Even freshwater lakes possess molluscan deposits, producing a white limestone of their own. Where streams flow over pure pre-existing limestone, there is no alluvial mud to choke the basins. In the hard lake-waters, gastropods such as Limnæa and Planorbis, and a few bivalves, can then flourish freely, and a "shell-marl" accumulates at the bottom, unmixed with sediment. Limestone of this type is conspicuous in hollows in the Dinaric Alps, which were once occupied by lakes, and is often found beneath peat in the limestone lowland of central Ireland. In older days, two groups of organisms, now relatively unimportant, had a powerful place. The brachiopods, including in early Palæozoic times an interesting series of thin shells largely composed of calcium phosphate, were for long the predominant shell-bearing organisms. The stout Spiriferidæ and the well-known _Productus giganteus_ of the Carboniferous period illustrate their dominance. The group became much restricted in variety in Jurassic times; but even then Terebratula and Rhynchonella occurred so abundantly that they now fall out of many rock-faces like pebbles from a loose conglomerate. The sea-lilies have similarly lost their place as limestone-builders, though their "ossicles," notably from their stems, furnish crinoidal or "encrinital" masses from Silurian to Carboniferous times. The broken portions of their stems, resembling tubes of tobacco-pipes, are conspicuous when they are weathered out on rock-surfaces or revealed in polished slabs of marble. The fact that each joint or ossicle, as is the universal case in the echinodermata, consists of a single crystal of calcite causes the fragments to break with the characteristic cleavage of that mineral. The smooth glancing surfaces thus seen on fractured specimens readily call attention to them in a rock. Those humble colonial organisms, the compound corals, have so special a place as limestone-formers that they have been reserved for more detailed treatment. The accumulation of their skeletons, and the fact that they may form large continuous masses by their very mode of growth, promotes the formation of solid rock at an unusual rate. Von Richthofen long ago pointed out how foraminifera and other drifted material became caught in the interstices of coral, producing even a stratified structure in the hollows of a reef; and subsequent research has shown the composite character of reefs in various portions of the tropic seas. Calcareous algæ as already remarked, and the massive and often encrusting skeletons of hydrozoa, such as Millepora, are freely associated with the products of true corals.