The geologist finds the remains of all these classes in the different rock-formations of the crust of the earth. They flourished so abundantly in the earliest seas, that the first geological period has sometimes been called the Age of Molluscs; and, during all the subsequent eras, they held a prominent place among the inhabitants of the deep. Let us look for a little at their development in the times of the Carboniferous system. As the Carboniferous group of rocks exhibits the remains of ocean-bed, lake-bottom, and land-surface, so we find in it shells of marine, fresh-water and (though rarely) terrestrial mollusca. The marine genera greatly predominate, just as the shells of the sea at the present day vastly outnumber those either of lakes or of the land. In England they occur chiefly in the lower part of the formation, giving a characteristic stamp to the deep series of beds known as the mountain limestone. There they are associated with the corals and stone-lilies already described--all productions of the sea. In Northumberland, however, and generally throughout Scotland, they occupy a somewhat different position. The great mountain limestone of central England gets split up into subdivisions as it proceeds northward, and beds of coal, full of land plants, become mingled with the ordinary marine strata. Sometimes we may find a group of brachiopods scattered over the macerated stem of a stigmaria; and the writer has himself collected a sigillaria in a limestone crowded with stone-lilies and _producti_. But this intermingling is still further carried on in the upper part of the series. The coal-beds, with their underclays and stigmaria rootlets, evidently representing ancient vegetation with the soils on which it grew, are succeeded by beds of limestone, full of marine mollusca; and these, again, are erelong replaced by sandstones, shales, and ironstones, charged with land-plants and fresh-water shells. To this curious blending of very different organic remains, I shall have occasion to refer more at large in a subsequent chapter. I mention it now as a sort of apology for the dryness of details which it is necessary to give, in order to complete our picture of the carboniferous fauna, and to understand the principles upon which the ancient history of the earth is deciphered. [Illustration: Fig. 21.] Of the _Pteropoda_, we have, as yet, but one carboniferous genus, the _conularia_ (Fig. 21). It was a slim delicate shell, in shape an oblong cone, having four sides, finely striated with a sort of zig-zag moulding like that of the Norman arch. Each of the four angles was traversed along its whole extent by a narrow gutter-like depression, and this style of fluting, combined with the markings on the sides, imparted no little elegance to the shell. The conularia is not a common fossil. It has been found among the coal-bearing strata of Coalbrook-Dale, and was noticed long ago by Dr. Ure in his _History of Rutherglen_.