nacre and foliated calcite may have developed rapidly and convergently in different early molluscan lineages.

The origin of the more complex crossed- lamellar structure is more difficult to understand, but it may be a modification of the tangentially arranged fibrous structures seen in pelagiellid shells (Fig. 7). These structures resemble dendritic growth in a two- dimensional space (Jackson, 1967) and presumably represent a structural alternative to the prismatic ultrastructures described above.

The phosphatic replicas suggest that most of the common molluscan microstructures (spherulitic prismatic aragonite, tangentially arranged fibrous aragonite, crossed-lamellar aragonite, nacre and foliated calcite) had developed by at least the beginning of the Middle Cambrian. As most of these materials are found in Cambrian monoplacophorans, it is no longer possible to regard a particular microstructure as characteristic of any major higher taxon of the Mollusca. However, microstructural similarities and differences are valuable characters at lower taxonomic levels, and they should prove to be increasingly useful as more data are assembled.

Unless otherwise indicated, all specimens are housed in the Department of Geology & Geophysics, University of New England.

Acknowledgements I thank Stefan Bengtson and Yti Wen for the gift of specimens, E A. Shaw for assistance with the photography, M. D. Speak and P. R. Garlick for help with the electron microscopy and R. K. Vivian for typing the manuscript. The work was financed by the Australian Research Grants Scheme and the University of New England.

REFERENCES ALMGREN, F. J., & TAYLOR, J. E., 1976. Geometry of soap

films and soap bubbles. Scientific American 235, 82-93.

BATHURST, R. G. C., 1971. Carbonate Sediments and Their Diagenesis. Elsevier, Amsterdam, 620 p.

BATTEN, R. L., 1982. The origin of gastropod shell structure. Third N. Amer. Paleontol. Conv. Proc. 1, 35-38.

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