PALAEONTOLOGY OF IOWA. 617 



Geological position and locality. In the Keokuk limestone : Nauvoo, 

 Illinois. 



Actinocriims pernodosus, var. ? 



PLATE XVI. FIG. 7. 



This fossil bears so close a resemblance to Jl. pernodosus, Plate xv, fig. 3, that I 

 can find no means of specific distinction : the basal plates are thinner at their mar- 

 gins, and the nodose plates more angular in their prominences, while the third radial 

 plates are not bent outward as in the larger individual. The details of the structure 

 and arrangement of the plates of the body, and the bases of the arms, are essentially 

 alike in the two specimens, and this one is probably only the young of that species. 



Fig. 7. The anterior side of the specimen. 



Geological position and locality. In the Keokuk limestone at Warsaw, 

 Illinois. 



Agaricocrinus tuberosus. 



PLATE XVI. Fia. 2 a, b, c. 

 A garicocrinus tuberosus : TROOST, in MS. and published Catalogue of Crinoideae. 



Amphoracrinus americanus? RCEMER, Bronn's Lethea Geognostiea, Vol. ii, p. 250; 



Tab. iv, f. 15 a, b. 



BODY robust, subconical or hemispheric, with strong arms : 

 base more or less deeply concave, the concavity involving 

 the entire radial series of plates. Basal plates very small, 

 rarely projecting beyond the circumference of the column. 

 First radial plates wider than long, the postero-lateral ones 

 largest. Second radials quadrangular, wider than long. Third 

 radials larger than either of the others, pentagonal, and 

 supporting on each upper sloping side a wide brachial plate, 

 which is succeeded by a second and sometimes a third si- 

 milar plate, all projecting at nearly right angles to the axis 

 of the fossil ; the outer limit of the basal concavity being at 

 the third radial plate, and usually near the summit of the 

 same. Interradial plate single, twelve-sided, extremely elon- 

 gate, resting on the first radials, and its lower lateral edges 

 sustained by the second and third radials, while the upper 

 lateral edges are bordered by the first or by the first and 

 second brachial plates : its upper narrow wedgeform ex- 

 SURVEY.] 78 



