606 GEOLOGICAL SURVEY OF IOWA. 



CRINOIDEJ; OF THE KEOKUK LIMESTONE. 



THE fossils of this family in the Keokuk limestone, al- 

 though of the same genera as those of the Burlington lime- 

 stone, nevertheless present in general aspect a striking 

 contrast with those of that formation. The species of PLATY- 

 CRINUS are far less abundant in proportion to the other forms ; 

 while ACTINOCRINUS is presented in numerous strong, rude 

 and extravagant forms, attaining apparently its maximum 

 of development as far as regards size, strength, and excess 

 of ornament and appendage : at the same time, while the 

 number of species is less than in the Burlington limestone, 

 certain forms of the ACTINOCRINUS of that formation are fully 

 represented in the Keokuk by analogous forms. The genera 

 CYATHOCRINUS and POTERIOCRINUS are more fully developed 

 than in the Burlington limestone. The other genera already 

 noticed are, with few exceptions, represented in the Keokuk 

 limestone by distinct and characteristic species. 



Pentremites wortheni ( n. s.). 



PLATE XV. FIG. 1. 



BODY elongate subfusiform ; length of base and distance 

 from base of radials to base of pseudo-ambulacral area, and 

 length of the latter, about equal to each other : base trian- 

 gular. Basal plates very gradually spreading ; upper margins 

 concave for the reception of the radials. Radial plates long, 

 narrow, almost linear; branches lanceolate. Interradial plates 

 very small, acutely lanceolate, reaching very nearly to the 

 summit. Pseudo-ambulacral areas narrow, linear, extending 



