In coiled mollusc shells, the apex typically is located near the centre of a whorl spiral and the aperture at its outer perimeter (exceptions do occur in molluscs with determinate or periodic growth, but they do not invalidate this general principle). This geometry satisfies simple growth and functional requirements. The Recent vermetid Dendropoma sp. defies these requirements with its inside-out shell geometry, in which the aperture is located along the axis of the spiral and earlier whorls coil around it. In addition, this species is unique among the Vermetidae in being fully endolithic in the adult, and is one of very few endolithic molluscs with the shell cemented to the substrate during growth. While Dendropoma is typically semi-endolithic, several species appear to have secondarily returned to epifaunal coiling as a response to crowding of the substrate. In D. sp., this was prevented by the immediate environment, subjected to a high rate of erosion. This is likely the factor that triggered the onset of endolithic coiling into the substrate. The change from a semi-endolithic to a fully endolithic life habit in this form was probably sudden, since the preadaptations of Dendropoma prevent a functioning intermediate stage.

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The ornamentation and dorsal wall structure of Vidrioceras (Cycloloboidea, Goniatitina) in the early ontogenetic stage are described on the basis of specimens from the Upper Pennsylvanian in Kansas, USA. The exposed surface of the embryonic shell is smooth, without any trace of ornamentation or growth lines. Regularly spaced lirae abruptly appear on the early postembryonic shell just adoral of the primary constriction. The inner surface of the dorsal wall in the embryonic and early postembryonic stages exhibits a distinct ornament consisting of evenly spaced, longitudinal ridges, which are replaced adorally by the typical wrinkled ornament in the subsequent stage. Our observations are in accord with those of goniatites from the Upper Carboniferous Buckhorn Asphalt of Oklahoma, suggesting that in the Goniatitina, the outer surface of the embryonic shell is smooth. Comparison with the embryonic shell formation of extant Nautilus suggests that in the Goniatitina, the embryonic shell was uniformly secreted by the shell gland on the posterior side of the embryo.

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The shell and opercular structures of Nerita (Theliostyla) albicilla and Cinnalepeta pulchella were described and compared with those of other extant members of Neritopsina. The shell of N. (T.) albicilla is composed of four layers : the outermost prismatic layer, followed by the simple crossed-lamellar layer, the myostracum, and the inner complex crossed-lamellar layers. The operculum consists of three prismatic layers deposited on both sides of an organic layer. C. pulchella also has a four-layered shell, but lacks an operculum. The outer layer is a homologous structure. The shells of Recent neritopsine families can be categorized into a four-layered group (Neritiliidae, Neritidae, and Phenacolepadidae) and a three-layered group (other families). In contrast, opercular structure is markedly variable in the Neritopsina, and little correlation can be established in the light of phylogenetic evolution or adaptation.

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The distribution patterns, sizes and nucleation sites of aggregated prisms on the outer shell surface were examined in 16 species of Bivalvia and modeled theoretically. Biometric analysis shows a negative correlation between the median size and variation of sizes of calcitic simple prisms. In species with aragonitic vertical composite prisms, instead, the density of prisms tends to decrease when their nucleation sites are randomly distributed. Comparison of the results of computer simulations with those of biometric analyses reveals the following : 1) a positive correlation between growth rate of prisms and the probability of nucleation for simple prisms, and 2) a limit of the number of nucleations per unit time in vertical composite prisms. Prism size correlates with the growth rate of the entire shell or prisms, and increases as the shell grows faster or prisms grow slower.

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Eight species and one subspecies of two genera of Collignoniceratinae, Collignoniceras and Prionocyclus, are analysed with respect to standard morphological distance measures and ribfrequencies, methods of multivariate statistical analysis, including canonical variate ordination, principal components and generalized distances. It was found that the biostratigraphically inferred evolutionary sequence, as currently perceived, is upheld in detail for the Collignoniceras data. The 'nearest-neighbour' relationships between the Prionocyclus part of the sequence is less complete, although links in main branches are supported. The other aspect given consideration, that of "gracile" and "robust" shells (based on visual inspection of the conch), yielded the result that the subjective assignation of shells to gross morphological type is largely, though not unequivocally, upheld by the statistical analysis. Illustrations of typical representatives of the species analysed are provided.

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The symbiosis (defined as a strict interspecific association) between bivalves and other organisms is examined. Microsymbiosis (i.e., symbiosis with microorganisms) is frequent among Recent bivalves, and has been proposed to explain the unusual characters of several fossil bivalves. However, a critical review of the morphological criteria used to infer microsymbiosis in fossil bivalves shows that their application is likely to result in a large number of false positive and false negative results. Symbiosis with macroscopic organisms (i.e., macrosymbiosis), on the other hand, has a better chance of being recognised correctly in fossils, although direct preservation of the associated organisms remains the only completely safe criterion. Recent and fossil instances of macrosymbiosis are reviewed, and new evidence is presented to clarify the adaptive significance of some of these associations.

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