* 



Manual of the Paleontology of the Cincinnati Group. 127 



marginal ; ambulacral grooves deep, wide or narrow, and bordered by 

 strong spines; pores penetrating the plates of the upper surface. 

 (Pal. of New York, vol. 2, p. 247.) 



Remarks. — This genus is represented by quite a number of spe- 

 cies in our section. An effort was made to arrange them in some sort 

 of scheme by which their identification could be facilitated. Owing 

 to the fact that in many cases only imperfect specimens are known, or 

 else only the dorsal or ventral surface is known, and to the fact that 

 the descriptions given by various authors do not always mention simil- 

 ar characters, this has been impossible. All that has been attempted, 

 therefore, is to group the species according as the ambulacral groove is 

 wide or narrow. In two cases the ventral side is unknown, so that 

 these have been placed by themselves. 



A. — Ambulacral grooves wide. 



1. — P. jamesii Dana, 1863. 



Pentagonal; rays five, about two inches long from center of disc; 

 body about one and one-quarter inches in diameter; marginal plates 

 probably from twenty to fifty alternating with the adambulacral plates, 

 which are about three times as wide as long ; flat at the outer ends 

 and raised into a conspicuous elevation near the ambulacral furrow, 

 apparently about twenty-five in number, the inner edges interlocking 

 in a peculiar manner; ambulacral groove wide : oral plates ten, in five 

 pairs, each irregular in form, longer than wide, flattened and depressed 

 at their outer ends ; elevated into crest-like prominences farther in, 

 and with a lateral process or thickening on the outer side of each ; 

 dorsal surface unknown. (Am. Jour, of Sciences, 2dser., vol. 35, p. 

 265, as Palceasterina? : Pal. of Ohio, vol. 1, p. 62.) 



Locality. — Cincinnati, O. 



Remarks. — This species has attained notoriety by being figured in 

 Dana's Manual of Geology and other places. It is one of the largest 

 species of the genus and is well characterized and easily separable 

 from all other species by the peculiar form of the oral plates The 

 type specimen is in the Walker Museum of Chicago University. 



2. — P. magnificus S. A. Miller, 1884. 



Pentagonal : rays (if perfect) about two and three-eighth inches 

 long ; disc one and one-quarter inches in diameter ; marginal plates 



