44 ICHNOGRAPHS OF THE 



semifluid mud, there is a settling or flowing back into the track, that obliterates 

 the phalangeal impressions, and converts the impress of the toes into grooved 

 lines merely. If this variety of impress happens when the strata are thin and 

 fissile, the foot often penetrates several laminae, and leaves a rude outline of the 

 toes in each, often as many as five or six in succession, and by splitting the 

 strata asunder, each shows a linear impression of the toes, and the lowermost of 

 the series sometimes retains a rude form of the articulation of the toes. Some- 

 times, too, the upper series exhibits the articulations partially, but as a general 

 rule there is a total absence of the true markings of the foot. In some locali- 

 ties the footprints are ruined in this way, the entire surfaces of the fossil strata 

 being cut up into innumerable impressions. These linear footprints often occur 

 upon strata that are remarkable for perfect impressions also, the first being 

 impressed before, and the latter after, consolidation ensued. Strata often occur 

 with footprints of different degrees of perfection, in proportion as the original 

 clay was soft or hard. 



It requires some discrimination to distinguish between the genuine impression 

 and that modified by changes occurring subsequently to the planting of the foot; 

 but if it be recollected that the perfect impression should invariably bear the 

 markings of the joints, claws, etc., no error need be committed on this point. 

 It is not difficult to believe that the examples upon the Plate are due to birds, 

 but it is impossible to prove them to be so. They have the trifid character 

 and general symmetry of the feet of birds, and there the comparison ends. 

 Their imperfection does not admit of specific description. It is not possible to 

 distinguish the innumerable forms that these linear imprints assume. Those 

 delineated upon the Plate are fair specimens of this class, but others occur, so 

 defective as to bear little resemblance to footprints. 



PLATES XXI. AND XXII. 



These were drawn from the footprints of existing reptiles, and are given for purposes 

 of comparison. Those on Plate 21 represent the tracks of a living alligator. 



PLATE XXIII. 



In the collection of Mr. Roswell Field, at Turner's Falls, is a stone having nine 

 tracks of an animal undoubtedly of the same species as the one of which footprints are 



