204 



PALEONTOLOGY OF ILLINOIS. 



The summit is composed of irregular, convex pieces, of various 

 forms, and provided with a slender, subcentral proboscis, rising 

 rather abruptly from its surface. Height to summit of vault, 

 0.6G inch ; do. to top of third radials on the anterior side, about 

 0.50 inch. Breadth of base, 0.35; height of do., 0.14 inch. 

 Breadth across the summit, between the rays, 0.61 inch. 



Fig. 12. 

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Fig. 13. 





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W do ^q> 



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Actinocrinus scitulus. (Natural size.) 

 Diagram showing the'anatomy out to 

 the second divisions of the rajs. 



A. Sillimani. (Enlarged two diameters.) 

 Cut showing the anatomy of this form, for com- 

 parison with A. scitulus. The specimen being in 

 a better condition, shows the secondary and 

 Tertiary radials more satisfactorily. 



This species is nearly related to A. verrucosus, Hall {Iowa Report, vol. 1, 

 part 2, pi. 10, Jig. 7), but differs in having its summit much more depressed, 

 and composed of less tumid plates, while its proboscis is more slender, and rises 

 more abruptly from the vault. It also differs in having but five, instead of 

 seven or eight, anal pieces below the horizon of the arms, and the nodes on its 

 body plates are more prominent, and of different form. It is true most of these 

 peculiarities are of a kind, in which individuals of the same species in this 

 genus sometimes vary to a greater or less extent, but in the depression of its 

 vault, and the less tumid character of the pieces composing it, as well as in the 

 slenderness of its proboscis, the individuals of the form under consideration 

 vary but little, and are readily distinguished from A. verrucosus. 



On comparing this species with Prof. Hall's description of his .4. rusticus, 

 and with specimens identified with it by Mr. Wachmuth, of Burlington, Iowa, 

 we have scarcely a doubt of their identity. Farther comparisons of additional 

 specimens, since describing our A. Sillimani, have also nearly convinced u.3 

 that the latter was likewise founded upon a variety of this variable species. 

 The type of J.. Sillimani has the body and base more depressed, and the nodes 

 on its body plates much more prominent; it also differs in having a distinct 



