THE FOSSIL FOOTMARKS OF THE UNITED STATES. 177 



the Strutliionidae have that low organization which might have en- 

 abled them to live almost as early as reptiles, renders it not improb- 

 able that these birds belonged to that family. 



Though several facts as above stated afford a presumption that 

 these animals were birds, yet the new developments that have come 

 to my knowledge on this subject have left that opinion to rest 

 mainly on one argument, namely, the number of phalanges in the 

 toes ; which, if we admit two phalanges to have made but one tu- 

 bercular impression at the extremities of the toes, correspond to 

 the feet of birds, and to those of no other animals. I should once 

 have relied much on the mere fact that these animals were bipeds, 

 to prove their ornithic type, taking existing animals as the basis of 

 judgment. But, as I shall show farther on, we now know that 

 some of these biped animals were probably batrachians, — cer- 

 tainly not birds. The trifid character of the toes in front is an- 

 other character which in existing animals is confined to birds, with 

 two or three unimportant exceptions. But, in one of the species 

 to be described in this paper, we have a distinct tridactyle charac- 

 ter to the fore foot, and yet we can prove beyond all question that 

 it belonged to a quadruped. Upon the whole, though the evidence 

 of the ornithic character of this group is narrowed down, it is still 

 firm and substantial. 



Sub-Group. 



Characters. — Toes and claws winged. Other characters the 

 same as the general group. 



Genus II. ^THYOPUS. 



Foot tridactylous, expanded, winged : phalangeal impressions in 



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