482 THE CRINOIDEA CAMERATA OF NORTH AMERICA. 



the back, very stout, widest at midway, somewhat tapering at both ends, the 

 tips infolding. The free arm plates are elevated in the centre, and produced 

 into transverse angular ridges, which in the upper part of the arm are broken 

 into nodes. There is but a single interbrachial at the regular sides; the 

 anal side has two, which enclose a second anal. The first anal is a little 

 longer than the radials. The exact arrangement of the plates in the tegmen 

 cannot be accurately ascertained from any of the specimens. Column com- 

 paratively strong ; the nodal joints a little highest and widest, and rounded 

 at their edges. 



IIo7'izon and LocdlUy. — Waverly sandstone; Richfield, Summit Co., and 

 Royalton, Cuyahoga Co., Ohio. (Fig. 2 is made after Hall, Fig. 5 drawn from 

 a gutta percha cast made in a natural mould.) 



Aorocrinus coneavus (Meek and Woethen). 

 Plate XLII. Figs. 13a, b, c. 



1861. Adinocrinus {Amphoracriims) concavits — Meek and Worthen ; Proceed. Acad. Nat. Sci. Pliila., 

 p. 132. 



1865. Aetinocriims {^Sphtprocriims) cofiravus — M. and W. ; ibid., p. 154. 

 18G5. Actiiiocrimts {Ccelocriiius) coiicavus — M. and W. ; ibid., p. 273. 



1866. Calocriims concams — M. and W; Geol. Rep. Illinois, Vol. II., p. 215, Plate 15, Figs. lOc, b, c. 

 1881. Dori/crimis coiicavus — W. and Sp. ; Revision Palteocr., Part II., p. 179 (Proceed. Acad. Nat. Sci. 



Phila., p. 353). 



A small species. Calyx subglobose, wider than high ; the ventral disk 

 almost flat. The sides of the dorsal cup rise nearly vertically from the 

 middle of the radials to the arm regions ; the lower half curving abruptly 

 inward, and forming with the basals the bottom of the cup. The plates are 

 nearly flat and devoid of ornamentation. 



Basals small, spreading almost horizontally, and forming a shallow in- 

 verted basin, which is slightly notched at the sutures. Eadials extremely 

 large, fully twice the size of both costals together. Costals very small, quad- 

 rangular and pentangular. Distichals 1x2, suppiorting the arms, except in 

 the posterior rays, in which the divisions next to the anal area support pal- 

 mars, and there are three arms to the ray in place of two in the others. Arm 

 openings small, directed outward ; the structure of the arms not known. 

 The interbrachials consist of one large plate and two smaller ones, the latter 

 being on a level with the arm bases. First anal larger than the radials ; 

 followed by three rows of three plates each, which decrease in size upwards, 

 and connect with the anal opening. Tegmen very short, a little depressed 



