When we come to the consideration of actual footprints of vertebrate animals having limbs, the information we can obtain is of a far more definite character. This has already been referred to in treating of the first Air-breathers in a previous chapter. One very curious example we may close with. It is that of the celebrated "bird tracks" of the sandstone quarries in the Trias of Connecticut and Massachusetts. These tracks, of immense size, as much as eighteen inches in length, and so arranged as to indicate the stride of a long-legged biped, were naturally referred to gigantic birds, allied to modern waders. But when it was found that some of them showed a central furrow indicating a long tail trailing behind, this conclusion was shaken, and when in tracing them along, places were found where the animal had sat down on its haunches and the end of its tail, and had brought down to the ground a pair of small fore feet with four or five fingers, it was discovered that we had to deal with biped reptiles; and when the tracks were correlated with the bones of the extinct reptiles known as Dinosaurs, we found ourselves in the presence of a group of the most strange and portentous reptilian forms that the earth has ever known. Marsh has been enabled, by nearly perfect skeletons of some allied reptilian bipeds found in the West, to reproduce them in their exact forms and proportions, so that we can realize in imagination their aspect, their gait, and their gigantic proportions. Examples of this putting together of footprints and osseous remains of vertebrate animals are not rare in the history of geology, and show us how the monsters of the ancient world, equally with their human successors, could leave "footprints on the sands of time." The Dinosaurs which have left their footprints on the sandstones of Connecticut and Massachusetts are, however, greatly more numerous than those known to us by osseous remains. Thus footprints have the further use of filling up the imperfections of our geological record, or at least of pointing out gaps which but for them we might not have suspected. The remarkable inferences of Matthew already referred to, respecting cuttle-fishes in the Cambrian period, constitute a case in point. Footprints of Batrachians in the Carboniferous rocks were known before their bones. The strange hand-like tracks in the Trias were known before we knew the Labyrinthodon that produced them. We are still ignorant of the animals whose tracks in the old Potsdam sandstones we name Protichnites.