58 ]F. Ii. Jillson — YeTtehrate Footprints. 



bottom crawling. Detailed anatomical studies of these tracks 

 are planned, and the results with a series of measurements and 

 outline diagrams will be presented in a later paper. 



One of the striking features of this slab of sandstone casts is 

 the absence of the impressions of the feet of one side of the 

 animal in both series of tracks. This peculiarity is generally 

 to be accounted for in one of two ways. First, the animal may 

 '' back track " over one of the lines of fresh impressions with 

 the result that the superimposed body weight flattens out and 

 destroys the new undried tracks. In this case, however, the 

 double track of the last movement should be left undisturbed. 

 The specimen, however, does not show this double track. 

 Second, an explanation for the single series of impressions is 

 frequently found in the steeply dipping surface of muds on 

 which the animal crawled along. Such a condition would of 

 course give a series of good impressions on the lower side of 

 the body, but on the upper side of the animal there would be 

 very poor impressions or none at all. Tracks made under such 

 conditions might be expected to show pronounced evidence of 

 the fact in their increasing outward impression. A strong heel 

 impression ought also to occur. ISTeither of these character- 

 istics can be said to occur in these tracks, and as a result pres- 

 ent speculations as to the reason for the absence of the 

 complementary series of tracks have led to no definite conclu- 

 sions. Only the very slightest and most occasional tail groov- 

 ings could be detected, indicating very possibly the comparative 

 physical insignificance of the caudal appendage. 



The block herein described has been presented by the writer^ 

 at the suggestion of Drs. M. G. Mehl and K. L. Moodie, to the 

 Department of Geology of the University of Oklahoma. 



Yale University, New Haven, Conn. 



