128 



Cincinnati Society of Natural History. 



hexagonal, each with several small pits for articulation of spines; 

 ambulacral groove wide; ambulacral plates in two rows, about fifty in 

 each row, with their greatest diameter across the rows; adambulacral 

 plates also fifty in number, hexagonal, much wider than long, more 

 numerous than the marginal plates, alternating with them near the 

 ends of rays, and having pits for articulation of spines ; dorsal surface 

 covered in part with convex, spine-bearing plates, generally a single 

 spine to each plate; a single series of highly convex plates down the 

 centre of each ray, and one on either side, the spaces between filled 

 with smaller plates so arranged as to run diagonally from the side to 

 the central row, and forming an angle with each plate in the central 

 series. (Jour. Cin. Soc. Nat. Hist., vol. 7, p. 16.) 

 Locality. — Waynesville, Ohio. 



3. — P. dyer 1 Meek, 1872. 



Pentagonal ; rays and body large, the former probably one and 

 one-half inches long; body about two inches in diameter; marginal 

 plates small, tumid, nearly square, alternating with a row of similar 

 but slightly smaller adambulacral plates, the number in each series 

 being about the same ; both series roughened by course granules and 

 possessing a pit for the insertion of a spine; spines smooth, straight, 

 rounded, thickened at the attached end and tapering to a blunt point ; 

 ambulacral groove wide ; dorsal side of disc and rays composed of 

 numerous small pieces, with large pores between them, touching at 

 three or four salient points, and thus forming a reticulated structure ; 

 each piece with a central tubercle having a minute pit for the insertion 

 of a small, short spine ; madreporiform tubercle flat, obtusely subtrilo- 

 bate, with striations like the nervation of some ferns. (Am. Jour. 

 Science, 3d ser., vol. 3, p. 257 Pal. of Ohio, vol. 1, p 58.) 



Locality. — Cincinnati, Ohio. 



4. — P. granulosus Hall, T.866. 



Pentagonal; rays five, a little more than twice as long as their 

 breadth at the base, obtuse ; body medium size ; marginal plates small, 

 tuberculose, about twenty-five on each side of a ray one and one- 

 quarter inches long ; ambulacral grooves broad, with two series of 

 ambulacral plates slightly curved, each marked by an elevated ridge 

 along its entire breadth ; adambulacral plates smaller than the mar- 



