Ascending to the next group of rocks, we find the traces of life become more abundant, the number of species extended, and important additions made in certain vestiges of fuci, or sea-plants, and of fishes. This group of rocks has been called by English geologists, the _Silurian System_, because largely developed at the surface of a district of western England, formerly occupied by a people whom the Roman historians call Silures. It is a series of sandstones, limestones, and beds of shale (hardened mud), which are classed in the following sub-groups, beginning with the undermost:—1, Llandillo rocks, (darkish calcareous flagstones;) 2 and 3, two groups called Caradoc rocks; 4, Wenlock shale; 5, Wenlock limestone; 6, Lower Ludlow rocks, (shales and limestones;) 7, Aymestry limestone; 8, Upper Ludlow rocks, (shales and limestone, chiefly micaceous.) From the lowest beds upwards, there are polypiaria, though most prevalent in the Wenlock limestone; conchifera, a vast number of genera, but all of the order brachiopoda, (including terebratula, pentamerus, spirifer, orthis, leptæna;) mollusca, of several orders and many genera, (including turritella, orthoceras, nautilus, bellerophon;) crustacea, all of them trilobites, (including trinucleus, asaphus, calamene.) A little above the Llandillo rocks, there have been discovered certain convoluted forms, which are now established as annelids, or sea-worms, a tribe of creatures still existing, (nereidina and serpulina,) and which may often be found beneath stones on a sea-beach. One of these, figured by Mr. Murchison, is furnished with feet in vast numbers all along its body, like a centipede. The occurrence of annelids is important, on account of their character and status in the animal kingdom. They are red-blooded and hermaphrodite, and form a link of connexion between the annulosa (white-blooded worms) and a humble class of the vertebrata. {62} The Wenlock limestone is most remarkable amongst all the rocks of the Silurian system, for organic remains. Many slabs of it are wholly composed of corals, shells, and trilobites, held together by shale. It contains many genera of crinoidea and polypiaria, and it is thought that some beds of it are wholly the production of the latter creatures, or are, in other words, coral reefs transformed by heat and pressure into rocks. Remains of fishes, of a very minute size, have been detected by Mr. Philips in the Aymestry limestone, being apparently the first examples of vertebrated animals which breathed upon our planet. In the upper Ludlow rocks, remains of six genera of fish have been for a longer period known; they belong to the order of cartilaginous fishes, an order of mean organization and ferocious habits, of which the shark and sturgeon are living specimens. “Some were furnished with long palates, and squat, firmly-based teeth, well adapted for crushing the strong-cased zoophytes and shells of the