Carboniferous Rocks of the Mississippi Valley. 283 



lose and regularly arranged, one over the centre of each 

 ray, and the largest at the top is sub-globose and sur- 

 rounded by a ckcle of smaller plates. The plates of the 

 flattened base are slightly convex ; surface of all the body 

 plates granulose or striato-granulose. 



This species differs from all others in the abrupt depres- 

 sion in the base. 



Geological formation and locality. In cherty layers in 

 the upper part of the Burlington limestone. Brush Creek 

 and Flint River, Iowa. 



Agaricocrinus [Amphoracrinus) cornigatus (n. s.). Body 

 irregularly flattened, discoid to the arm bases, radial series 

 and arm bases projecting below ; the upper part of the 

 interradial spaces somewhat contracted, and the centre 

 of the disc or base of the cup impressed ; the articular 

 facet for the attachment of the column more deeply de- 

 pressed. Basal plates small, in the bottom of the column 

 cavity, and concealed by the first plates of the column. 

 First radial plates hexagonal and heptagonal, nearly three 

 times as broad at the upper as at the lower margins. Sec- 

 ond radial plates short, quadrangular, width twice the 

 length. Third radial plates short, broad, pentangular, 

 wedge-form above, supporting on each side a series of two 

 or more short, broad plates, on which rest the first arm 

 plates. 



First interradial plates ten or eleven-sided, irregularly 

 ovate, attenuated above. Sometimes only one plate in the 

 second range, narrow and elongate ; in other spaces the 

 adjacent arm plates unite above the first interradial plate. 

 First anal plate seven-sided, ( or six-sided, from the 

 straightness of the lower side,) succeeded by three small- 

 er plates in the second range. Dome and arms unknown. 

 Surface of plates marked by small, irregular pits near the 

 margin, with elevated ridges between. These pits and 

 ridges produce a strongly corrugated appearance. The 



