CARBONIFEROUS SYSTEM. 213 



the same manner that the stone of the oolitic limestone at 

 Burlington does. These layers, however, as well as the 

 greater part of the others, make excellent lime, so that the 

 whole region occupied by this formation is well supplied with 

 that indispensable article. 



Fossils. The only vertebrate remains in this formation 

 are, as might be expected, those of fishes. These are so 

 frequently met with that with the perceptibly waning promi- 

 nence of the crinoidal characters of this limestone, as 

 compared with those of the Burlington epoch, they constitute, 

 next to the crinoids, the most prominent palseontological 

 feature of the formation. 



So far as yet discovered, the fish remains of the Keokuk 

 limestone all belong to the order Selachians, and consist of 

 both teeth and spines; the latter being referred wholly to 

 that order, because no undoubted Ganoid remains are found in 

 the formation. Some of the teeth and spines are of great 

 size, indicating that their owners probably reached a length 

 of twenty-five or thirty feet. 



None of those distinct layers of rock crowded with fish 

 remains, such as were before mentioned, as occurring in the 

 Burlington limestone, have been discovered in the Keokuk 

 limestone within the limits of Iowa, but such remains are 

 more generally distributed throughout the strata of the forma- 

 tion. The genera thus far recognized in this formation are 

 Cladodus, Petalorliynclius, Cliomatodus, Antliodus, Helodus, 

 Orodus, Psomimodus (?) Cochlwdus, Pcecilodus, Deltodits, 

 Sandalodus, Trigonodus y Oracantlius, and Drepanacantlius. 



Articulates. Two species of the genus PMllipsia are 

 the only representatives of this sub-kingdom yet found in the 

 Keokuk limestone in Iowa. 



Molltisks. No Cep7ialopods have yet been recognized in 

 this formation within our State; Gasteropods are rare, and 

 the Heteropod division of the latter class has not been 

 recognized. A few LamellibrancMates are found, and Bra- 

 cliiopods and Polyzoans are quite abundant. Among the 

 latter is the form supported upon a spiral axis, known by 



