Carboniferous Rocks of the Mississippi Valley. 2t)3 



plates of the dome without nodes, numerous, small, and 

 polygonal. 



Arms slender, round at base, becoming gradually flat- 

 tened and broader on the back above, at the widest part 

 more than double their width at base, composed of a 

 double series of very short plates ; each of these bears on 

 its lateral angles a short point or sub-spine, which becomes 

 gradually longer towards the flattened portion of the arms. 

 Column round, of medium size, composed near its junc- 

 tion with the body of strong plates. 



This species resembles in general expression A. turbina- 

 tus and its variety eleg-ans, (Geol. Rep. Iowa, pi. 11, figs. 1 

 and 5,) but differs in the arm formula, and in the flatten- 

 ing of the arms. The dome also of this is hemispherical, 

 while in that species it is depressed-convex, with strong 

 nodose plates. 



Geolog-ical formation and locality. Burlington lime- 

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

 Burlington. 



Actinocrinus eqnibrachiatus var. alatus. Body variable 

 in form, sometimes broadly turbinate, the depth below the 

 arms equal to half the transverse diameter, and sometimes 

 so short as to appear scarcely more than a stellate disc ; 

 the dome above the arm bases is usually nearly equal in 

 height to the body below the arm bases. In some speci- 

 mens the interbrachial and anal spaces are constricted half 

 the depth from the periphery to the centre, giving a deeply 

 pentalobate form, while in others they are scarcely de- 

 pressed. 



This species has the same arm formula and general 

 structure as A. eqjubrachiatus, McChesney, and the deep 

 constrictions between the rays give it a stellate or pentalo- 

 bate form, not noticed in typical specimens of the species. 

 This variety ditfers from A. simiosus (which it resembles 

 in the sinuosities or constrictions between the rays) in 



