PALAEONTOLOGY OF IOWA. 699 



Archeeocidaris keokuk ( n. s.). 



PLATE XXYI. FIG. 2 a, b. 



BODY unknown ; plates hexagonal, with a prominent ele- 

 vated nodulose rim. : central tubercle papilliform, the space 

 between it and the surrounding annulation slightly depressed. 

 Surface outside the annulation marked by a somewhat flat- 

 tened elevation, and between this and the nodulose rim di- 

 stinctly concave. Spines round, of moderate length, muricate 

 with elongate ridges and spiniform tubercles; spines ex- 

 panding near the base into a subangular annulation ; con- 

 cavity in base of spine large. 



Fig. 2 a. A single plate with the spine, as it occurs on the surface of the stone. 

 The figure is very slightly enlarged. 



Fig. 2 b. Profile view of a single plate enlarged. 



Geological formation and locality. In the Keokuk limestone : Warsaw, 

 Illinois. 



Archaeocidaris slramardana ( n. s.). 



PLATE XXVI. FIG. 3 a, b, c, d. 



BODY unknown ; plates hexagonal, except adjacent to the 

 ambulacral field : central tubercle subtubuliform or papilli- 

 form, with usually a deep space between it and the annula- 

 tion, which is abrupt toward its upper margin and gradually 

 swelling out below, and expanding in a slightly convex rim, 

 between which and the elevated nodulose margin the surface 

 is somewhat abruptly concave ; margin elevated, the nodes 

 somewhat rounded. Spines short, small, smooth or finely 

 striated longitudinally near the base, muricate, with elon- 

 gated ascending nodes ; the articulating extremity abruptly 

 contracted below and above the annulation. 



This species differs from the A. keokuk in the smaller and proportionally narrower 

 plates and smaller spines, as well as in the articulating processes of the plates and 

 spines. 



