Geology, of all the sciences, addresses itself most powerfully to the imagination, and hence one main cause of the interest which it excites. Ere setting ourselves minutely to examine the peculiarities of these creatures, it would be perhaps well that the reader should attempt realizing the _place_ of their existence, and relatively the _time_--not of course with regard to dates and eras, for the geologist has none to reckon by, but with respect to formations. They were the denizens of the same portion of the globe which we ourselves inhabit, regarded not as a tract of country, but as a piece of ocean crossed by the same geographical lines of latitude and longitude. Their present place of sepulture in some localities, had there been no denudation, would have been raised high over the tops of our loftiest hills--at least a hundred feet over the conglomerates which form the summit of Morvheim, and more than a thousand feet over the snow-capped Ben Wyvis. Geology has still greater wonders. I have seen belemnites of the Oolite--comparatively a modern formation--which had been dug out of the sides of the Himalaya mountains, seventeen thousand feet over the level of the sea. But let us strive to carry our minds back, not to the place of sepulture of these creatures, high in the rocks,--though that I shall afterwards attempt minutely to describe,--but to the place in which they lived, long ere the sauroid fishes of Burdie House had begun to exist, or the corallines of the mountain limestone had spread out their multitudinous arms in a sea gradually shallowing, and out of which the land had already partially emerged. A continuous ocean spreads over the space now occupied by the British islands: in the tract covered by the green fields and brown moors of our own country, the bottom, for a hundred yards downwards, is composed of the debris of rolled pebbles and coarse sand intermingled, long since consolidated into the lower member of the Old Red Sandstone; the upper surface is composed of banks of sand, mud, and clay; and the sea, swarming with animal life, flows over all. My present object is to describe the inhabitants of that sea.