224 



PALAEONTOLOGY OF ILLINOIS. 



summit of the interaxillary openings. They arc all pierced 

 directly through the middle by small perforations. 



The summit is very slightly convexj and composed of une- 

 qual plates, the larger of which are tumid, and the smaller less 

 convex. Extending from each false arm towards the middle, 

 there is a depression, occupied by smaller pieces, while the 

 larger and more convex plates occupy the prominences between. 

 Height, to top of first false arm-pieces, 0.48 inch; do. to the 

 true arm-openings, 0.39 inch; do. to the top of the vault, 0.56 

 inch. Breadth of body below the arms, 0.51 inch. 



Fig. 20. 



On 



SS8m 





Gilbertsocrinus fiscellus. (Magnified two diameters.) 

 In this diagram, showing the structure of the body out to the false-arm 

 bases, the anal area, and a part of each ray on that side, are restored. It 

 is probable, however, that the anal area may have more plates than here 

 represented, as we have reason to believe it was not isolated from the 

 vault, like the interrarlial areas. The last piece of the divisions of each 

 ray, marked a, connect laterally over each lmVrradial space, and form the 

 bases of the false arms. The dark spots are the true arm-openings. 



The specimen from which our description is made out is defective on the 

 anal side, but we have seen another, apparently of the same species, consisting 

 of the vault and the false arm-bases, together with the sides down to the true 

 arm-openings, inclusive. In this the anal area is clearly not insolated from the 



