62 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



each of which is the proximal plate of a supramarginal column. 

 The rays are margined by the inframarginal plates. There are 10 

 plates in each radial and 9 in each supramarginal column. 



Madreporite abactinal, situated interradially and dis tally against 

 the large interradial plate of the second ring, which is in front of the 

 anal opening, and between two of the proximal supramarginal plates. 

 It is trapezoidal in form, flat, and marked by very fine, dichotomous, 

 radial striae. 



Inframarginal plates completely inclosing all other plates and com- 

 mon to both the abactinal and actinal surfaces. Usually there are 

 9 or 10 in a column, not counting the axillary plates. Spines are 

 present at least along the inner edges of the inframarginals. 



Abactinally the axillary marginals are not prominent but actinally 

 they are very conspicuous. No spines appear to be connected with 

 these ossicles. 



Adambulacral plates small, subquadrate, usually 2 to each 

 inframarginal or from 18 to 22 plates in each column. From 4 to 

 6 are situated around the inner edge of each axillary, of which the 

 2 central ones are largest, being one of the 5 pairs of plates 

 making the oral armature. The adambulacral plates originally bore 

 on their lateral and inner edges numerous, very short, thick spines. 



Ambulacral plates nearly entirely concealed by the adambulacral 

 plates, apparently as numerous as the latter, and, so far as can be 

 determined, like those in P. matutinus Hall. 



Locality and formation. In the lower, middle, and upper beds of 

 the Cincinnatic (Ordovicic). Ten specimens have been studied: 

 Four in the Harris collection in the United States National Museum 

 (No. 40882) from the Richmond formation (Waynesville division) 

 about Waynesville, Ohio; another from the same place and formation 

 in the University of Chicago Museum (Gurley collection, No. 10977); 

 one from the hill quarries in the Maysvillian series (Vaupel collection, 

 U. S. Nat. Mus., No. 60616) back of Cincinnati; another, said to be 

 from Cincinnati, in the University of Toronto (Walker collection, 

 No. 691 H. R.); the type-specimen, now in the Museum of Com- 

 parative Zoology, also from Cincinnati; one in the Ulrich collection 

 of the United States National Museum (No. 60617), found back of 

 Covington, Kentucky, in the Maysville formation, about 315 feet 

 above low-water mark in the Ohio River; and the type of Palxaster 

 simplex Miller, found near Raysville, Ohio (Faber collection, Uni- 

 versity of Chicago Museum, No. 8830). The holotype of P. darkana 

 Miller is in Professor Harper's collection at Cincinnati, while another 

 specimen is in the Ulrich collection of the National Museum (No. 

 60618). 



Remarks. The writer has seen the type-specimen of Palseaster 

 simplex Miller, which agrees with the description and figure given 



