678 GEOLOGICAL SURVEY OF IOWA. 



CMNOIDEJ! AND OTHER ECHINODERMATA OF THE KASKASKIA 



LIMESTONE. 



THE Crinoideaa most characteristic of this rock are the 

 ZEACRINUS of TROOST, and the SCAPHIOCRINUS, already described 

 under the Burlington limestone ; genera having a very simi- 

 lar fundamental structure, but differing essentially in general 

 form and habit. Several species of PLATYCRINUS and CYATHO- 

 CRINUS are likewise known in this rock ; but so far as my 

 observation has extended, they do not constitute an equal 

 proportion of the whole, when compared with the Crinoidese 

 of the other rocks of the Carboniferous period. Far from 

 having made a selection for contrasting with those of the 

 preceding formations, I have taken the most common and 

 prevailing forms ; and it is extremely interesting to compare, 

 in the successive epochs of the Carboniferous period, so well 

 marked in physical characters over so wide an area of the 

 Mississippi plateau, the development and expression of this 

 family of fossils. 



It is likewise in this rock that the Genus PENTREMITES 

 becomes so conspicuous *, and from this formation were ob- 

 tained the typical species of the genus, described by SAY. 

 Although at the present time there are perhaps an equal 

 number of species known in the Burlington limestone, they 

 are mostly different in form, and far from being as numerous 

 individually as those of the Kaskaskia limestone. 



GENUS SCAPHIOCRINUS. 



( For generic description, see pp. 549 and 550 of this report.) 



