344 PALEONTOLOGY OF ILLINOIS. 



radials. Base depressed, or about four or five times as 

 wide as high, with slightly expanded margins notched at 

 the sutures. First radial pieces of moderate size, a little 

 wider than long, and as usual, two heptagonal and three 

 hexagonal. Second radial pieces a little smaller than the 

 first, more or less regularly hexagonal, the superior lateral 

 sides, however, being sometimes very short. Third radials 

 about as large as the second, normally pentagonal, but 

 sometimes with the lateral angles truncated so as to give 

 them an irregular heptagonal outline. Secondary radials 

 resting one upon each superior sloping side of each third 

 primary radial, which they exceed in length; more or less 

 prominent, curving outward, and generally constricted and 

 rounded in the middle, though not entirely free from the 

 walls of the body, excepting on the upper side; supporting 

 on each of their superior (free) sloping sides the first 

 divisions of the arms. First interradial pieces about as 

 large as the first radials, hexagonal, and supporting two 

 smaller pieces in the next range, which connect with others 

 above, belonging apparently more properly to the vault. 

 First anal piece nearly as large as the first radials, hep- 

 tagonal in form, and supporting in the second range two 

 pieces, one of which (in the typical specimen) is as large 

 as the first; above these there are four or five smaller 

 irregular pieces in the third range, and above the latter 

 others belonging apparently to the vault. 



Arms slender, rounded, and composed below of distinct- 

 ly constricted pieces longer than wide, and after the first 

 division on the secondary radials the inner ones bifurcating 

 on the second piece, and one or both of these subdivisions 

 divide again on the second piece, above which they still 

 continue to be composed of a single range of rounded pieces 

 for a short distance, and then pass gradually through a 

 few wedge-formed pieces into a double series of alternating 

 pieces; above this they are all a little stouter than below, 

 and show a very slight tendency to become somewhat flat- 



