458 PALEONTOLOGY OF ILLINOIS. 



presenting a curious upward flexure or geniculation in tlie 

 middle, so as to give a zigzag appearance to the transverse 

 sutures between them. Often at this point of flexure, one 

 of the pieces, on each side of the mesial longitudinal zigzag 

 suture, is divided into two, so as to show a tendency to 

 break up into four alternating rows of very small inter- 

 locking pieces to each arm. Pinnule in close contact, and 

 composed of joints from three to four times as long as wide 

 on their outer surface. 



Vault unknown.* Surface nearly smooth or only finely 

 granular. 



Hight of body to top of first radial pieces, 0.84 inch; 

 greatest breadth immediately below the sinuses for the 

 second radials, 1 inch; length of arms from the first divi- 

 sion of the rays to their extremities, 3.30 inches; breadth 

 of same near the middle, 0.12 inch. 



The body of this species can be distinguished from that of any of the 

 other smooth species of this genus yet known, from the Burlington 

 beds, by the protuberant character of the facet for the attachment of its 

 column, and its slightly tumid basal pieces. Its most marked character, 

 however, is the peculiar geuiculation, and duplication of its arm pieces, 

 on each side of the mesial line. This character is so marked that even 

 a fragment of one of its arms, from near the middle, could be readily 

 distiu guished from a corresponding part of any of the other species 

 known to us. 



We have figured the beautiful specimen represented on plate 3, 

 because it shows the arms and body complete; while the specimen 

 originally described by Prof. HALL consisted merely of a detached 

 base. 



Locality and position Upper bed of the Burlington group, of the 

 Lower Carboniferous, at Burlington, Iowa. The specimen figured is 

 numbered 213, in Mr. WACHSMUTH'S collection. 



*Mr. WACHSMUTH thinks this species has a simple lateral opening in the vault. 



