It is probable that the limpet takes several years to attain full growth, and during that period it frequents the same spot, which becomes gradually sunk below the surrounding surface, especially if the rock be carbonate of lime.
Close to them is the remarkable dart-sac ps, a thick-walled sac, in the lumen of which a crystalline four-fluted rod or dart consisting of carbonate of lime is found.
In other slugs, namely, Limax and Anion, the shell-sac remains permanently closed over the shell-plate, which in the latter genus consists of a granular mass of carbonate of lime.
It is not at all certain that the masses on which coral reefs are built consist entirely of the remains of the skeletons of reef-forming organisms and it is probable that chemically precipitated carbonate of lime predominates.
The remarkable " stone reefs " of the north-east coast are ancient beaches hardened by the infiltration of carbonate of lime.
