298 TWENTIETH REPORT ON THE STATE CABINET. 



EOCIDARIS DRYDENENSIS. 



PLATE 



Echinus drydenensis : Vanuxem, Report of Third Geol. District, p. 184. 1842 

 jirchcBocidaris (?) drydenensis : Vanuxem. Shumard, Catalogue of Palaeozoic 



Fossils. 1865. 



Body spheroidal, having the poles deeply impressed. Ambulacral areas 

 comparatively wide, contracted in the upper part, composed of very short 

 broad plates, about five in the space of a tenth of an inch ; each pierced 

 by two small pores, making four rows of pores to each ambulacral field. 

 The adjacent ends of the plates are depressed, forming a longitudinal 

 groove passing along the middle of the field, with a slight ridge in the 

 centre. The ranges of pores have an undulating direction, corresponding 

 with the curving edges of the adjacent plates of the interambulacral 

 areas. 

 Interambulacral areas, in their widest part, composed of seven ranges 

 of plates, two of them being pentagonal and five hexagonal. These 

 decrease in size towards the ends of the areas ; and the central ranges 

 become obsolete in turn, until at the summit of the area the outer ranges 

 only exist. Each plate of the interambulacral areas, so far as can be 

 determined, is characterized by a central tubercle for the attachment of 

 a single spine. 



The spines, as seen scattered over the surface of the rock, are slender, 

 and vary from one-half to three-fourths of an inch in length, with a slight 

 annulation around the lower end for the attachment of the muscles. 



Summit structure not known. The oral aperture, to judge from the im- 

 pressions of the oral ossicles left on the rock, has been comparatively large. 

 One specimen, as it occurs flattened upon the stone, is nearly two and 

 three-fourths inches in diameter. 



The specimen described by Mr. Vanuxem is upon a thin slab of shaly 

 sandstone of about ten by eleven inches ; one of the angles, being nearly a 

 fourth of the area, having been broken off. Upon this slab is one specimen 

 better preserved than the others, from which the characters have been 

 mainly derived. There are three other individuals possessing the form and 

 showing the ambulacral fields ; while there are parts of four others, with 

 multitudes of slender spines scattered over the surface. 



Geological formation and locality. In the shaly sandstones of the Che- 

 mung group, in the town of Dryden, Tompkins county, N. Y. 



The position of this fossil is in the lower part of the Chemung group, 

 and probably not more than one thousand feet above the upper beds of the 

 Hamilton group. Up to the present time the species is not known in any 

 other locality, and it remains the earliest known form of this group of 



fossils. 



18 



