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 seems to me that we should turn to the facts we have in our possession for the explanation of these mysterious footprints, rather than endeavor to supply by assumption those which we have not. As there are no bones found in connection with these tracks, the only way to arrive at their true character, in the present state of our knowledge, is by comparing them with bones found in other localities in the deposits of the same period in the world's history. Now there have never been found in the Trias any remains of Birds, while it contains innumerable bones of Reptiles; and therefore I think that it is in the latter class that we shall eventually find the solution of this mystery. [Footnote 10: For all details respecting these tracks see Hitchcock's _Ichnology of New England_. Boston, 1858. 4to.]