At the bottom, then, of twenty miles of rocks the seeker for the progenitor of the great family of backboned animals finds the scant remains of fish-like animals that the cautious naturalist, who is much given to "hedging," terms, not vertebrates, but prevertebrates or the forerunners of backboned animals. The earliest of these consist of small bony plates, and traces of a cartilaginous backbone from the Lower Silurian of Colorado, believed to represent relatives of Chimaera and species related to those better-known forms Holoptychius and Osteolepis, which occur in higher strata. There are certainly indications of vertebrate life, but the remains are so imperfect that little more can be said regarding them, and this is also true of the small conical teeth which occur in the Lower Silurian of St. Petersburg, and are thought to be the teeth of some animal like the lamprey. A little higher up in the rocks, though not in the scale of life, in the Lower Old Red Sandstone of England, are found more numerous and better preserved specimens of another little fish-like creature, rarely if ever exceeding two inches in length, and also related (probably) to the hag-fishes and lampreys that live to-day. These early vertebrates are not only small, but they were cartilaginous, so that it was essential for their preservation that they should be buried in soft mud as soon as possible after death. Even if this took place they were later on submitted to the pressure of some miles of overlying rock until, in some cases, their remains have been pressed out thinner than a sheet of paper, and so thoroughly incorporated into the surrounding stone that it is no easy matter to trace their shadowy outlines. With such drawbacks as these to contend with, it can scarcely be wondered at that, while some naturalists believe these little creatures to be related to the lamprey, others consider that they belong to a perfectly distinct group of animals, and others still think it possible that they may be the larval or early stages of larger and better-developed forms. Still higher up we come upon the abundant remains of numerous small fish-like animals, more or less completely clad in bony armor, indicating that they lived in troublous times when there was literally a fight for existence and only such as were well armed or well protected could hope to survive. A parallel case exists to-day in some of the rivers of South America, where the little cat-fishes would possibly be eaten out of existence but for the fact that they are covered--some of them very completely--with plate-armor that enables them to defy their enemies, or renders them such poor eating as not to be worth the taking. The arrangement of the plates or scales in the living Loricaria is very suggestive of the series of bony rings covering the body of the ancient Cephalaspis, only the latter, so far as we know, had no side-fins; but the creatures are in no wise related, and the similarity is in appearance only.