12 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. ']'] 



After careful consideration of the characters briefly reviewed above, 

 the weight of evidence seems to favor the reference of the present 

 specimen to the genus Nanopus. Three species have previously been 

 described, A'', caiidatus Marsh, N. obtusus Matthew, and N . quad- 

 ratus Matthew. 



The specific distinctness of Nanopus incrriaini from N. caiidatus 

 is shown by the more slender form of the digits terminated by sharp 

 claws, relatively shorter soles, smaller size, and lack of tail trace. 

 The last mentioned feature is probably unimportant, for the dragging 

 of the tail must often have depended on the occupation of the animal. 

 The lack of claws, strongly divergent outer toe, unequal length of 

 the two middle digits of the pes, heavier digits, quadrate form of 



Fig. 7. — Barillopiis arcfw.? Matthew, a, Left hindfoot; b, left fore- 

 foot. About twice natural size. (After Matthew.) 



the sole, and forefoot placed behind the hind, effectually distinguish 

 the Canadian species from Nanopus merriami. 



No tracks referable to this species were found in the higher track- 

 bearing levels of the Coconino sandstone, but larger collections 

 are necessary before one can be assured that they are confined to 

 the lowermost part. 



Marsh was of the opinion that Nanopus caudatus in all probabil- 

 ity favored a reference to the Amphibia, but the nature of the animal 

 indicated by the impressions of N. merriami, although a matter of 

 conjecture, might with equal probability be considered reptilian. 



The species is named for Dr. John C. Merriam, president of the 

 Carnegie Institution of Washington, who was instrumental in bring- 

 ing about the arrangements whereby this excellent series of foot- 

 prints was acquired for the national collections. 



