238 PALEONTOLOGY OF ILLINOIS. 



sides, while the sutures between the first and second radials, 

 as well as between the second radials and the first arm pieces, 

 are generally more gaping. In most of the species all the arm 

 joints are proportionally longer, and more wedge-shaped than 

 in the characteristic species of Zeacrmus. From the typical 

 forms of Poteriocrinus it differs chiefly in having but two 

 primary radial pieces in the posterior and lateral rays, and in 

 the gaping character of the sutures between those pieces. 



Prof. Hall ranges this group as a subgenus under Poteriocrinus. If Troost's 

 genus Zeacrinus is to be retained as distinct from Poteriocrinus, it seems to us 

 from tbe species we have had an opportunity to examine, that Scaphiocrinus 

 might, with about as much propriety, be placed under the former, since the 

 species show no fundamental differences of structure from those of that group. 

 In order to fully comprehend the close relations between these groups, in the 

 number and arrangement of their parts, it is but necessary to compare the dia- 

 gram of Zeacrinus ramosus, Hall (Iowa Report, vol. I, part ii,p. 549), with his 

 Scaphiocrinus divaricatus (Sup. Iowa Repot, pi. 3, Jig. 6.) 



As already suggested in another place, it is not improbable that the type of 

 de Koninck and Lehon's genus Graphiocrinus, will be found to possess five 

 small basal pieces hidden by the column, within the range regarded as basals 

 by those authors. If so, we can scarcely doubt that Zeacrinus and Scaphio- 

 crinus should either be grouped together as a subgenus under Graphiocrinus 

 (which has priority), or stand under it as two distinct subgenera; unless all 

 three should be included as sections of Poteriocrinus. 



Scaphiocrinus decadactylus, M. and W. 



PL 11, Fig. 6. 



Poteriocrinus {Scaphiocrinus) decadactylus, Meek and Worthen, Sept., 1860. Proceed. 

 Acad. Nat. Sci., Philad., p. 394. 



Body rather small, broad obconical below the second radial 

 pieces, expanding somewhat rapidly from the base; plates 

 rather thick, and nearly or quite smooth; sutures between the 

 first and second radial plates, and between the second radials 

 and the first arm-pieces, gaping. Base small, convex, or about 

 half as high as wide, pentagonal in outline, as seen from below. 



