354 PALAEONTOLOGY OF ILLINOIS. 



radials one-half to two-thirds as large as the first, wider 

 than high, hexagonal, or in part (sometimes) pentagonal, 

 the superior lateral sides being short. Third radials wider 

 than long, pentagonal, hexagonal, or irregularly heptagonal, 

 and supporting on each superior sloping side a secondary 

 radial, each of which gives origin to brachial pieces leading 

 to an arm on the outer side, and supports on its inner slop- 

 ing side a tertiary radial, which gives origin to two arms, 

 thus making three arms to each main division, or six to 

 each ray, and thirty to the entire series. 



Eirst anal piece a little longer than wide, hexagonal, and 

 supporting two pieces of about half its own size in the next 

 range; between the superior sloping inner sides of these 

 there is one small piece, and above these three or four other 

 small pieces, one or two of which extend up so as to sepa- 

 rate slightly the outer brachial pieces of the adjacent rays. 

 Interradial pieces, four or five to each area, the upper of 

 which is narrow and extends up so as to separate slightly 

 the brachial pieces above. In each interaxillary space there 

 is usually an elongated intercalated piece, sometimes large 

 enough to truncate slightly the upper margin of the third 

 primary radial, while it continues upward so as to separate 

 the brachial pieces above. 



Vault much depressed, and composed of numerous, irregu- 

 lar, slightly convex pieces of moderate size, and provided 

 with a rather stout sub-central proboscis, composed near 

 the base of unequal pieces, some of which are distinctly 

 protuberant. 



Surface of all the body plates moderately convex, or 

 sometimes a little angular in the center, and ornamented 

 with distinct radiating costse, extending from the middle to 

 the sides of each piece, so as to divide the whole into nu- 

 merous triangles. These costse are also more or less com- 

 pound, so as to form a secondary or (below the middle of 

 the first radials) a tertiary series of smaller, less defined 

 triangles within those formed by the principal eostse. 



