emble
those of the Carboniferous epoch, belonging principally to the
Selachians and Ganoids. They have, however, approached somewhat toward a
modern pattern, the lobes of the tail being more evenly cut, and their
general outline more like that of common fishes. The gigantic marsh
Reptiles have become far more numerous and various. They continue
through several epochs, but may be said to reach their culminating point
in the Jurassic and Cretaceous deposits.

I cannot pass over the Triassic epoch without some allusion to the
so-called bird-tracks, so generally believed to mark the introduction of
Birds at this time. It is true that in the deposits of the Trias there
have been found many traces of footsteps, indicating a vast number of
animals which, except for these footprints, remain unknown to us. In the
sandstone of the Connecticut Valley they are found in extraordinary
numbers, as if these animals, whatever they were, had been in the habit
of frequenting that shore. They appear to have been very diversified;
for some of the tracks are very large, others quite small, while some
would seem, from the way in which the footsteps follow each other, to
have been quadrupedal, and others bipedal. We can even measure the
length of their strides, following the impressions which, from their
succession in a continuous line, mark the walk of a single animal.[10]
The fact that we find these footprints without any bones or other
remains to indicate the animals by which they were made is accounted for
by the mode of deposition of the sandstone. It is very unfavorable for
the preservation of bones; but, being composed of minute sand mixed with
mud, it affords an admirable substance for the reception of these
impressions, which have been thus cast in a mould, as it were, and
preserved through ages. These animals must have been large, when
full-grown, for we find strides measuring six feet between, evidently
belonging to the same animal. In the quadrupedal tracks, the front feet
seem to have been smaller than the hind ones. Some of the tracks show
four toes all turned forward, while in others three toes are turned
forward and one backward. It happened that the first tracks found
belonged to the latter class; and they very naturally gave rise to the
idea that these impressions were made by birds, on account of this
formation of the foot. This, however, is a mere inference; and since the
inductive method is the only true one in science, it