NO. 8 



GRAND CANYON P^OSSIL FOOTPRINTS GILMORE 



II 



bearing sandstone in the middle Snpai appears to be identical, but 

 the extreme hardness of the sandstone resisted all attempts to collect it. 

 These were the only trails of this particular kind ol)served in many 

 days of prospecting in this formation. The trail illustrated (see fig. 5) 

 is impressed on the surface of a pinkish sandstone and has a length 

 of approximately 370 mm. The specimen, which is the positive slab, 



\ ^1 



Fig. 5. — Invertebrate trail from Supai formation. 

 U. S. N. M. About 4 natural size. 



No. 11,740, 



has been cast, and the replica affords all the evidence of the original. 

 The trackway as a whole is asymmetrical, brought about, it would 

 seem, by the failure of the appendages of the right side to leave their 

 imprints. Two faint impressions on the right side near the midlength 

 lend support to this view. (See pi. 4, fig. i.) These are elongated 

 depressions set diagonally to the line of movement, and in nearly every 

 way conform to those forming the outer row on the left side of the 

 trackway. If this supposition is correct, the normal trail would have a 

 width of about 46 mm. The longitudinal row of tracks of the left 



