1867.] 



SALTER — TRACKS OF PTERASPIS. 



335 



nor equidistant, nor alike in their depth or direction; nor is the 

 number of imprints the same in each track. So that we are at once 

 relieved of the supposition that a single individual with three series 

 of appendages could have left this as its compound track ; and we 

 are at liberty to treat each as independent, though their main direc- 

 tion is the same — namely, along the line of the tide-wash, as indi- 

 cated by the stripe A to A*, B to B*, &c. 



rig. 1. 



Two of these, at least, are the track of the same creatui'e, repeated 

 in its double journey from water to shore or vice versa ; and I think 

 (but am not sure) the third track is e\idence of another advance of 

 the same individual in a parallel direction. We may suppose, then, 

 the effort of the animal (whatever it was) to reach the deeper water 



