But this discussion has led us far away from our starting point, and interrupted our walk along the Silurian beach; let us return to gather a few specimens there, and compare them with the more familiar ones of our own shores.  I have said that the beach was a shelving one, and covered of course with shoal waters; but as I have no desire to mislead my readers, or to present truths as generally accepted which are still subject to dispute, I would state here that the parallel ridges across the State of New York, considered by some geologists as the successive shores of a receding ocean, are believed by others to be the inequalities on the bottom of a shallow sea.  Not only, however, does the general character of these successive terraces suggest the idea that they must have been shores, but the ripple-marks upon them are as distinct as upon any modern beach.  The regular rise and fall of the water is registered there in waving, undulating lines as clearly as on the sand-beaches of Newport or Nahant; and we can see on any one of those ancient shores the track left by the waves as they rippled back at ebb of the tide thousands of centuries ago.  One can often see where some obstacle interrupted the course of the water, causing it to break around it; and such an indentation even retains the soft, muddy, plastic look that we observe on the present beaches, where the resistance made by any pebble or shell to the retreating wave has given it greater force at that point, so that the sand around the spot is soaked and loosened.  There is still another sign, equally familiar to those who have watched the action of water on a beach.  Where a shore is very shelving and flat, so that the waves do not recede in ripples from it, but in one unbroken sheet, the sand and small pebbles are dragged and form lines which diverge whenever the water meets an obstacle, thus forming sharp angles on the sand.  Such marks are as distinct on the oldest Silurian rocks as if they had been made yesterday.  Nor are these the only indications of the same fact.  There are certain animals living always upon sandy or muddy shores, which require for their well-being that the beach should be left dry a part of the day.  These animals, moving about in the sand or mud from which the water has retreated, leave their tracks there; and if, at such a time, the wind is blowing dust over the beach, and the sun is hot enough to bake it upon the impressions so formed, they are left in a kind of mould.  Such trails and furrows, made by small Shells or Crustacea, are also found in plenty on the oldest deposits.

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 kinds 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