290 PALEONTOLOGY OF ILLINOIS. 



axillary appendages (wings of Lyon *), and a central or subccntral opening of 

 the vault, with a small first anal, and short first radial pieces. The other sec- 

 tion consists of species with the interaxillary appendages small or wanting, the 

 opening of the summit lateral, and the first anal pieces nearly or quite as large 

 as the first radials. Of the latter group, Dichocrinus cornigerous, of Shumard, 

 is an example ; it will probably also include D. sexhbatus, of Shumard. 



It will be observed that the last mentioned section bears somewhat similar 

 relations to the typical species of this genus, that Pleurocrinus, of Austin, bears 

 to Platycrinus. 



All the species of this group known to us, are from the Chester division of 

 the Subcarboniferous series, or near that horizon. 



Pterotocrinus crassus, M. and W. 



PI. 23, fig. 2 a, 2 b. 



Dichocrinus (Pterotocrinus) crassus, Meek and Worthen, Sept., 1860. Proceed. Acad. 

 Nat. Sci, Philad., p. 382. 



Body below the arms basin-shaped, more than twice as wide 

 as high; widening rapidly upwards from the base; composed 

 of thick, apparently smooth plates, connected by linear sutures. 

 Base comparatively large, about four times as wide as high, dis- 

 tinctly concave below, and widening upwards from its round- 

 ed lower margin; slightly sinuous at the extremities of the 

 suture; columnar facet round, concave, and about one-fourth 

 as wide as the base. First radial plates broader than long, a 

 little convex, widening regularly upwards from the base, and 

 concave on the upper side, which is longer than either of the 

 others; four of them subquadrangular, with the lateral angles 

 slightly truncated, and one on the anterior side, subpenta- 

 gonal in consequence of having a fifth obscure angle at the 



* Mr. Lyon seems to think these appendages were articulated, or movable, so as to 

 bave been used in seizing and crushing food. In this view, however, we cannot con- 

 cur. So far as we have been able to see, these appendages appear to us to be enor- 

 mously developed vault pieces, analogous to the great spines seen rising from the vault 

 of Dorycrinus. In some species they were directed obliquely outwards, and in others 

 they were more erect, but we cannot believe they were movable like the arms. 



