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If the bed of the sea were now raised round the British Isles, so as to expose its deeper parts, we should have some differences in the shells laid bare according to the original depths at which they lived. We should distinguish, by the aid of Forbes' s dredging, the old littoral or tidal zone everywhere by patella vulgaris; if rocky, by littorinæ; if sandy, by cardia, tellinæ, solenæ; if gravelly, by mytili; if a clay bottom, by lutrariæ and pullastæ. The laminarian or subtidal zone, extending to 15 fathoms, would give us lacunæ and rissoæ, patella pellucid and p. lævis, trochus ziziphinus, and various modiolæ. The coralline zone, extending to 50 fathoms, would show a great variety of carnivorous mollusca; fusus antiquus, pullastra virginea, pecten maximus, and pleurotoma teres being characteristic. And in the deepest of the zones, that of deep sea corals, which goes beyond 100 fathoms, we should have a more contracted series of mollusca, but including brachiopoda and ditrupa.

What in fact we have in our raised beaches, and shelly gravels and clays, is a series of littoral, subtidal, and shallow sea shells, without the deep sea corals. They do not, however, lie as shells might be found in undisturbed parts of the old sea bed; on the contrary, they are frequently rolled, and much broken, as if drifted by water, and crushed by pressure.

We may compare with these results what has been learned in late years concerning the circumstances under which mineral matter is aggregated and the conservable parts of living beings preserved under the sea.