THE FOSSIL FOOTMARKS OF THE UNITED STATES. 161 



probably, when tracks of the same form differ a good deal in size, 

 they are made by different species, perhaps of the same genus. 

 Yet in view of the difficulty of proving this, I have avoided de- 

 pending upon this character alone, except, perhaps, in the single 

 case of the Steropezoum elegantius ; and as to this species I feel 

 no great confidence. Nevertheless, the tracks of many species, 

 and even genera, of living animals differ less than the S. elegans 

 and elegantius. 



21. Character of the integuments of the foot. — In a few instan- 

 ces, the ridges, furrows, pits, and anfractuosities of the animal's 

 feet are exhibited upon its tracks. As yet, however, I have not 

 been able to employ this character as a distinctive mark of 

 the nature of the animal, "^ partly, perhaps, because I have not 

 had opportunity to make extensive comparisons with the feet of 

 living animals on this point. 



22. Coprolites. — A few coprolites have been discovered of one 

 species of these animals, the Argozomn Redfieldianuin ; and Dr. 

 Dana has deduced from their analysis a beautiful argument to show 

 the nature of the animal that produced them. But its elucidation 

 has * been presented fully in the American Journal of Science, 

 Vol. XLVIII. p. 46. 



23. Means of distinguishing between the tracks of bipeds and 

 quadrupeds. — Persons who have never turned their attention to 

 this subject will probably suppose that this is a very easy matter. 

 But they would think otherwise should they attempt to make the 

 distinction ; especially in many cases of fossil footmarks, where 

 imperfect specimens are often all that can be obtained. And 

 even in studying the tracks of living animals, we shall sometimes 

 be liable to confound those of bipeds and quadrupeds. Thus the 

 dog, for instance, sometimes moves along without bringing all his 



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