Admitting it, then, to be a beach, let us begin with the lowest type of the Animal Kingdom and see what _Radiates_ are to be found there. There are plenty of _Corals_, but they are not the same kind of _Corals_ as those that build up our reefs and islands now. The modern Coral animals are chiefly _Polyps_, but the prevailing _Corals_ of the _Silurian_ age were _Acalephian Hydroids_, animals which indeed resemble _Polyps_ in certain external features, and have been mistaken for them, but which are, nevertheless, _Acalephs_ by their internal structure. Of the _Echinoderms_, the class of _Radiates_ represented now by our _Star-Fishes_ and _Sea-Urchins_, we may gather any quantity, though the old-fashioned forms are very different from the living ones. The _Mollusks_ were also represented then, as now, by their three classes, _Acephala_, _Gasteropoda_, and _Cephalopoda_. The _Acephala_ or _Bivalves_ we find in great numbers, but of a very different pattern from the _Oysters_, _Clams_, and _Mussels_ of recent times. Of the _Silurian Univalves_ or _Gasteropods_, there is not much to tell, for their spiral shells were so brittle that scarcely any perfect specimens are known, though their broken remains are found in such quantities as to show that this class also was very fully represented in the earliest creation. But the highest class of _Mollusks_, the _Cephalopods_ or _Chambered Shells_, or _Cuttle-Fishes_, as they are called when the animal is unprotected by a shell, are, on the contrary, very well preserved, and they are very numerous. Of _Articulates_ we find only two classes, _Worms_ and _Crustacea_. Insects there were none--for, as we have seen, this early world was wholly marine. There is little to be said of the _Worms_, for their soft bodies, unprotected by any hard covering, could hardly be preserved; but, like the marine _Worms_ of our own times, they were in the habit of constructing envelopes for themselves, built of sand, or sometimes from a secretion of their own bodies, and these cases we find in the earliest deposits, giving us the assurance that the _Worms_ were represented there. I should add, however, that many impressions described as produced by _Worms_ are more likely to have been the tracks of _Crustacea_. But by far the most characteristic class of _Articulates_ in ancient times were the _Crustaceans_. The _Trilobites_ stand in the same relation to the modern _Crustacea_ as the _Crinoids_ do to the modern _Echinoderms_. They were then the sole representatives of their class, and the variety and richness of the type are most extraordinary. They were of nearly equal breadth for the whole length of the body, and rounded at the two ends, so as to form an oval outline.