i 4 o POPULAR SCIENCE MONTHLY. 



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. 



Classification of the Trades. 



The footprints may be classified under three groups, according to 

 the mode of progression or the posture of the maker. First the im- 

 pressions 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 addi- 

 tion 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 impres- 

 sions 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 impres- 

 sions 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 evi- 

 dently formed by living organisms, are certainly not those of verte- 

 brates or backboned animals, and, if one may judge from their appear- 

 ance, may have been made by creeping worms or by crab- or centipede- 

 like creatures or possibly by insects. 



Interpretation of the Footprints. 



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 



