FAUNAL EVIDENCE AND CORRELATION. 267 



very base. I feel.little disposed to defer the evidence of other species to Chonetes 

 mesoldbtis, though the range of the latter in Kansas is confirmed by many observa- 

 tions. Compromising- the somewhat conflicting evidence of the Rico genera and 

 species and holding in view that of the Hermosa fauna, I think that the age of the 

 Rico can hardly be earlier than the lola limestone (No. 16) nor older than the Esk- 

 ridge shales (No. 37). I am disposed to think that its age is somewhat intermediate 

 between these two limits, or about the horizon of the Deer Creek, Hartford, and 

 Howard formations, though this opinion is set down with diffidence and with a reali- 

 zation of the ver^^ slender evidence upon which it rests. 



Comparison between the faunas of Colorado and those of the Mississippi Valley 

 with a view to correlation presupposes that both areas were connected with the same 

 zoological basin, so that the same faunas had access to each simultaneouslJ^ The 

 occurrence in Colorado of so many species identical with those in the Mississippi Valley 

 leaves little doubt that this was the case. If land existed in Colorado during Upper 

 Carboniferous time, or between Colorado and the Mississippi Valley, it could hardly 

 have been sufiiciently extensive to form a barrier to the migration of species from 

 one to the other, or to shut ofl' either area so that it was subjected to peculiar 

 conditions checking or diverting the course of faunal evolution. 



