574 GEOLOGICAL SURVEY OF IOWA. 



interbrachial. Anal plates in series of one, three, five, and 

 an interrupted series of six surrounding the aperture, which 

 lies at the base of a distinct series extending from the apicial 

 node, below the plane of the range of the arms. Arms two 

 from each ray. Summit composed of numerous small plates ; 

 the apicial plate a little excentric posteriorly, and extended 

 into a conspicuous tubercle. Plates of the body all tubercu- 

 lous. Surface granulose. 



This species resembles Jl. brevicornis ; but the plates differ slightly in form and 

 arrangement, with fewer anal plates, and the anal aperture placed much lower on the 

 posterior side. 



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

 lington, Iowa. 



Actinocrinus symmetricus ( n. s.). 



PLATE X. FIG. 8 a, b. 



BODY rotund subpyriform ; summit rounded ; base narrowly 

 truncate. Basal plates large, heptagonal. First radials large, 

 hexagonal and heptagonal, a little wider than long. Second 

 radials short, hexagonal, the second interradial plates resting 

 on their upper oblique sides. Third radials short, pentagonal, 

 supporting on each of their upper oblique sides a supraradial 

 plate, on which rests a second supraradial plate supporting 

 brachial plates ; those of the anterior ray, above the third 

 radial, being simple brachial plates. First interradial plate 

 hexagonal, supporting two other smaller hexagonal plates, 

 and these two other elongate plates which lie between the 

 groups of arms. First anal plate heptagonal, succeeded by 

 a second and third series of three each, which are mainly 

 hexagonal ; two or more ranges of small plates surrounding 

 the aperture, which is situated about midway between the 

 bases of the arms and the summit. Apicial plate subcentral, 

 tuberculiform. 



ARMS from the anterior ray in a single pair, from each of 

 the antero-lateral rays two pairs, and from the postero-lateral 

 rays three arms, or sixteen arms altogether. Surface granu- 



