382 PALAEONTOLOGY OF ILLINOIS. 



pieces in the third range, connecting with others extending 

 up to the opening of the vault. First interradials nearly 

 half as large as the first radials, heptagonal in form, and 

 bearing two smaller pieces in the second range, above 

 which one or two small pieces intervene to separate the 

 brach'ial pieces, and connect with the vault. 



Vault about two-thirds as high above the arm openings 

 as the hight of the body below, provided with a single, 

 rather pointed and prominent central node, that may be in 

 some instances developed into a short spine. Opening 

 with margins a little projecting and situated in a slightly 

 impressed area above the horizon of the arm openings. 



Body plates convex, separated by deeply canaliculated 

 sutures, and roughened by a peculiar shallow pitting over 

 the entire surface, but which is larger and deeper at the 

 edges of the larger plates, to which it imparts a slightly 

 crenate appearance. The plates of the vault are also 

 defined by the same deeply canaliculate sutures, and 

 roughened by similar pitting to that on the body plates, 

 though they are not convex like the latter. 



As we have not seen the arms of this species it is barely possible 

 that it may be more properly an aberrant Agaricocrinus than a true 

 Dorycrinus. Its arm bases, however, or rather the brachial pieces, 

 have not the breadth and stoutness seen even in the most aberrant spe- 

 cies of the former group, such as Agar. corniculus (= Act. corniculus, 

 Hall), and from their appearance there is little room for doubting that 

 it had two slender arms from each arm opening, instead of a single 

 stout one as in Agaricocrinus, which, so far as we are aware, never has 

 more than three arm openings to each posterior ray, and two or three 

 to each of the others. It is the only species we have ever seen of the 

 Dorycrinus group with the peculiar sculpturing of its body plates 

 already mentioned. This sculpturing, however, is very different from 

 that seen on Agaricocrinus cormculm, which the species most nearly 

 resembles in several respects, being a peculiar pitting of the whole sur- 

 face of each individual plate, with a few larger marginal indentations. 

 Its greater number of arm openings (four to each posterior ray, and 

 three to the anterior one, instead of two to each all around), would 

 alone at once distinguish it from that species, even in specimens with- 

 out the arms. 



