Among the crustaceans, or soft shell-fishes of the Silurian, we meet with the _Trilobites_, continued from the Primordial in great and increasing force, and represented by many and beautiful species; while an allied group of shell-fishes of low organization but gigantic size, the _Eurypterids_, characteristic of the Upper Silurian, were provided with powerful limbs, long flexible bodies, and great eyes in the front of the head, and were sometimes several feet in length. Instead of being mud grovellers, like the Trilobites and modern king-crabs, these _Eurypterids_ must have been swimmers, careering rapidly through the water, and probably active and predaceous. There were also great multitudes of those little crustaceans which are inclosed in two horny or shelly valves like a bivalve shell-fish, and the remains of which sometimes fill certain beds of Silurian shale and limestone. No remains found in the Silurian rocks have been more fertile sources of discussion then the so-called _Graptolites_, or written stones--a name given long ago by Linnæus, in allusion to the resemblance of some species having rows of cells on one side, to minute lines of writing. These little bodies usually appear as black coaly stains on the surface of the rock, showing a slender stem or stalk, with a row of little projecting cells at one side, or two rows, one on each side. The more perfect specimens show that, in many of the species at least, these fragments were branches of a complex organism spreading from a centre; and at this centre there is sometimes perceived a sort of membrane connecting the bases of the branches, and for which various uses have been conjectured. The branches themselves vary much in different species. They may be simple or divided, narrow, or broad and leaf-like, with one row of cells, or two rows of cells. Hence arise generic distinctions into single and double graptolites, leaf and tree graptolites, net graptolites, and so on. But while it is easy to recognise these organisms, and to classify them in species and genera, it is not so easy to say what their affinities are with modern things. They are exclusively Silurian, disappearing altogether at the close of this period, and, so far as we know, not succeeded by any similar creatures serving to connect them with modern forms. Hence the most various conjectures as to their nature. They have been supposed to be plants, and have been successively referred to most of the great divisions of the lower animals. Most recently they have been regarded by Hall, Nicholson,[J] and others, who have studied them most attentively, as zoophytes or hydroids allied to the Sertularise, or tooth-corallines and sea-fir-corallines of our coasts, to the cell-bearing branches of which their fragments bear a very close resemblance. In this case, each of the little cells or teeth at the sides of the fibres must have been the abode of a little polyp, stretching out its tentacles into the water, and enjoying a common