30 ICHNOGRAPHS OF THE 



the foot, its inclined position causing a projected impression from the heel, corre- 

 sponding in length to the depth to which the foot itself penetrated. The fourth 

 toe sometimes leaves its impress standing out transversely from that produced 

 by the sinking leg. Such multiplied impressions, although they illustrate tbe 

 history of the footprints, do not convey accurate ideas of structural forms, and 

 their imperfection excludes their specific arrangement. 



The perfect footprints, then, were impressed when the consistence of the stratum 

 was such that it prevented the sinking of the foot, yet yielded sufficiently to 

 take its accurate form; and some of the finest examples therefore are those in 

 which the impress is superficial (PI. 11). Sometimes the resistance opposed to 

 the weight of the animal produced a flattened impress (PI. 10, fig. 1). When 

 the drying process had not reached that point, the articular impress is concave 

 and tuberous (PL 10, fig. 2), and in proportion as the stratum was softer the 

 impress becomes more imperfect, until it can barely be detected, or it results 

 in mere fissured lines that represent the toes. 



Another form of imperfect footprints occasionally happens. If a slight earthy 

 deposit falls upon and adheres to a surface having a footprint, the general aspect 

 of this footprint will be preserved by the adventitious layer, but there will be 

 an entire absence of the finer markings of the foot. If such deposits accpuire 

 considerable thickness, all signs of the original impress disappear, and the result 

 is a stratum with a relief cast upon its inferior surface. If also the layer upon 

 which the bird trod be very thin, the impress of its foot may be transmitted 

 to the layer beneath; but the impression thus resulting will not be accurate. 

 It is the stratum upon which the bird steps, and that only, that reflects the 

 true features of the foot : the impress presents a smooth, compressed surface, 

 a clear and unequivocal expression, and sharp and well defined ridges, that sepa- 

 rate both the toes and articulation. The perfection of the footprint is in pro- 

 portion to the smoothness, fineness, stability, and plastic property of the stratum, 

 and when these conditions concur the impress is absolutely perfect, whether of 

 the feet of colossal birds or of insects; while, on the contrary, footprints impressed 

 upon soft mud or beneath water will be so modified as simply to be distinguished 

 by linear depressions that represent the several toes. 



If the direction in which the animal moved be parallel to the margin of the 

 water line, there is no essential difference iu the character and perfection of the 

 consecutive footprints; but if the direction be transversely, or into or from the 



