594 GEOLOGICAL SURVEY OF IOWA. 



Fig. 4 a. Anal side of specimen. Fig. 4 b. Anterior side of same. 



Fio. 90. 



-3289 



& 



r) O CW\0 "^e accom P an yi n g diagram is a strict illustra- 



v "\pL/ tion of the plates composing the body of the spe- 



r-\o~ cimen figured, showing the form and proportions 

 L_JO>^^" 



of the plates as far as the brachial plates upon 



\\>^s>w ~ f^~\ ^ff^OCS which the arm-plates commence. 



W Obp SB? 



Geological formation and locality. In the Burlington limestone : Bur- 

 lington, Iowa. 







Actinocriims discoideus ( n. s.). 



BODY extremely short, spreading, discoid : base distinctly 

 hexagonal : dome depressed-convex, with a subcentral pro- 

 boscis. Basal plates short, projecting in a somewhat rounded 

 rim. First radial plates prominent, much wider than long, 

 transversely tubercular. Second radial plates very short, 

 quadrangular, and sometimes nearly obsolete. Third radials 

 small, pentagonal, supporting on each oblique upper side a 

 range of two or three diverging supraradials which are suc- 

 ceeded by brachial plates in first and second series, from 

 each of which arise a series of arm-plates ; giving four arms 

 from the anterior and each antero-lateral ray, while each 

 postero-lateral ray is modified so as to support five arms, 



4 

 making altogether 4 4 = 22 arms. 



T T 



Interradial plates large, ten- or twelve-sided ; sides below 

 flanked by the second and third radials, and above by the 

 first and second supraradials : no second interradials. First 

 anal plate and second range of three plates large, prominent ; 

 third range of three plates holding the position of subbrachial 



