LOWER SILURIC SHALES OF THE MOHAWK VALLEY 87 



Thecal plates irregular polygonal, mosaic, disposed without order 

 and of small number; their surface granulose. Usually three or 

 four large plates in the interambulacra, the marginal plates not seen. 



Rays five, four solar and one contrasolar, broad and nearly 

 straight in the proximal half, the extremital part rather abruptly 

 bent and terminating within the disk subparallel to the margin. Oral 

 aperture elongate, two rays arising from one, and three from the 

 other end, surrounded by (five?) triangular oral plates with raised 

 lateral edges. Ambulacra covered by two rows of alternating, in- 

 wardly converging, contiguous narrow plates with flat, horizontally 

 extended tops. Subambulacral plates polygonal, as the thecal plates, 

 but smaller, apparently hexagonal (the outer margins not clearly 

 seen). Cover plates granulose, as the thecal plates. Anal pyramid 

 small, consisting of four triangular plates, situated as in Agelacri- 

 nites, between the distally approaching or overlapping pair of solar 

 and contrasolar rays. 



Horizon and locality. Snake Hill shale and sandstone at Snake 

 hill, Saratoga county, N. Y. 



Remarks. This small and interesting type occurs only in molds 

 in graywacke sandstone, which has been found loose in slabs under 

 the cliff on the shore of the lake but not in place. Altogether four 

 specimens were obtained by me. 



The species is clearly congeneric with E . b i g s b y i Billings, 

 from the Trenton of Ottawa. It agrees exactly with that species, 

 and since the latter is the type of the genus, also with the genus, in 

 the form of the ambulacra and their coverplates, the form of the 

 tegal plates and their sculpture. The oral and ambulacral plates, as 

 well as the anal pyramid ofEdrioaster bigsbyi have not 

 yet been described and it is therefore still possible that they differ 

 in the genotype, and the reference of our species to Edrioaster is 

 doubtful in that respect. 



Billings described E . bigsbyi as possessing four rows of 

 pores in the ambulacra. Jaekel (title 43, page 44) has pointed out 

 that these pores, if they existed, had probably nothing to do with 

 the pores of the Cystoidea, but only served for the entrance of 

 water into the vectacle furrows or for the discharge of sexual pro- 

 ducts. In E. saratogensis the cover plates fit snugly 

 against each other wherever well preserved and have straight lateral 

 sides, leaving no room for pores. In a few places, however, where 

 the plates are apparently somewhat abraded, they are contracted 

 in the middle, leaving long elliptic depressions between them. 



