338 PALAEONTOLOGY OF ILLINOIS. 



the spine. These nodose spinules continue down over the middle 

 portion of the spine, but they are smaller and less numerous than 

 on the clavate portion above. The lower part of the long spines 

 are nearly or quite destitute of spinules, but on the shorter ones 

 they continue nearly to the basal ring. This ring is nearly twice 

 the diameter of the articulating extremity, and smooth. 



Interambulacral plates hexagonal, with a nodose rim around their 

 margins, and there are indications of low granules scattered spar- 

 ingly over the outer surface of some of them, but they are highly 

 silicified, and their external markings thereby obscured. 



Central tubercle strongly elevated, papilliform, with a rather deep 

 space between it and the annulation, the latter forming a distinct 

 and slightly projecting rim for its support. 



This species may be readily distinguished from other Coal Measure 

 species by the spino-clavate character of its spines. 



Position and localities: Middle and lower Coal Measures, St. Clair 

 and Marshall counties, Illinois. 



No. 2,404 Illinois State collection. 



ARCILEOCIDARIS ILLINOIENSIS, Sp. nov. 



PL XXXI, Fig. 1, a. b. 



Primary spines rather stout, cylindrical, tapering very gradually 

 above the articulating ring to their extremities. Articulating ring 

 subangular, and from this the spines contract rapidly to the articu- 

 lating extremity. Just above the articulating ring the spines are 

 slightly curved, and above the curvature they are studded with short 

 sharp spinules that are directed outward and slightly upward 

 towards the extremity of the spine. The articulating end is perfo- 

 rated by a round aperture, which extends about to the articulating 

 ring. 



The only specimens of this species yet obtained consist of pri- 

 mary spines, and some fragments of the inter -ambulacra! plates, 

 the latter too imperfect for an accurate description. 



Position and locality: From the black beds of the St. Louis lime- 

 stone, near the Illinois Furnace, Hardin county, Illinois. 



No. 2,475, of the Illinois State collection. 



