ECHINODERMA TA—ECHINOIDEA . 



579 



of pores; plates irregular, imbricating towards the mouth; inter- 

 ambulacra with 4-7 columns of large, thin plates, the median ones 

 six-sided, those bordering the ambulacra five-sided or rounded; 

 median plates beveled slightly over those on either side and these 

 over others, to the ambulacral edge. Each interambulacral plate 



Fig. 1912. Archceocidaris agassizi^ spine, X 2. (After Hall.) 



with a large primary tubercle on a conical elevation, and sur- 

 rounded by a large flat areola and a circlet of granules; primary 

 spines large, usually serrated (the spines and single plates are 

 frequently the only parts known). Mississippic-Carbonic. 



2. A. agassizi Hall. (Fig. 1912.) Mississippic-Carbonic. 



Plates small, hexagonal, except those adjacent to ambulacral 

 area; central tubercle slender, elongate, projecting above surround- 

 ing, abruptly elevated annulation; a low annular ridge outside of 



Cs 



Fig. 1913. ArchcEocidaris shutnardana, plates and spine, X 2. (After Hall.) 



this; spines elongate, compressed, contracted below, with greatest 

 diameter one third from base; spiniform tubercles except on lower 

 part. 



Burlington of Iowa and Missouri; Upper Coal Measures of 

 Kansas. 



3. A. shumardana Hall. (Fig. 1913.) Mississippic. 



Smaller ; tubercles of plate more closely surrounded by annulus ; 

 spines slender, scarcely swelling, spinules more distant, larger; 

 section of spine round. 



Keokuk and Warsaw of Illinois, Missouri ; Eureka of Nevada. 



