558 GEOLOGICAL SURVEY OF IOWA. 



six-sided, the upper side concave : second radials very wide, 

 irregularly six-sided, the lateral angles being truncated : 

 third radials five-, six- or seven-sided, depending on the 

 truncation of the lateral angles. Arm-plates quadrangular, 

 resting on the upper sloping sides of the third radials. 

 Structure beyond the first arm-plates unknown. 



This species is noticed here for the purpose of calling attention to the occurrence 

 of the genus in carboniferous rocks, the only species hitherto known being in rocks 

 of Silurian age*. 



FIG. 75. 



The accompanying figure illustrates the form and proportions of the 

 plates re 11. aining in this specimen, affording the means of comparison 

 with the other known species. 



Geological formation and locality. In the Burlington limestone : Bur- 

 lington, Iowa. 



* Prof. TROOST has described a species of this genus under the name of Cyathocrinus 

 tiartzformis = Ichthyocrinus tiarceformis, from White's Creek Springs in Davidson 

 county, Tennessee, which may be presumed to be of Carboniferous age. 



Ichthyocrinus tiaraeformis (TROOST). 



DESCRIPTION. Entire body globose, narrower at the base and somewhat flattened on 

 the summit. Column large : basal plates not visible. First radial plates adjoining and 

 surrounding the column : first and second radials somewhat hexagonal, concave above : 

 third radials six-sided, with the upper central angle extended in a projecting point be- 

 tween the bases of the first arm-plates; the anterior side with four radial plates. Arms 

 dichotomizing on the third radial, and again on the third or fourth plate above, making 

 twenty arms at their summit. 



FIG. 74. 



The accompanying diagram illustrates the form and pro- 

 portions of the plates to the first arm divisions, affording 

 the means of comparison with the Niagara and Burlington 

 species. 



The base is more rotund, and the column much larger and more impressed within the 

 base, than in the Burlington species; while it differs conspicuously from 7. I avis of the 



