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"Calcareous algae and stromatolites" is shorthand for a wider array of organisms and fabrics that also includes calcified cyanobacteria, plus thrombolites and other microbial carbonates. Composition is the link: these are all important components of CaC0 sediments, from 3 Archaean to present and from the ocean floor to streams and lakes. It is hardly possible to examine limestones of any age without en countering them. Simultaneously they are fossils, sediments, and en vironmental indicators. It is the range of significance, coupled with the breadth of their distribution in time and space, which compels their study. Modern calcareous marine algae mainly include reds (corallines, squamariaceans, and the nemalialean Galaxaura) and greens (dasy cladaleans, udoteaceans, halimedaceans). Blue-greens, of course, are cyanobacteria and not algae, and significantly, although they are largely responsible for Recent tidal flat stromatolites, they are not calcified in the same way that pre-Cenozoic marine blue-greens are. It is in the freshwater environment of calcareous streams and lakes that we find modern calcified cyanobacteria, and they are commonly associated with the only major group of non-marine calcareous algae, the charophytes. However, in the past, and especially in the Palaeozoic and Mesozoic, things look radically different. Mingling with the ancestors of the modern flora are distinct, and often problematic, organisms.