Many marine molluscs produce complex shells of calcium carbonate. These shells are formed under strict biological control to provide a range of functions to ensure the survival of the living organism. These inspiring biomineral structures can also provide an archive of environmental change via proxies such as δ18O and δ13C within the shell carbonate. However, the intimate relationship between the biological and environmental controls influencing biomineral production can often obscure the environmental signal, making it difficult to interpret environmental information from shell proxies. Understanding the design of biomineral structures will further our knowledge of biomineralisation as a whole, while understanding the controls that influence shell production will ensure that shell proxies applied to palaeoenvironmental studies are accurate. Oysters are sessile bivalve molluscs that have evolved since the Triassic and expanded to occupy a range of habitats with almost global distribution, providing an example of a highly successful biomineral system. This study investigates the ultrastructure, crystallography and stable isotope composition of the Pacific oyster, Crassostrea gigas from estuarine and marine environments. The method by which oysters adhere to hard substrates is also investigated. Both valves of the oyster shell are composed predominantly of low Mg calcite in three forms; an outer prismatic region, an inner foliated structure and chalk lenses which appear sporadically throughout the valves. Aragonite is restricted to the myostracum and parts of the hinge structure. Crystallographic analysis shows that, despite variations in structural morphology, the superimposed layers of the oyster shell maintain a single crystallographic orientation with the crystallographic c-axis orientated perpendicular to the outer shell surface. Varying crystal morphology, while maintaining crystallographic unity, may be a deliberate design to provide the oyster shell with both strength and flexibility. In general, there is no difference in the overall ultrastructure or microstructural arrangement of estuarine and marine oysters. Estuarine oysters contain significantly more chalk than marine equivalents suggesting that the appearance of chalk lenses is, to some degree, triggered by an external environmental stimulus. Stable carbon and oxygen isotope analysis of folia and chalk provides further insights into the appearance of chalk in the oyster shell structure. There is no significant difference in the isotope composition of folia or chalky layers, although patterns of δ13C and δ18O of folia and chalk reveal key differences between the two structures. Folia display a narrower range of δ18O values compared with chalk and exhibit significant interspecimen variation with respect to δ13C. Interspecimen variation, with respect to either δ18O or δ13C is absent in chalk samples. These patterns suggest that secretion of folia requires a more specific environmental stimulus and a greater input of metabolic carbon than chalk. Chalk is apparently secreted across a greater range of environmental conditions, with less metabolic regulation. Deviation from optimal environmental conditions, for example during periods of reduced salinity and/or cold or warm temperatures, may reduce metabolism causing the oyster to deliberately default from folia to chalk secretion. In general, folia and chalk in both estuarine and marine oysters is secreted in oxygen isotope equilibrium with the ambient environment. Another aspect of the oyster biomineral system is their ability to adhere tightly, and usually permanently, to a range of hard substrates by cementation of the left valve. Investigation of the contact zone between oyster shells and biological (other oyster shells) and inorganic (rock) substrates reveals the influence of both biogenic and non-biogenic processes in oyster cementation. Original adhesion is brought about by secretion of an organic adhesive which acts as a nucleating surface onto which crystals precipitate which have random orientation and are composed of high Mg calcite. The lack of orientation and elevated Mg content suggests that these crystals are nucleating outwith the biological control experienced by the shell biomineralisation process and are formed from inorganic precipitation from seawater. It is proposed that oysters do not control, or secrete, the crystalline cement but instead they secrete an organic film onto which crystals precipitate from seawater. Oysters thus provide excellent examples of both biologically induced and biologically controlled mineralisation.

This thesis provides detailed studies of the five stropheodontid species which occur commonly in the Ludlow rocks of the Welsh Borderland. These are Shaleria ornatella (Davidson), Leptostrophia filosa (J. de C. Sowerby), Pholidostrophia (Mesopholidostrophia) lepisma (J. de C. Sowerby), Amphistrophia funiculata (McCoy), and Strophonella euglypha (Dalman). About 2250 specimens have been collected from well-spaced geographical localities in the shelf facies of the Welsh Borderland (i.e. Ludlow, Abberley Hills, Woolhope inlier and Usk inlier) and, where possible, from different levels in the Ludlow succession. In addition, a large collection of Shaleria aff. ornatella Bassett & Cocks from Gotland, Sweden plus museum and other specimens of the Wenlock forms of S. euglypha and A. funiculata were studied statistically for comparison with the Welsh Borderland Ludlovian forms. After preparation, accurate measurements were made of: shell length and width, pedicle muscle field length and width, angle of divergence of the pedicle valve muscle field, width of denticulation, the number of ribs per mm at 5 or 10mm from the dorsal umbo, and some other special morphological features. All five species are fully described and illustrated: some previously undescribed morphological features have been recognised, discussed and figured. For each species, also, statistical analyses of the range of variation of the most significant morphological features have been carried out. In the case of P. (M.) lepisma, the emphasis is on the full description of the species and its variation as it has not previously been adequately described and figured. The study of L. filosa also concentrates on the description and the range of variation, together with a comparison with Wenlock forms of the same species. Shaleria ornatella is fully described and its range of variation is such that there seems little doubt that the Gotland form Shaleria aff. ornatella should be included in this species. The small but consistent difference in the Gotland form can be related to environmental rather than genetic controls. The intraspecific variation in a present-day species of Brachiopoda; Kraussina rubra (Pallas) from one locality was also studied; the range of variation is comparable to that in S. ornatella, thereby supporting the palaeontological interpretation of this (& other) species. In both S. euglypha and A. funiculata, particular interest lies in the morphological differences between the Wenlock and Ludlow forms of these species. These changes have been analysed in detail and attention directed to the containing sediments in order to determine whether changes of substrate or other environmental factors might explain the vertical changes. The accumulated evidence, however, seems to favour gradual evolutionary changes from the Wenlock to the Ludlow although there are too many gaps in the record to be certain. Finally, the functional morphology and palaeoecology of stropheodontid brachiopods in general, and of these five Ludlow specimens in particular, are discussed.