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616 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 
not known, but they must have been quite delicate. Regular interbrachials : 
1, 2,2, 2,1; the upper one narrow and entering the disk. Anal plate 
narrower than the radials; followed by 2, 3, 8,2, and 2 plates. Interdis- 
tichals one. Ventral disk hemispheric to low-conical; the anal tube sub- 
central, moderately small, rising abruptly from the tegmen. Plates of the 
ventral disk proportionally large, and convex. 
Horizon and Locality. — Lower Burlington limestone ; Burlington, Iowa. 
Type in the (Worthen) Illinois State Collection, Springfield. 
femarks. —The specimen with arms figured by Hall in the Bulletin of 
1872 is totally different from the type in the Worthen collection at Spring- 
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field. In the former the arm-bearing plates are horizontal; while in the 
latter they do not expand at all, and the arms were evidently quite delicate. 
Hall describes the anterior side of the type specimen as extending out more 
prominently, and he regards this as possibly of specific value. This promi- 
nence, however, occurs in the left posterior ray, and not in the anterior one, 
and is incidental, and not of structural importance. 
Cactocrinus extensus W. and Sp. (nov. <pae 
Plate LVII. Figs. 6, 7. 
(Figured by Hall, 1872, in Bull. I., New York State Mus. Nat. Hist., as Actinocrinus sexarmatus.) 
Closely allied to C. thetis, but smaller and the plates more nodose. 
Dorsal cup gradually spreading to the top of the costals, thence rapidly, 
almost horizontally, to the base of the free arms. The plates of the rays 
rising into transverse, angular tubercles, with obscure striz. Interbrachials 
and interdistichal spaces deeply depressed; the plates of the former cov- 
ered with round nodes. 
Basals short, forming a slightly projecting, trilobate disk, deeply indented 
at the suture lines, and excavated on the bottom; axial canal small and pen- 
tangular. Radials larger and more prominent than the costals, once and a 
half as wide as long. The two costals of equal size, both narrower than the 
radials. Distichals as large as the costals; all axillary. Palmars consider- 
ably smaller, the inner ones axillary and supporting two arms, the outer ones 
a single arm. Arm openings thirty, almost equidistant. Arms rather heavy 
and closely packed; they pass out almost horizontally from the calyx, then 
curve upward, infolding at the tips. Interbrachial spaces: 1, 2,1, and a 
minute piece on a level with the arm bases. Anal plate succeeded by 2, 2, 
