THE FOSSIL FOOTMARKS OF THE UNITED STATES. 165 



plied to fossil footmarks, however, the only result of the mistake 

 would be to lead us to describe too few genera; that is, to confound 

 more than one genus under one name, — an error far more venial 

 in natural history than its opposite. 



6. To what species. And since a specific description embraces 

 the whole animal, — or, in the present instance, its whole track, — 

 I think we can be more sure of being led right by these characters 

 as to species, than as to genera. 



Adopting these principles as my guide, I have arranged the fos- 

 sil footmarks of the United States, mainly of New England, ac- 

 cording to the following synopsis. I have no great confidence in 

 the arrangement into groups, except in a few instances ; and only 

 in a few cases have I ventured to attach names to the groups. In 

 the genera and species I have more confidence. 



Group I. (STRUTHIONIDiE ?) 



Genus 1. BRONTOZOUM (BpoVr^f and fio./). 



1. B. giganteum. 



2. B. Sillimanium. 



3. B. loxonyx (Xofo's, oblique, and oi/u|, a claw). 



4. B. expansum. 



5. B. gracillimutn. 



6. B. parallelum. 



Genus 2. ^THIOPUS {aWma, fuUca, and n-oOf). 



1. JE. Lyellianus. 



2. M. minor. 



Group II. 



Genus 3. STEROPEZOUM (STep67rvs and fSov). 



1. S. ingens. 



2. S. elegans. 



3. S. elegantius. 



