AFTER the obituary notices of eminent Fellows lost during the past year, who were more than usually numerous, the president confined his own special address to the consideration of the structure and origin of limestones, relying mainly on his own observations, but incorporating general facts derived from other sources. Since, in order to properly understand the nature of the various constituent fragments of which many limestones are composed, it is necessary to know the organic and mineral constitution of the various different living calcareous organisms, this question was first considered from a somewhat novel point of view, and they were regarded, not merely as living tissues, but also as mineralised organisms, much attention being paid to their special optical characters. Much attention had been also paid to their true mineral constitution, so as to ascertain in which groups the carbonate of lime exists in the form of calcite, and in which as aragonite. The results are in some cases remarkable, even in relation to biology, and are of great interest and importance in the study of limestone rocks and their included fossils, since subsequent changes depend mainly on whether the original material was calcite or aragonite. This is due to the fact that calcite is in a state of stable equilibrium, and cannot be changed to aragonite, whereas aragonite is relatively in a state of unstable equilibrium, can be changed to calcite, and usually has so changed in limestone rocks. This circumstance has given rise to a complete difference in the state of preservation of many fossils. When they were originally calcite, they may have been further consolidated, but retain their original structure and optical properties, whereas when they were aragonite they have sometimes been completely removed by solution, and in other cases are usually changed into a mass of crystals of calcite, and have lost their original microscopical and optical characters. The general structure of various recent and fossil organisms was then considered, and it was shown how and to what extent they could be distinguished, when occurring as minute fragments in thin sections of limestones.