352 PALEONTOLOGY OF ILLINOIS. 



ing to an arm, and on its outer a small quaternary radial, 

 bearing on its outer side a brachial piece, and on its inner 

 another auxiliary piece, giving origin to two arms, thus 

 making, as far as can be determined, ten arms to each ray, 

 or fifty to the entire series. 



Eirst anal piece as long as the first radials, but narrower, 

 heptagonal in form, and supporting one small piece over 

 its middle and a larger one on each upper sloping side in 

 the second range ; in the third range there are three, in the 

 fourth two pieces, and above these three smaller pieces ex- 

 tending up so as to connect with the vault. First interra- 

 dial pieces of about the size of the third radials, hexagonal 

 in form, and bearing two smaller pieces in the second range, 

 above which there are some five or six very small, irregular 

 pieces, some of the upper of which extend up and connect 

 with the vault. 



Vault rather ventricose, or more than one-third as high, 

 near the middle, as the body below the horizon of the arms ; 

 composed of irregular, nearly flat pieces, of moderate size ; 

 opening apparently simple, at the highest point about one- 

 third of the distance from the middle towards the posterior 

 side. 



All the body plates are protuberant and sharply angular 

 in the middle, the angular part, especially on the larger 

 plates, forming a sharp transverse ridge. From the under 

 side of this transverse ridge two or three others extend 

 downward on the first radial and first anal pieces, so as to 

 connect with others on the basal pieces. The second and 

 third radial pieces have each about three small pointed 

 nodes, arranged transversely, while from the third radials 

 narrow ridges extend up to the secondary radials, thence 

 up all the branches to each arm-base. All the smaller 

 body plates also have a more or less projecting angular 

 point in the middle. 



Notwithstanding the number of arms, the narrow anal 

 and interradial sinuses extend in so as to divide the disc 



