Page:Popular Science Monthly Volume 66.djvu/144

140 that we owe the founding and development of a study which soon rose to the dignity of a science. The interest of President Hitchcock ceased only with his death in 1865, and his tireless energy resulted in the bringing together of a magnificent collection of track-bearing slabs and in the description and publication of more than one hundred species of track-making organisms.

The impressions, while mainly of footprints, also give evidence of dragging tails and other portions of the body or of the armoring and texture of the skin. Other attendant phenomena have left their records also, such as the rainprints of a summer shower, ripple and other beach marks, and the shrinkage cracks which are found in sun-dried mud. These are preserved with wonderful fidelity and minuteness of detail.

The footprints may be classified under three groups, according to the mode of progression or the posture of the maker. First the impressions of true bipeds, those whose very bird-like imprints gave the popular name of bird tracks to the phenomena as a whole. Here may be, though rarely, the trace of a dragging tail, but with this exception the hind feet alone leave their record on the rocks.

The second group is as truly bipedal as the first in gait, and the footprint is usually as bird-like as before, but there may occur in addition to an occasional tail trace the impressions of little five-fingered hands placed just in front of those of the feet, while from the rear of the footprint often extends that of a long slender heel. These impressions of the hand and heel were only formed when the creature rested, for, except for differences in shape, the footprint of the moving form can not be distinguished from those of the first group.

The third are the true quadrupeds, not alone in resting posture, but during locomotion as well, in some cases with feet whose impressions are full of character, which gives some clue to their maker's affinities; others slender toed and obscure, whose true relationship with known creatures it is difficult to conjecture.

There remains yet a fourth group of imprints, which, while evidently formed by living organisms, are certainly not those of vertebrates or backboned animals, and, if one may judge from their appearance, may have been made by creeping worms or by crab-or centipedelike creatures or possibly by insects.

Even to the layman the mounted skeleton of an extinct animal is something tangible which the imagination can readily clothe with flesh and endow with life; but the first impression which the mind receives of the footprints is like that of an ancient inscription whose characters