In the seas of the Carboniferous time, we notice that the ancient life of the earth is passing away.  Many creatures, such as the trilobites, die out, and many other forms such as the crinoids or sea lilies become fewer in kind and of less importance.  These marks of decay in the marine life continue into the beds just after the Carboniferous, known as the Permian, which are really the last stages of the coal-bearing period.

When with the changing time we pass to the beds known as the Triassic, which were made just after the close of the Carboniferous time, we find the earth undergoing swift changes in its life.  The moist climate and low lands that caused the swamps to grow so rapidly have ceased to be, and in their place we appear to have warm, dry air, and higher lands.

On these lands of the Triassic time the air-breathing life made very rapid advances.  The plants are seen to undergo considerable changes.  The ferns no longer make up all the forests, but trees more like the pines began to abound, and insects became more plentiful and more varied.

[Illustration:  FIG. 3.  CYCAS CIRCINALIS, AKIN TO HIGHEST PLANTS OF COAL TIME.]

Hitherto the only land back-boned animal was akin to our salamanders.  Now we have true lizards in abundance, many of them of large size.  Some of them were probably plant-eaters, but most were flesh-eaters; some seem to have been tenants of the early swamps, and some dwelt in the forests.

The creatures related to the salamanders have increased in the variety of their forms to a wonderful extent.  We know them best by the tracks which they have left on the mud stones formed on the borders of lakes or the edge of the sea.  In some places these footprints are found in amazing numbers and perfection.  The best place for them is in the Connecticut Valley, near Turner’s Falls, Mass.  At this point the red sandstone and shale beds, which are composed of thin layers having a total thickness of several hundred feet, are often stamped over by these footprints like the mud of a barnyard.  From the little we can determine from these footprints, the creatures seem to have been somewhat related to our frogs, but they generally had tails, and, though provided with four legs, were in the habit of walking on the hind ones alone like the kangaroo.  A few of these tracks are shown in the figure on this page.

[Illustration:  FIG. 4.  FOOT-PRINTS, CONNECTICUT SANDSTONES.]

These strange creatures were of many different species.  Some of them must have been six or seven feet high, for their steps are as much as three feet apart, and seem to imply a creature weighing several hundred pounds.  Others were not bigger than robins.  Strangely enough, we have never found their bones nor the creatures on which they fed, and but for the formation of a little patch of rocks here and there we should not have had even these footprints to prove to us that such creatures had lived in the Connecticut Valley in this far-off time.