!(>(> PALAEONTOLOGY OF ILLINOIS. 



tributed at greater intervals and are longer. The two species 

 also differ in their style of ornamentation. The calyx plates of 

 A. nodobraclriatus, are strongly convex, the suture lines deeply 

 excavated, and the radiating ridges which cover the plates are 

 not continuous. The ridges, though generally well defined along 

 the edges of the plates, either disappear entirely in the central 

 portion, or become indistinct even in the best specimens, and 

 appear as if they had been worn off by attrition. The specimens 

 are light colored with a slight brownish tint. 



Dorsal side of the calyx bell-shaped, sides moderately convex, 

 spreading on approaching the arm bases. Basals short, ex- 

 panding into a conspicuous rim with a sharp edge, which pro- 

 jects beyond the limits of the column. Interbasal sutures 

 deeply notched, making the basal disk distinctly tripartite. 



Primary radials decreasing in size upward; the first plate as 

 high or higher than wide; the second wider than high, hexangu- 

 lar, exceptionally pentangular, or even quadrangular when not 

 in contact with the interradials of the second range; the third 

 twice as high as wide. Secondary radials 1X2X5, almost as 

 large as the preceding axillaries. There are 1X2X10 tertiary 

 radials; the inner ones axillary, giving off t\vo arms, one from 

 each side; the outer ones truncate; supporting a single arm; 

 thus giving six arms to every ray or thirty to the species, with oc- 

 casional exceptions. The higher radials retain the embryonic 

 character of arm plates; they are cuneiform, and immediately 

 on becoming free change into two series of transverse pit M-OS. 



Arms equidistant, crowded at their bases, and taking an out- 

 ward direction ; they are very long, rounded on the back, taper- 

 ing upward, the tips deeply infolded, sometimes reaching down 

 to the calyx. They are biserial, the places of each series united 

 longitudinally by straight sutures. Upon the back of the arms 

 there is a series of large tooth-like nodes, whose sharp points 

 are directed upwards. They are larger, but not so regularly 

 arranged as those of the preceding species, and are farther 

 apart, occurring usually either on the sixth, eighth or tenth 

 joint. These nodes are not confined to one plate only, but 

 generally extend over two or more, sometimes covering even a 



