PALAEONTOLOGY OF IOWA. 591 



stinctly hexagonal : plates of the body turgid, and obtusely 

 subangular in the middle. Basal plates thick, strong, truncate 

 below and projecting in a thickened riin over the column, 

 broadly and deeply notched at the suture lines, the groove 

 reaching to the summit of the plates : suture lines marked 

 in the base of the plates by a narrow elevation on each side, 

 giving a slight prominence in the bottom of the notch. First 

 radials as long as wide ; joining to the base and among them- 

 selves, as well as with those above, by very short angular 

 processes from the margins. Second radials hexagonal, as 

 wide as long. Third radials heptagonal (those of the postero- 

 lateral rays hexagonal ), and supporting on each of the 

 superior oblique sides a series of four or five supraradials in 

 direct line, diverging from each other so as to leave room 

 for a series of superinterradials, as one, two, and two ; the 

 third series supporting brachial plates. The outer oblique 

 margin of each first supraradial supports a hexagonal plate, 

 and this in turn a second of similar form : these support on 

 their upper edges the brachial plates. The third supraradial 

 supports on its outer sloping side a brachial plate ; and the 

 fourth supraradial supports on its outer sloping margin a 

 simple brachial plate, and on its inner margin a bifurcating 

 plate, beyond which the specimen is imperfect. These divi- 

 sions give eleven or twelve arms from each ray. 



First interradial plate hexagonal, as large as the second 

 radial, and supporting two smaller plates, one hexagonal and 

 one pentagonal ; and above this a third series of two similar 

 plates, succeeded by two or three smaller plates which lie 

 in a plane with the first supraradials : above these are two 

 ranges of two each, nearly as large as the second and third 

 interradials, which sustain the brachial plates of the inter- 

 mediate arms as already described. Anal plates in series of 

 one, two, four, and two, the latter in a plane with the first 

 supraradials; to which succeed two series of two each, as 

 between the other rays, to the brachial plates. 



