CONTRIBUTIONS TO PALEONTOLOGY. 343 



EOUIDARIS DRYDENENSrS. 



Echinus drydenmsis, Vanuxbh. Report of Third Geol. District of N. Y., p. 184. 1842. 

 jirchtEociduris (.') drydenmsis, Vandsbm; in Shumaed's Cat. 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 intei'ambulacral areas. Interambulacral 

 areas, in their widest jsart, 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 attach- 

 ment 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 

 impressions of the oral ossicles left on the rock, has Iseen 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, and 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 

 Chemung group, in the town of Dryden, Toinpkins county. New York. 

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



