Carboniferous Rocks of the Mississippi Valley. JJ03 



idly expanding, and differs materially in the proportions of 

 the plates, especially the arm plates. It resembles in form 

 and proportions some of the Homocrini in the lower for- 

 mations. 



Geological formation and locality. Bm-lington lime- 

 stone. Burlington, Iowa. Collection of C. A. White, 



Pole riocr inns Barrisi (n. s.). Body turbinate. Basal 

 plates unknown. Sub-radial plates of the usual form 

 and proportions, sub-equal. First radial plates wider than 

 high ; articulating scar for the attachment of the arms 

 very small. One anal plate only observed. 



Arms slender, elongated, bifurcating several times, com- 

 posed of cylindrical joints, which are once and a half as 

 long as wide, and deeply grooved for the attachment of 

 tentacula. One of the postero-Iateral rays shows four 

 joints below the first bifurcation of the arm, the others are 

 obscure in this part of the specimen. 



Surface of body plates marked by a series of sharp ra- 

 diating ridges, one from the centre of each side of the 

 plate joining in the centre of the plate, and the interme- 

 diate spaces occupied by similar ridges extending from 

 each margin and converging so as to meet others from the 

 adjacent margin on the line from the pentre to the angles 

 of the plate, forming a series of gradually diminishing 

 rhombs; or, taking the lines upon the adjacent plates, 

 forming a series of gradually diminishing triangles, one 

 within another. 



This species more nearly resembles P. radiatns of Aus- 

 ten, than any American species which I know ; but the 

 radiating ridges are more nearly equal in strength and 

 more numerous than represented in the figures of Austen 

 or De Koninck, while there is no space near the angles of 

 the plates free from these ridges, as shown by these au- 

 thors. 



Geological formation and locality. Burlington lime- 



