animals appear to have been able to move very quickly; others would roll themselves up like hedgehogs to defend themselves against attack; and some of the larger ones were from eighteen inches to two feet in length. Next to these in interest were the cephalopod types, marine animals, that may have resembled the swimming nautilus of to-day in some of their developments. They attained to enormous sizes, some of the shells being twelve to fifteen feet in length and a foot in greatest diameter. From this maximum they ranged down to forms smaller than a pipe-stem. Their habits are to be gathered only from their structure and from the habits of their relations in the present seas. Perhaps they floated, shell uppermost, or crawled upon the bottom and preyed on a variety of the weaker forms of life. There appear to have been fewer worms, perhaps because the muddy and chalky sea bottoms of the Ordovician period were less congenial to them than the Cambrian sands. The changes in the structure of the earth's crust which brought the Ordovician period to an end marked also the beginning of the Silurian period. These changes affected sometimes small areas and were very intense; sometimes they affected larger areas more slightly. It must not be assumed, however, that these changes were necessarily sudden or violent. In examining the rocks now, we see merely the effects, and of these effects it is the more remarkable alone which have survived the march of ages. There was more water on the earth's surface then than now; and side by side with continuous storms of tropical violence, it is extremely likely that volcanoes and earthquake movements were more frequent and more considerable in their effects. The tide of movement by material things may have been faster; and certain it is that the land was now lifting itself up above the shallow seas. Mountains were being built along the coast-lines; behind the coast-lines the continents were shouldering their way upwards in large land areas. North America began to show in this period the first signs of becoming a continent; Europe's countries, or some of them, assumed a distinct existence. With the advent of mountains came streams and rivers; and the streams, fed by the abundant rainfalls, rushed down to the seas in torrents that performed the work of erosion with a rapidity perhaps unequalled by even the greatest rivers of our present-day knowledge--though at first the land areas were not large enough to give rise to streams as long as great rivers like the Amazon or Mississippi. We cannot say exactly what the areas and localities of the water and the land were; but it is safe to assume that at the beginning of the Silurian period beds of sediment brought down by the rivers and the rain were accumulating about the borders of the land, and as far out as the waves and currents were able to convey the earth materials. The climate was still equable and was much the same over great areas of