PALAEONTOLOGY OF IOWA. 569 



Fig. 5 a. Anal side of specimen. Two arm-bases only on each side are shown in the 



figure. 



Fig. 5 b. Anterior view of same specimen. 

 Fig. 5 c. Base of the same. 



FIG. 80. 



CQ 



The accompanying diagram illustrates the struc- 

 ture of the body of this species, to the summit of 

 (^^/^/^^(^j^ the brachial plates. The narrow second radials form 

 a peculiar feature. 



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

 lington, Iowa. 



Actiiiocrinus tricornis (n. s.). 



BODY subglobose, truncate at base and spiniferous at the 

 summit. Basal plates concave in the centre, with projecting 

 margins, and broadly notched at the sutures ; with edges 

 between the sutures emarginate in the middle, but less 

 deeply than at the sutures. First radial plates thick, turgid, 

 tuberculiform, and abruptly truncated on the upper sides ; 

 width about once and a half the length. Second radials very 

 short, more than twice as wide as long. Third radials short, 

 pentagonal, each side with a simple plate on the upper ob- 

 lique side, which supports a brachial plate ; those adjoining 

 the anal side supporting two brachial plates. First interradial 

 plate ten-sided, presenting the form of an angular tubercle, 

 resting on the upper sloping edges of the first radials, and 

 supported on the sides by the second and third radials ; its 

 supralateral edges supporting the supraradial plates, and its 

 superior plates supporting two narrow second interradials 

 which lie between the brachial plates. First anal plate large, 

 tuberculiform : second range consisting of three plates, the 

 [ IOWA SURVEY.] 72 



