NO. 9 GRAND CANYON FOSSIL FOOTPRINTS GILMORE 3I 



some doubt as to the length of stride and the relative position of 

 the imprints as I'epresenting the normal gait. For example, the im- 

 print regarded as having been made by the manus falls behind and 

 slightly inside the line of the larger impression made by the pes. 

 In Allopus Uttoralis, as interpreted by Marsh, the positions are re- 

 versed. 



The less number of digits on the manus and greater on the pes 

 serves at once to distinguish this species from A. Uttoralis v^^ith its 

 five and four respectively. However, until the detailed structure of 

 the feet of this new form is more completely and positively known, 

 it appears best to refer it to this established genus. 



Alarsh regarded the tracks of Allopus as having been made by a 

 iarge labyrinthodont animal but the reduced number of digits in the 

 manus does not suggest their assignment to any of those forms known 

 from their skeletons. 



As noted above, at the time this series of tracks was made the 

 sand must have been thoroughly saturated with water as evidenced 

 by the fact that it flowed back into the tracks from both sides, leav- 

 ing a narrow longitudinal depression at the center where the flows 

 failed to merge. Furthermore, on the down-hill side of the imprints, 

 especially those made by the pes, the displaced sand has flowed down- 

 ward for a distance of 200 to 225 millimeters. Three successive flows, 

 one above the other, are registered, as indistinctly shown in figure 2, 

 plate II. 



These features raise the question as to how an aeolian deposit 

 of sand on a slope of 30 degrees could become so fully saturated 

 with water. It could hardly be accounted for by submergence for 

 under that condition the smaller tracks would hardly be registered 

 so distinctly as many of them are. It permits of the suggestion that 

 perhaps a further study of their origin, in the light of this new evi- 

 dence, may bring about a modified conception of the aeolian theory 

 as accounting for the original deposition of these sandstones. 



Genus PALEOHELCURA, new genus 



Generic characters. — Foot apparently tridactylous ; long axis of 

 each cluster of three placed strongly diagonal to direction of move- 

 ment. Tail trace. 



PALEOHELCURA TRIDACTYLA, new species 



Plate 12, fig. I 



Type. — Catalogue number 11,145, U. S. N. AI. Consists of a slab 

 about 560 mm. long, having a trail traversing the entire length. 



3 



