ACTINOCRINID^. 613 



of nearly equal size, and as large as the first costals. Occasionally there is 

 another small elongate jDlate between the arm bases, but more frequently 

 this is absent, and the palmars are in contact laterally. Anal interradius 

 formed of six to seven plates; the upper one very elongate, restino- between 

 the palmars. Interdistichals one. Ventral disk covered by numerous small 

 and extremely irregular pieces, which enclose somewhat larger spinous 

 plates, probably representing the orals, and radial dome plates of a first, 

 second, and third order. Column large ; axial canal very wide and obtusely 

 pentangular ; the joints are long, the third from the calyx wider and longer 

 than the surrounding ones, in some specimens almost twice as wide, and 

 having a knife-like edge. 



Horizon and Locality. — Lower Burlington limestone ; Burlington, Iowa. 



Txjpe in the University Museum at Ann Arbor, Mich. 



Bemarks. — This species is well characterized by the form of the calyx, 

 and by the smooth and heavy arms without nodes or spines. It was described 

 by Hall as having six arms in each ray, which is certainly incorrect. We 

 have examined seven specimens, in all of which the antero-lateral rays have 

 but five arms, while the others have six. 



Cactocrinus obesus Keyes (MS.). 



Plate L V. Figs. 9a, b. 



A large species of the type of C. clarus, but more elongate, much more 

 nodose, and having but four arms to the ray. Calyx once and a half as wide 

 as high, broadly and sharply truncated at the base; the plates thick and 

 heavy. Dorsal cup gradually expanding to the top of the distichals, then 

 bending abruptly outward. The plates of the dorsal cup extremely heavy, 

 highly elevated, and produced into rather sharp, round nodes, especially the 

 radials, which are decidedly more prominent than the succeeding plates ; the 

 suture hues traversed by short, obscure ridges. Ventral disk high-conical, 

 as high as the dorsal cup, its sides but very slightly convex ; orals and first 

 radial dome plates wedge-shaped, and produced into very long, sharp, broadly 

 transverse tubercles, which stand out conspicuously (5 to 7 mm.) from the 

 tegmen. 



Basal cup short but wide, much wider than the column; sub-cylindrical; 

 the lower end abruptly truncated, forming a sharply angular edge at the 

 bottom; the suture lines not grooved. Eadials a little wider than long. 



