CONTRIBUTIONS TO PALEONTOLOGY. 335 



outside of the disc, where it measures seven-sixteenths of an inch. 

 The ventral side of the ray shows three series of plates — ambulacral, 

 adambulacral and marginal — on each side of the centre ; of these at 

 least seven ranges have been included within the disc. The ambu- 

 lacral plates are a little wider than long, arranged in alternating- 

 order. The pores penetrate the interstices near the outer extremity 

 of the plates, while the partial or obsolete pores are obscure, becoming- 

 deeper and more conspicuous towards the extremity of the ray. 

 Near the base of the ray there are twelve plates in the length of 

 an inch, and fourteen plates in the same distance in the central 

 portion, while towards the extremities there are twenty or more in 

 the same space. The marginal plates are subhexagonal, a little 

 longer than wide, and ornamented by long slender spines on their 

 outer margins. 



This is a much larger and more robust species than the Eugaster logani, 

 and in its entire condition it is the largest star-fish known in our palteozoic 

 rocks. 



Geological Formation and Localitij. — Tn the Chemung group. The speci- 

 men is in a brownish-gray sandstone, which weathers to a very lio-ht 

 ashen color. It was received from Henry S. Randall, Esq., of Cortland- 

 ville, in the vicinity of which place it was obtained. 



Note. — It is only as these pages are going to press that my attention 

 has been directed to the similarity of structure in the ray of Pal^coma 

 of Salter with that of the proposed Genus Ptilonaster. In the illus- 

 trated species of the former genus {Palceocoma marstoni) the disc is 

 proportionately larger, and the rays much shorter, while the outer ranoe 

 of plates is represented as imbricating; but being characterized by a 

 double row of plates bordering the ambulacral area, it must be regarded 

 as very nearly related if not generically identical with Ptilonaster. 



GENUS PROTASTER, Forbes. 



Peotastbk, Forbes; in Memoirs of the Geolog. Survey of Great Britain, Decade i. 1849. 



The description of the genus is as follows : 



"Body circular, covered with squamiform plates; genital openings in the 

 angles of junction of the arms beneath ; arms simple, formed of alternatino- 

 ossicula." 



