THE FOSSIL FOOTMARKS OF THE UNITED STATES. 141 



It has been thought by some, that the difference between these 

 two sorts of tracks was the result, not of a difference in the feet of 

 the animals, but of the state of the mud impressed by them ; that 

 is, in the case of the narrow-toed tracks, the mud is supposed to 

 have slid back so as to narrow the impression. That the mud 

 did thus more or less collapse, in some cases, is evident. But 

 it will not, in my opinion, explain the broad difference between 

 these two sorts of tracks ; and for the following reasons. 



This supposition regards all the tracks as made by thick-toed 

 animals. If so, only the mud near the surface would slide back 

 and bring the margins of the impressions near together ; and 

 where that impression extends some inches in depth, as it does 

 sometimes, the inferior layers of the narrow-toed tracks ought to 

 be broader ; but this is never the case to any great extent. As 

 the track is at the surface (in respect to the width of the toes), so 

 it is on all the layers. Secondly, no sliding back of the mud, 

 after a thick-toed animal trod upon it, would obliterate the distinct 

 phalangeal protuberances, without distorting the track in other re- 

 spects. Thirdly, both sorts of tracks are not unfrequently found 

 upon the same layer of rock, as at Wethersfield, Northampton, and 

 Gill ; and each exhibits its peculiar characteristics. Fourthly, the 

 feet of living animals exhibit similar differences. Compare, for 

 instance, the feet of the Struthionidse with those of the Ardea, 

 or Charadrius ; or those of the thick-toed frogs with those of the 

 Iguana, &c. Why, then, should we not look for diversities equally 

 great among the fossil animals ? 



This character is a very important one in the classification of 

 these animals. The group which I have denominated Struthi- 

 onidae is beautifully distinguished from all others in this way ; 

 they being all pachydactylous. For a long time I had supposed 



