SS yy 
Y 
HEX ACRINIDZ. 797 
the anal plate; the upper faces at both ends are slightly excavated for the 
reception of the two outer palmars of the proximal row, and there are similar 
excavations at the middle portions for the costals and distichals. The anal 
plate, which is placed within a notch formed by the basals, is either pentan- 
gular or triangular; when pentangular, its lower part rests between the 
radials, the upper between two of the palmars; when triangular, the two 
posterior radials meet over its apex. Costals small, trigonal. Distichals as 
Jarge as the fixed palmars. The latter, of which there are three plates to 
each series, are wider than high, and quadrangular, except the outer ones 
of the first row, which are either pentagonal or hexagonal. Arms short, - 
slightly tapering, flattened on the back, and biserial from the fourth or fifth 
plate. They are composed of very short transverse pieces, which are united 
laterally by a sharply zigzag suture. Pinnules strong and closely packed. 
Ventral disk conical, once and a half as high as the dorsal cup; the plates 
arranged similarly to those of P. pyramidalis ; but the posterior oral, instead 
of being central, is pushed completely to the anterior side by the anus. The 
anal opening is located at the top of a small elongate cone or short tube, 
which occupies almost the centre of the disk. The interradial spaces are lon- 
gitudinally grooved to their full length. Those of the four regular sides are 
constructed of three good-sized interambulacral pieces, succeeded by the oral 
plates. The posterior side has 2, 3, and 4 pieces, followed by numerous 
smaller ones, which support the anal tube. The sockets for the reception of 
the winged appendages are narrow, the appendages leaf-like, perfectly flat on 
their lateral faces, and thickest at the place of attachment, whence they thin 
out gradually into a sharp, knife-like edge. As seen in the specimen, they 
look like five immense blades, which project out in all directions far beyond 
the limits of the arms. The interdistichals and secondary radial dome plates 
are comparatively small, the former less protuberant than in the preceding 
species. | 
Horizon and Locality. — Kaskaskia group; Grayson, Edmonson and Pu- 
laski Cos., Ky. 
Types in the Lyon collection. 
