INVERTEBRATES. 303 



arranged in alternate series of thick and thinner joints. This beau- 

 tiful species, the first discovered in the Keokuk limestone in Illinois, 

 I take pleasure in dedicating to its discoverer, Mr. L. A. Cox, of Keo- 

 kuk, to whom I am indebted for the use of the typical specimen. 



Position and locality: Upper beds of the Keokuk limestone, Ham- 

 ilton, Illinois. 



No. 400 of Mr. Cox's collection. 



ZEACRINUS KEOKUK, Worthen. 



PI. XXVIII. Fig. 3. 



Zc.aci'inun Keokuk, WOBTHEN, February, 1882. 



Bulletin No. 1, of the Illinois State Museum of Natural History, p. 28. 



Body of medium size, shallow, basin-shaped, about twice as wide 

 as high. Base depressed and basal plates concealed by the first 

 columnar joints. 



Subradials on the anterior side hexagonal, curving below into the 

 basal concavity, their upper angles extending up about half the 

 length of the radial plates. 



Radials pentagonal, nearly once and a half as wide as long. 

 Brachials single on four of the rays, pentagonal, a little larger 

 than the radial below, and supporting the first divisions of the 

 rays. The anterior ray has four brachials, the first as long as both 

 the succeeding ones, quadrangular and as wide below as the radial 

 on which it rests. The two succeeding brachial plates are short and 

 quadrangular, a little narrower than the first. The fourth is also 

 short, subtriangular, and supports the first divisions of this ray. 

 Both branches of the anterior ray divide again on the tenth plate, 

 and the outer division the second time on the tenth plate above, 

 making six arms to this ray. One of the antero-lateral rays, after 

 its first division on the brachial plate, sends off an arm from each 

 division on the sixth plate, the outer branches dividing again on the 

 eighth to the tenth plate, making eight arms to this ray. The arm 

 plates are but slightly rounded exteriorly, and nearly quadrangular 

 in form. Anal plates unknown. 



Column round, and composed near the body of rather thick, alter- 

 nating with very thin plates. 



Position and locality: Upper part of the geodiferous shales of the 

 Keokuk group, one mile below Keokuk. 



No. 217 of Mr. L. A. Cox's collection. 



