528 NORTH AMERICAN INDEX FOSSILS. 



and nearly central anal tube. Ornamentation of dorsal cup similar 

 to that of Actinocrinus and Cactocrinus but coarser, with nodes 

 more conspicuous than the striations and often obscuring them. 

 Arms simple, closely crowded, and rather small. Ventral disk 

 convex, in form of a lo-rayed star, its inner floor strengthened by 

 braces. Column covered by rows of angular processes. 

 Mississippic (Burlington of Mississippi Valley). 



93. T. liratus (Hall). Mississippic. 

 Larger than T. umbrosus, with more elongate calyx and higher 



tegmen. Plates slightly convex, covered with radiating ridges 

 which, in parallel sets of three or four, unite at the middle of the 

 plates in nodes. 



Upper Burlington of Missouri, Illinois and Iowa. 



94. T. umbrosus (Hall). (Fig. 1855.) Mississippic. 

 Plates varying from nearly smooth to extremely nodose, but 



usually nodose in the middle and ridged at the margins. Column 

 small, covered with small overhanging processes. (Type of genus.) 

 Upper Burlington of Missouri, Illinois and Iowa. 



XXXVII. Steganocrinus Meek and Worthen. 



Like Actinocrinus in general structure but rays produced not 

 into mere lobes but into arm-like tubular extensions which rise 



to the full height of the calyx, 

 giving off armlets alternately 

 from opposite sides. There 

 are either one or two of these 

 brachial extensions to the ray, 

 depending whether they origi- 

 nate from the costals or from 



Fig. 1856. Steganocrinus sculptus, show- ^j^^ distichals. Arms given 



ing arms, V K- ( After Meek and Worthen. ) ^^ , . , ^ , , 



^ ^^ ^ ^ off at the sides of these trunks 



and much smaller. Specimens are usually found with these trunks 

 broken off. Mississippic. 



95. S. araneolus Meek and Worthen. Mississippic. 

 Resembling vS. pentagonus but only half or one fourth its size, 



more depressed, with more convex and strongly ridged plates. 

 Lower Burlington of Missouri, Illinois, Iowa and New Mexico. 



