FOSSIL CRINOIDS. 155 



POTERIOCRINUS MAGNIVENTRUS, Sp. nOV. 



PL V, figs. Ifl, 6, 2, 3; VI, figs. 1, 2, 3. 



A very large species, length of maximum crown probably ten or twelve inches; 

 height and width of calyx probably about equal, but specimens are all so much 

 flattened that measurements are only approximate. IBB about as wide as long; 

 BB longer, and RR shorter, than wide. Primibrachs usually 2, sometimes 

 3 or 4, not necessarily in the anterior ray. Calyx plates folded into large ridges 

 distributed as in P. doris, with deep triangular and diamond shaped pits inter- 

 vening; convex part of ridges perfectly smooth, but short, shallow grooves, 

 with tooth-like folds between originating near the margins, pass from plate to 

 plate across the sutures both at the main ridges and in the depressions, giving 

 a finely serrated edge to the plates when exposed. Plates very thin, and meeting 

 by edges as thin as paper, affording little surface for ligaments, which must 

 have had their attachments in the small folds or grooves at the inside. Radial 

 facets oblique, facing more upward than outward. Arms long and heavy, 

 bifurcating more than once with intervals of 12 to 15 plates; composed of short, 

 cuneiform brachials; pinnules small, appearing relatively larger in the distal 

 parts. Ventral sac of enormous size, very wide below, and tapering suddenly 

 toward the distal end of the arms, and then gradually to a small point; it is 

 composed of longitudinal rows of very short, wide plates, with a median ridge 

 or rib, which projects above the folds, and resembles a small arm; these ribs 

 run the full length of the sac, converging at the distal end; there are transverse 

 folds in form of sharply elevated, rounded ridges, one to each plate from opposite 

 sides, with a broad furrow, obliterating all sutures except those between the 

 longitudinal rows of plates, which are faintly visible. Tegmen leading to base 

 of sac anteriorly and laterally composed of a thickly plated skin, connecting 

 with the distal edge of the radials. Column round, composed of very short, 

 alternately projecting columnals. 



Types. Yale University Museum; except Fig. 3, PI. V, and Figs. 2, 3, PI. VI, 

 which are in the author's collection. 



Horizon and Locality. Keokuk Group, Crawfordsville, Indiana. 



Several years ago, while visiting Dr. Charles Schuchert at the Yale Uni- 

 versity Museum, he called my attention to a large crinoidal specimen hanging 

 upon the wall in a frame, being among material collected for Professor Marsh 

 many years before by Mr. Bradley, at the celebrated locality of Corey's Bluff, 

 near Crawfordsville, Indiana. It was at once seen to be a flattened ventral sac 



