186 
IOWA ACADEMY OF SCIENCES. 
Known only from loose plates and spines. Interambnlacral 
plates are of medium size, hexagonal, except the marginal ones, 
which are snbpentagonal. Central tubercle large, occupying 
two- thirds of 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 mar- 
ginal series of elongated confluent nodes. Spines long, stout, 
somewhat 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 in quincunx 
order. 
Horizon and localities. Lowmr Carboniferous, Burlington 
limestone: Hannibal, Missouri, Burlington, Iowa. 
Archa:ocidaris shumardiana Hall. 
Arclueocidaris sJiumardina Hall, 1858; Geology Iowa, vol. I, p. 
669, pi. xxvi, figs. Sa-d. 
Archceocidaris shumardina Keyes, 1894; Missouri Geol. Sur.,. 
vol. IV, p. 128. 
Spines and plates as in A. agassizi, but only about one-third 
as large. 
Horizon and localities. Lower Carboniferous, Keokuk lime- 
stone: LaGrange, Lewis county, Missouri; Warsaw, Illinois. 
ArCH.^OCIDARIS KEOKUK Hall. 
Archceocidaris keokuk Hall, 1858; Geology Iowa, vol. I, p. 699, 
pi. xxvi, figs. 2a-lj. 
Eocidaris hlairi Miller, 1892; Geol. and Nat. Hist. Indiana, 17th 
Ann. Rep., p. 683, pi. xii, figs. 1-2. 
Archceocidaris keokiik Keyes, 1894; Missouri Geol. Sur., vol. IV, 
p. 128. 
Known only from fragments. Plates with the marginal 
nodes more prominent and further apart than in A. agassizi, and 
the central tubercles also smaller. Spines less than two- 
thirds the size of those of the species just mentioned. 
Horizon and localities. Lower Carboniferous, Keokuk lime- 
stone: Clark county and Boone ville, Missouri, and Warsaw, 
Illinois. 
The form described as Eocidaris Uairi is too imperfect to 
deserve recognition and the figures show clearly that only the 
general characters were capable of being made out. From 
