HEXACEIjSriDiE. 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 

 large 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. 'pyramiclalis ; 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 bej'ond 

 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 ; Graj'son, Edmonson and Pu- 

 laski Cos., Ky. 



Types in the Lyon collection. 



