576 GEOLOGICAL SURVEY OF IOWA. 



Summit composed of numerous convex plates, symmetrically 

 arranged from the arm-bases to the subcentral plate : apicial 

 plate tuberculiform. Standing above and near the anal aper- 

 ture there is likewise on each side an elongated tubercle, 

 which rises from the upper side of each of the postero-lateral 

 arms, the three being nearly equidistant from the aperture. 

 ARMS two from the lateral and antero-lateral rays ; two 

 pairs from the right, and one pair and a single one, or 

 three arms, from the left postero-lateral ray. Surface finely 

 granulose. 



This species resembles the Jl. symmetricus in general form, though proportionally 

 broader and less convex on the summit : the base also is much larger, and the 

 number of arms quite different. In the prominent anal ridge, and the tubercles bor- 

 dering the anal opening, it resembles Jl. tricornis ; from which it differs essentially 

 in the smoother surface of plates, number of arms, and other characters. 



Fio. 83. 



C2 T )Q ^ 

 JC/xV^(^/O _/^j The accompanying diagram illustrates the form and 



fRO^-^-C-v^l\v3 A-Xj proportion, as well as the number of plates to the bases 



>A/ A QPTV /-V'S 



of the arms. 



Od ^85 



Geological formation arid locality. In the Burlington limestone : Bur- 

 lington, Iowa. 



Actinocriiius cornigerus ( n. s.). 



PLATE IX. FIG. 12 a, b, c. 



BODY urnshaped. Basal plates large, concave below, 

 spreading out in a broad thickened margin much beyond the 

 base of the radial plates, and overhanging the column. First 

 radial plates hexagonal, twice as wide as long. Second radial 

 plates quadrangular, and in the same proportions as the first. 

 Third radial plates somewhat pentagonal or irregular, very 

 short. First anal plate hexagonal, supporting on its upper 

 truncated edge another hexagonal plate, and on the sloping 



