sic 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.

[Illustration: FIG. 5. FOOT-PRINT, TURNER'S FALLS.]

But these wonderful forms are less interesting than two or three
little fossil jaw-bones that prove to us that in this Triassic time
the earth now bore another animal more akin to ourselves, in the shape
of a little creature that gave suck to its youn