of Burlington, loiva, and its Vicinity. 225 



In the Keokuk limestone, they reach a culmination of 

 rudeness and extravagance of form, which constitute a 

 prominent feature of most of the genera found in that 

 rock. 



Few if any species of these fossils are common to both 

 beds of the Burlington limestone, as before remarked, and 

 it is hardly probable that any will be found to be common 

 to the Burlington and Keokuk limestones, yet all the 

 genera discovered in the three beds are common to all, 

 except the genera Codaster and Spatangus (?), which have 

 been found thus far only in the Burlington limestone. 

 There are, moreover, certain groups, or subdivisions of the 

 more prominent genera, that comprise species from each 

 of the three beds. 



The conformity of all the beds exposed at Burlington, 

 together with the uninterrupted range of the fossils of the 

 lower beds until they reach a point in No. 7 where they 

 are gradually displaced by the Carboniferous species, with- 

 out any abrupt change in the lithological character of the 

 imbedding rock, seems to preclude the possibility of a 

 hiatus existing between the Devonian and Carboniferous 

 beds. From the evidence which the rocks themselves 

 present, backed by the opinions of the best geologists of 

 the country, we feel as much warranted in referring the 

 lower beds to the Chemung group of New York, as we 

 do in referring the upper ones to the Carboniferous lime- 

 stone ; notwithstanding the fact that another member of 

 the Devonian system overlies the Chemung group in 

 New York, the thickness of which is greater than that of 

 the entire development of both the Devonian and Carbo- 

 niferous systems in the Mississippi Valley, and underlies 

 the rocks which are there considered the equivalent of the 

 Carboniferous limestone of the West. 



An interesting question thus presents itself as to the 

 actual equivalency of these rocks, which it is not proposed 



JOUKXAL B. S. K. H. 29 



