Carboniferous Rocks of the Mississippi Valley. 305 



Geological formation and localily. Burlington Ihne- 

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



GENUS ZEACRINUS. 



Zeacrinus scoparius (n. s.). Body small, broadly cup- 

 shaped. Basal plates small, scarcely reaching beyond the 

 circumference of the column, forming by their union a 

 pentagon. Sub-radials small, height and breadth nearly 

 equal, three pentagonal and two heptagonal, (apparently 

 hexagonal from the straightncss of the lower sides). Ra- 

 dial plates four in the anterior ray and two in each of the 

 others. First radials pentagonal, proportionally large, 

 width twice the height. Second radial in the anterior ray 

 quadrangular, width and height sub-equal ; the third very 

 short, quadrangular ; the fourth pentagonal, like the sec- 

 ond in the other rays, but not so high. The last radials 

 support on their upper faces the arms, two from each ray. 

 The arms bifurcate on the sixth or eighth plate above, in 

 the postero-lateral and antero-lateral rays. The outer 

 branch of each division again bifurcates at about half the 

 length. In the anterior ray the arms bifurcate on the 

 eleventh or twelfth plate, apparently simple above. First 

 anal plate pentagonal, slightly elongated ; second and 

 third hexagonal ; form of fourth and fifth not determined. 

 Column round, slender, composed in the upper part of 

 short, equal plates, below often throwing out branches from 

 larger and thicker plates than the intervening ones. Sur- 

 face smooth or very finely granulose. 



This species resembles Zeacrinus eleg-ans and Z. ramo- 

 sus, (Iowa Geol. Rep. pi. 9, figs. 2 and 3,) but differs very 

 materially from either in the mode of growth and bifurca- 

 tion of the arms. 



Geological formation and locality. Burlington lime- 

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



JOURNAL B. S. N. H. 39 



