The mollusks, or shell-fish, with single or bivalve shells, with heads or without,[32] were, no doubt, larger and more abundant in the Silurian epoch than ever after; but they remain plentiful now, as all collectors of shells are glad to realize, and though in many orders they have decreased in size, they have increased in beauty of form and coloring. The crustaceans, or crab family, were among the earliest of living creatures. The class survives in many varieties, but some of its earliest families have entirely perished. In the age in which they first appeared, the crustaceans were the most numerous of living things. The crustaceans are of the ringed class of animals, to which also insects belong,[33] and here we find that the ringed creatures of the sea preceded by a long time the ringed creatures of the air and land; for crabs of many kinds crawled in the shallow seas and were cast upon the rocky shores of the earliest continents and islands long before a solitary dragon-fly or cockroach found life, or could have found a wooded or grassy piece of land upon which to rest. The first of the ancient crustaceans which we shall mention is represented by a very numerous class of fossils called trilobites. There are no living trilobites, but the creatures now nearest them are the isopods. The trilobite was more like an isopod in its larva state than like a full-grown one. The trilobite had a hard plate or shell over his head and the front part of his body. This piece of armor was shaped nearly like the large front portion of the shell of a horseshoe or king-crab. The king-crab uses this shell as a mud-plough to plough its way through the slime, where it finds its food and deposits its eggs. Some of the ancient trilobites were as small as a split pea, and some were a foot across their head-plate; but large or small they all had the head-shell and used it for the same purposes not only for protection, but as a mud-plough for digging burrows in the ooze below the shallow seas or on the shores. On the sides of this hard buckler were placed the eyes, which were large, prominent, and compound, or furnished with many lenses, as we find in the eyes of insects.[34] These round, prominent eyes are a very marked feature of a fossil trilobite; they seem to be staring at creation still, though the creature has been dead thousands of years.