CONTRIBUTIONS TO PALAEONTOLOGY. 331 



towards the extremity where they are rounded. All the marginal 

 plates are visible from the upper side, and usually appear as an addi- 

 tional range of plates on each margin of the ray, making five with the 

 three properly belonging to the upper surface. Those of the inner 

 range bordering the ambulacra (adambulacral plates) are smaller 

 than the marginal plates, about thirty-eight to forty in number ; the 

 basal or oral plates are triangular, those of the adjacent rays uniting 

 by their longer margins ; and with a single minute plate situated 

 at these points. The plates of the exterior surface, both upper and 

 lower, present a granulose or striato-granulose surface which appears 

 to have been produced by short setae or spines ; and at the angles of 

 the rays the marginal plates are armed by a few spines, which are 

 as long or longer than the transverse diameter of the plates. Ambu- 

 lacra composedof a double range of short broad poral plates (ossicula), 

 equal in number to the adambulacral plates ; their outer ends exca- 

 vated on the posterior border, forming a comparatively large pore, 

 just within its junction with the adambulacral plate. There appears 

 to have been but one range of pores in each set of ossicula, but 

 these are large, distinct, and pass between the plates. 



In the collection, there is an impression of a single ambulacral area of 

 this species, which is spread open laterally and measures about two and 

 a half inches in length by nearly three-fourths of an inch in width in 

 the middle, broadly petaloid in shape, and showing the form and number 

 of poral plates, with the position of the pores and their junction with 

 the adambulacral plates. 



This species differs very remarkably from any of the preceding, and 

 every other described species, in its robust form, its more numerous 

 and proportionately larger marginal plates, and in the large and deep 

 ambulacral grooves and poral plates. In the single large tuberculose 

 plate at the base of the marginal range it resembles the Niagara species ; 

 and in having more adambulacral than marginal plates, it resembles 

 P. granulosa, but differs from the last in the large plates of the dorsal 

 side. 



Geological Formation and Localities. — In rocks of the Hamilton group 

 near Hamilton, Madison county, and near Summit, Schoharie county ; 

 also from near Cooperstown, Otsego county, whence I received a specimen 

 retaining the impression of the lower side, from Paul F. Cooper, Esq., 

 of Albany. 



