The creatures that existed, though differing from those of the previous age, were still confined to the waters; as yet the dry land remained untenanted. The fishes give evidence of a higher organization, and many of them appear to have been of gigantic dimensions. Some teeth which have been found of one kind, the Megalichthys, equal in size those of the largest living crocodiles. There is one peculiarity respecting fossil fishes which is worthy of remark. It is that, in the lapse of time from one era to another, their character does not change _insensibly_, as in the case of many zoophytes and testacea; on the contrary, species seem to succeed species _abruptly_, and at certain definite intervals. A celebrated geologist[22] has observed, that not a single species of fossil fish has yet been found that is common to any two great geological formations, or that is living in our own seas. Continuing our investigation, we next find the fruitful coal era passing away; scarcely a trace of vegetation remains; a few species of zoophytes, shells, and fishes are to be found, and we observe the impression of footsteps, technically called _ichnites_, from the Greek _ichnon_, a footmark. These marks present a highly interesting memento of past ages. Persons living near the sea-shore must have frequently observed the distinctness with which the track of birds and other animals is imprinted in the sand. If this sand were to be hardened by remaining exposed to the action of the sun and air, it would form a perfect mould of the foot; this is exactly what occurred in these early ages, and the hollow becoming subsequently filled by the deposition of new sediment, the lower stone retained the impression, while the upper one presented a cast in relief. Many fossil footmarks have been found in the rocks belonging to this period. It is evident from the fact of footmarks being found, that creatures capable of existing on dry land were formed about this time, and we accordingly find the remains of a new order--Reptiles. These animals, which now constitute but a small family among existing quadrupeds, then flourished in great size and numbers. Crocodiles and lizards of various forms and gigantic stature roamed through the earth. Some of the most remarkable are those which belong to the genus Ichthyosaurus, or fish-lizard, so called from the resemblance of their vertebræ to those of fishes. This saurian Dr. Buckland describes as something similar in form to the modern porpoise; it had four broad feet, and a long and powerful tail; its jaws were so prodigious that it could probably expand them to a width of five or six feet, and its powers of destruction must have been enormous. The length of some of these reptiles exceeded thirty feet.