ART. 5 FOSSIL FOOTPRINTS FROM MONTANA GILMGRE 3 



sharp claws although deep scratches are recorded by the toes espe- 

 cially of the pes where they dragged with each step. These are 

 clearly indicated on the left side of the trackway shown in Plate 1. 

 In this same section of trackway a smoothing out of the surface be- 

 tween the two lines of tracks suggest a belly drag leading to the 

 inference that the animal was a low, wide-bodied, short-legged quad- 

 ruped. The presence of a heavy tail is clearly indicated by an in- 

 termittent but deep median groove. 



The digital fornnila of four and five toes, respectively, on manus 

 and pes, a close similarity in arrangement and in the relative 

 lengths of the digits, these tracks have their closest affinities with 

 Ammohatrachu^ turhatans Gilmore recently described ' from the 

 Supai formation ( ?Pennsylvanian) of the Grand Canyon. Their 

 specific distinctness, however, is at once indicated by the much 

 greater width of the trackway, longer stride, and more open spac- 

 ing of the fore and hind foot impressions. The larger foot measure- 

 ments of A. twrhatans suggests a bigger animal than the one mak- 

 ing the tracks considered here, which accentuates the importance of 

 the differences enumerated. 



Comp)arison of the pes tracks shows the Montana ichnite to have 

 relatively longer and more slender digits with a more strongly 

 divergent digit five. Contrasting the manus tracks the sole of 

 Cat. No. 7635, U. S. N. M., is shorter, the lateral digits relatively 

 longer, and the angulation of the imprint with toes directed strongly 

 inward toward the center of the trackway, at once distinguishes 

 these imprints from the forwardly pointed toes of A. tv;rl)atans. 



The digital formula strongly suggests this new species to be 

 of amphibian lineage, but in the absence of confirmatory evidence 

 there seems no way of definitely determining this point. Neither 

 does a review of the known fauna of this formation give any hint 

 as to the class or the kind of animal to which these tracks might 

 be attributed. There are no amphibians Imown from the Fort 

 Union and the only reptiles are small lizards, turtles, crocodiles, and 

 the Rhyncocephalian C hmnpsosaurus. The two last mentioned with 

 their elongated toes of the hind feet would be at once ruled out, 

 though there is the possibility of their being Chelonian in origin. 



2 Smith. Misc. Coll., vol. 80, 1928, No. 2956, p. 8, pi. 2. 



