614 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



once and a half as large as the first costals ; the second costals a little 

 smaller than the first, both nearly as long as wide ; the former hexangular, 

 the latter generally pentangular. Distichals nearly as large as the costal 

 axillary, but the nodes shorter. Palmars two in the calyx, short, rounded, 

 and both wedge-shaped, the narrower ends directed to the inner side of the 

 ray. Arm facets very large ; the ambulacral passage oblong ; the respiratory 

 pores proportionally small. Structure of arms not known, but, to judge 

 from the size of the facets, as large as those of C. clarus. Anal side not 

 observed, being covered by matrix. Plates of the ventral disk greatly vary- 

 ing ; gradually increasing in size and prominence upwards, those nearest the 

 arm bases being the smallest and least consf)icuous. Posterior oral very 

 large, erect, and forming a part of the base of the anal tube, its wedge- 

 shaped prominences directed transversely outward. The four other orals 

 a little smaller, and separated from each other, and from the posterior one, 

 by small flat pieces. Anal tube extremely large, subcentral; composed at 

 the base of very large and smaller pieces, the larger ones produced into 

 wedge-shaped nodes, similar to those of the orals, but somewhat smaller, the 

 others having a perfectly flat surface. 



Horizon and Locality. — Lower Burlington limestone, Hannibal, Missouri. 



Types in the Missouri Survey collection, and that of Wachsmuth and 

 Springer. 



Cactocrinus thetis Hall. 

 Plate LYI. Figs. 3 and 4. 



1861. Actinocrinus thetis — Hall; Descr. New Spec. Criu. (Prelim, notice), p. 11. 

 1893. Whitfield; Mem. Am. Mus. Nat. Hist. N. Y., Vol. I., p. 6, Hate 1, Fig. 10. 



Sjn. Actinocrinus securus Hall; Descr. New Spec. Criu. (Prelim, notice), p. 14. 



This species is remarkable for the uniformity of its plates, which decrease 

 but little upward. It is of the type of C. dams, but smaller, the plates less 

 robust, and it has six arms to each ray in place of five in the antero-lateral 

 rays. It also approaches C. opuscidus in the calyx, but that differs in having 

 flattened and highly ornamented arms. Doi-sal cup nearly once and a half 

 as wide as high, broadly truncate at the base, uniformly spreading to the 

 lop of the distichals, and thence abruptly to the arms. Plates slightly 

 convex, covered with fine, obscure ridges, with or without central nodes. 



Basals short, their lower margins crenulated, slightly projecting beyond 

 the sides of the column ; the suture lines distinct but not notched. Eadials 



