PALAEONTOLOGY OF IOWA. 701 



Fig. 4 e. Enlargement of one of the ambulacral areas and the adjacent plates in outline, 



showing the oval pores, etc. 

 Fig. 4/. A single spine, natural size; length a little more than two and a quarter inches, 



imperfect at the extremity. 



Fig. 4 g. Enlargement of a portion of a spine near the base, showing the crenulations on 

 the edge of the annulation. 



Geological formation and locality. In the St. Louis limestone : Near 

 St. Louis, Missouri. 



Arckseocidaris norwoodi. ( n . s.) . 



PLATE XXVI. Fia. 5 a - e. 



BODY unknown ; plates transverse hexagonal and penta- 

 gonal ; the central tubercle papilliform or subtubulifbrm with 

 sides thickened, elevated above the annulation ; surrounding 

 area slightly depressed : margin of the annulation thickened, 

 rounded and constricted below, spreading on the surface of 

 the plate into a small well-defined flattened disc ; the space 

 between which and the nodulose margin is slightly depressed, 

 and sometimes showing obscure radiating ridges. Spines 

 slender, elongated, slightly curving, longitudinally striate 

 below and muricate above with fine sharply elevated den- 

 ticles, and at intervals marked by short round diverging 

 spinules : intermediate surface cancellate or granulose. 



This species has usually been regarded as the JlrchaRoddaris nerii, but differs from 

 that species in the less transverse plates and the less acute lateral angles, as well as 

 in the radiating ridges surrounding the central disc, and the tubercles on the outside 

 of these : the spines of the two species are likewise quite distinctive. 

 Fig. 5 a. A fragment, showing several imperfect spines and a plate of the body. 

 Fig. 5 b. A plate enlarged, from the range adjacent to the ambulacral area. 

 Fig. 5 c. Profile view of the same. 

 Fig. 5 d . A spine nearly twice enlarged, showing the striate lower part, and the muricate 



and spinulose character of the middle and upper part. 

 Fig. 5 e. A farther enlargement of the lower part of the spine, showing the longitudinal 



stride, which are strongly marked on the annulation above the articulating 



surface. 



Geological formation and locality. In the Kaskaskia limestone : Chester, 

 Illinois. 



