The following figures of the Astræa geminata ([Fig. 27]) and the Astræa rotula ([Fig. 28]), showing their external character and the radiations as exhibited in section, are only intended to display the novel and elegant character which prevails through an almost infinite variety of these coralline forms.
These beautiful creations are produced by animals of the polyp kind, which, possessed of a power of separating the carbonate of lime from sea-water, are constantly engaged in building up around themselves those stone structures which, if not geometrical in all their arrangements, are strikingly varied and beautiful. The coral animal has left traces of its work on the earliest fossil rocks, but in the more recent or Oolitic series the corals are most abundant.
Fig. 27. Fig. 28.
It is almost impossible to select a specimen from any cabinet of the corals of the Oolitic period without being struck with the regularity of arrangement and the variety of beautiful forms produced. It is true that our existing corals bear a strong resemblance to those of the seas of the ancient world, but they differ in specific, and often in generic character, and the fossil remains present forms and dispositions of parts widely varied from those of the recent coral. It is curious and interesting to observe, however, in both species, the same contrivances adapted to provide that resistance to the waves so necessary for the protection of the coral animal, and which especially marks its work.
The extent to which these coralline formations have gone on will be indicated by the fact that the coralline crag at Oxford is exposed at the surface, and the bottom of it has not been reached at the depth of fifty feet. One of the limestone beds of the middle Oolite series of England is a continuous bed of petrified corals, retaining the position in which they grew at the bottom of the sea; and beside these we find scattered through our Oolitic formations an immense quantity of coral remains. Indeed, if we examine the stones of which some of our most admired churches are built, as at Oxford and Cambridge, we shall find that the firmly integrated mass is little else than shells and corals. Thus the labours of hosts of insect architects, working in the ocean which overflowed this island myriads of ages since, are now employed to form those temples which religion consecrates to the Creator of all things.
The elegance of these fossil remains is still further illustrated by the three cuts of the Pentacrinites subangularis, the sections of the Pentacrinites dubius, and of the Encrinites moniliformis ([Fig. 29]).