A knowledge of the form of the extremities of animals has been of great service to palæontology in another way—in proving the existence of certain species of animals at given periods of the earth’s formation, from the print of their footsteps left upon the sand or other material of the strata while it was yet in a soft state. Such traces were first observed by Dr. Duncan in Dumfries-shire. On examining a sandstone quarry, he found these prints not on one only, but on several successive layers of the stone; so that they must have been made at periods distant from each other. Similar impressions have been since observed in the Forest marble-beds near Bath, at Hessburg in Saxony, in the State of Connecticut, and in some other parts of the world. “The marks found in Dumfries-shire, of which there were as many as twenty-four upon a single slab, formed as it were a regular track, with six distinct repetitions of each foot, the fore and hind feet having left different impressions, and even the marks of the claws being discernible. They appear to have been made by some animal of the tortoise kind. At Hessburg the impressions were discovered in quarries of grey and red sandstone alternating; and the marks were both larger and more distinct than those found in Scotland. In one the hind foot measured twelve inches in length, and the fore foot always appeared much smaller than the hind. From this circumstance, and from the distance between the two being only fourteen inches, it is conjectured that the animal was a marsupial like the kangaroo, and it has been termed by Dr. Kaup _Chirotherium_, from the supposed resemblance of the four toes and turned-out thumb to a hand. In the State of Connecticut, near Northampton, footsteps differing exceedingly in size have been found in inclined strata of sandstone. They were evidently made before it assumed its present position. The marks are always in pairs, and the tracts cross each other like those of ducks on the margin of a muddy pond. One is of the length of fifteen or sixteen inches, and a feathery spur or appendage appears to have been attached to the heel, eight or nine inches long. The distance between the steps is proportioned to their length, but in every case the pace appears to have been longer than that of the existing species of birds to which they approach nearest—the ostrich, and the animal must consequently have been proportionably larger. How much larger he was than the ostrich may be gathered from this, that the large African ostrich has only a foot ten inches long, less than two-thirds that of this bird, and yet stands nine feet high. These proportions would give a height of fourteen feet to the extinct animal.”[4] The characters afforded by the foot have recently enabled Mr. Strickland to determine that the dodo, a bird now extinct, is not related either to the gallinaceous birds,—the vulture or the ostriches as some have conjectured,—but is closely allied to the pigeons.