EOHINOIDS. 127 



Test large, spherical. Interambulacral areas lanceolate 

 in outline, moderately convex, occupied below the middle by 

 nine vertical ranges of plates. Ambulacra! areas about half 

 as wide as the others, and nearly as convex, the broad, rounded 

 furrow on each side of the middle rather shallow. Pore-plates 

 in four rows, much wider than high, and somewhat irregular. 

 Surface covered with small tubercles at the spine bases. 



Horizon and localities — Lower Carboniferous, Keokuk 

 limestone : Felton ( St. Louis county ), Ourry ville ( Pike county ); 

 Keokuk (Iowa). 



Oligoporus parvus Hambach. 



Oligoporus parvus Harubach, 1884 : Trans. St. Louis Acad. Sci. , vol. IV, 

 p. 550, pi. C, fig. 3. 



Like 0. dance but somewhat smaller. 



Horizon and locality. — Lower Carboniferous, St. Louis 

 limestone : St. Louis. 



Archaeocidaris agassizi Hall. 



Plate XV, fig. 5. 



Arehceocidaris agassizi Hall, 1858: Geol, Iowa, vol. I, p. 698, pi. xxvi, 

 figs, la-d. 



Known only from loose plates and spines. Interambu- 

 lacral plates of medium size, hexagonal, except the marginal 

 ones, which are subpentagonal. Central tubercle large, occu- 

 pying two-thirds the superficial area of plate, rather high; base 

 small, perforated. Surface of plates smooth except along the 

 margins, which are deeply crenulated, or marked by a marginal 

 series of elongated confluent nodes. Spines long, stout, some- 

 what compressed, contracted below, bluntly pointed above ; 

 socket deep ; annulation rather coarsely striated. Surface of 

 the spine below, smooth ; above marked by numerous small 

 spinous tubercles, arranged in oblique rows, or quincunx order. 



Horizon and localities — Lower Carboniferous, Burlington 

 limestone: Hannibal; Burlington (Iowa). 



