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 conspicuous. 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. 
fHorizon and Locality. — Lower Burlington limestone, Hannibal, Missouri. 
Types in the Missouri Survey collection, and that of Wachsmuth and 
Springer. | 
Cactocrinus thetis Hatt. 
Plate LVI. Figs. 3 and 4. 
1861. <Actinocrinus thetis — Matt; Descr. New Spec. Crin. (Prelim. notice), p. 11. 
1893. WuuitrreLp; Mem. Am. Mus. Nat. Hist. N. Y., Vol. L., p. 6, Plate 1, Fig. 10. 
Syn. Actinocrinus securus Wari; Descr. New Spec. Crin. (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. clarus, 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. opuscudus in the calyx, but that differs in having 
flattened and highly ornamented arms. Dorsal cup nearly once and a half 
as wide as high, broadly truncate at the base, uniformly spreading to the 
top 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. Radials 
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