572 



GEOLOGICAL SURVEY OF IOWA. 



Fig. 4 a. Anal side of specimen. Fig. 4 6. Anterior side of the same. 



Fio.81. 



H The accompanying diagram illustrates the structure of 



^O V^rS\(y this species to the base of the arms and lower arm-plates, 



o^~ / ^'a!^~VQO o showing a slightly different structure in the postero- 



^-^vj f^^~J/ \ <QCn lateral rays. 



/ * ^"^O * ^ 



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

 lington, Iowa. 



Actinocrinus superlatus ( n. s.). 



BODY un symmetrically urnshaped, the crown very broad 

 and almost flat, gradually enlarging towards the bases of the 

 arms. Base pentagonal, depressed in the centre and elevated 

 between the centre and the margin, which is depressed. First 

 radial plates prominent, wider than long. Second radials 

 hexagonal. Third radials pentagonal, supporting on each of 

 the upper sloping sides a simple brachial plate, succeeded 

 by a second similar plate, which constitute a part of the 

 walls of the body. Interradial plates in successive series of 

 one, two, one, two, and two; the last lying in the inter- 

 brachial spaces. Anal plates consisting of a series of one 

 hexagonal, supporting one central heptagonal and two lateral 

 hexagonal plates; the third series consisting of four, and 

 above these are smaller plates surrounding a proboscidiform 

 aperture which lies a little below the plane of the arms, 

 causing a slight depression on that side of the summit. 

 Summit composed of numerous small plates ; the apicial 

 plate posteriorly excentric, extended into an elongated 

 tubercle. 



ARMS in pairs from each ray, each consisting of a double 

 series of slightly interlocking plates, which begin to diverge 

 from the plane of the body on about the sixth plate above 

 their bases. 



