346 PALAEONTOLOGY OF ILLINOIS. 



twice and a half as wide as high, not thickened or ex- 

 panded below, and but very slightly notched at the sutures ; 

 facet for the reception of the column large and moderately 

 concave, with a rather large central perforation. First 

 radials comparatively large, very nearly or quite as long as 

 wide. Second radials scarcely half as large as the first, 

 about as wide as long, quadrangular, pentagonal, or un- 

 equally six-sided. Third radials of about the same size as 

 the second, pentagonal, hexagonal, or heptagonal, and each 

 bearing on its superior sloping sides two secondary radials 

 of near its own size, each of which supports on its outer 

 sloping upper side, brachial pieces, leading to an arm, and 

 on its inner side, above, a small tertiary radial giving origin 

 to two arms, thus making three arms to each main divis- 

 ion, and six to each ray, or thirty to the entire series. 

 (Arms unknown.) 



Anal plates five or six, the first being of the same size 

 as the smaller first radials, hexagonal in form, and a little 

 longer than wide ; second anals two, about two-thirds as 

 large as the first, and irregularly heptagonal or octago- 

 nal ; above these there are two smaller pieces in the third 

 range, and one or two still smaller pieces above the latter, 

 the upper one being barely large enough to separate the 

 posterior lateral brachial pieces of the two posterior rays. 

 First interradial pieces about half as large as the first 

 radials, heptagonal, and supporting two smaller pieces 

 in the second range, above which there are one or two still 

 smaller pieces in the third, and a minute piece over the lat- 

 ter, wedged in between the brachial pieces. In each inter- 

 axillary space there are usually two intercalated pieces, the 

 lower of which is sometimes as large as one of the tertiary 

 radials, while the upper is very small, and wedged in be- 

 tween the brachial pieces. 



Vault conical, and nearly three-fourths as high as the 

 body below the arms, composed of irregular pieces of mode- 

 rate size, some of which project in the form of small pointed 



