454 THE CRINOIDEA CAMERATA OF NORTH AJVIERICA. 



DORTCRINUS Eoembr. 



1854. F. Roemeb; Arcbiv. f. Naturgescli., Jabrg. XIX., Baud 1, p. 207. 



1855. F. Roemee; Letbsea Geogii. (Ausg. 3), p. 219. 



1869. Meek aud Worthen ; Proceed. Acad. Nat. Sci. Pbila., p. 165. 



1873. Meek and Womhen ; Geol. Kep. IlliEois, Vol. V., p. 379- 



1878. W. and Sp. ; Proceed. Acad. Nat. Sci. Pbila., p. 264. 



1879. ZiTTEL (subgenus of Amphoracrimis) ; Handb. der Palaiont., Vol. I., p. 370. 



1881. W. and Sp. ; Revision PaliBocr., Part II., p. 176 (Proceed. Acad. Nat. Sci. Pbila., p. 350). 



Sjn. Amplioracrinus Meek and Wokthen (not Austin, 1866), Geol. Rep. lUiuois, A^ol. II., p. 209. 



Calyx decidedly bilateral, distinctly lobed at the arm regions, the inter- 

 radial spaces deeply depressed and sinuous. The dorsal cup broadly trun- 

 cate at the bottom, the plates heavy and frequently nodose. Basals three, 

 large, their sides produced into a rim which sometimes becomes highly con- 

 spicuous. Radials, as a rule, as large as the two costals together. First 

 costals quadrangular ; the second usually pentangular, exceptionally hexan- 

 gular or heptangular. Distichals two when there are no palmars, but when 

 the latter are represented, there is but one distichal, which is axillary, and is 

 followed by a single row of palmars. Arms in pairs, given off from a minute 

 axillary, which occupies the same facet with the proximal arm plates at each 

 side, and both arms have a common ambulacra! opening in the calyx. In 

 species with twenty arm openings, each ray has four pairs of arms, but when 

 there are less than twenty, the antero-lateral rays generally have but two 

 pairs, and the anterior one from two to four. The arms are rather short, 

 biserial, and more or less spinous. The spines are given off at intervals 

 from the sides, and are formed by the outward prolongation of the arm 

 l^lates. Pinnules of moderate size. The number of interbrachials is lim- 

 ited, there being rarely more than three in the dorsal cup, the two upper 

 ones at the level of the arm bases. The anal side is more or less flattened, 

 the median line elevated, the sides grooved. It consists of a longitudinal row 

 of anal plates, supporting a number of smaller pieces, which surround the 

 anus. At each side of the anal row there are one or more interbrachials, 

 somewhat depressed below the level of the anal plate between them. Ven- 

 tral disk highly elevated ; the posterior oral, and frequently also the first 

 radial plates above the ambulacra, extended into low spines. Anus excen- 

 tric, opening out laterally directly from the disk. Column round, the nodal 

 joints largely projecting over the others ; axial canal small, pentangular. 



Distribution. — So far as known, restricted to the two Burhngton beds, and 

 the Keokuk limestone. 



