584 GEOLOGICAL SURVEY OF IOWA. 



SUMMIT composed of a great number of small plates, rising 

 to a low dome, in the centre of which is a small aperture. 

 Broad shallow grooves pass downward from this dome, and 

 become deep defined channels between the sets of arms from 

 each ray. 



This is one of the most beautifully ornamented species of this genus, so far as 

 known among the American species of ACTINOCRINUS. 



Fig. 12. View of the anterior side, the specimen being slightly compressed. 



FIG. 86. 



5 ..-;,* The accompanying diagram is an 



illustration of the anterior and right 

 antero-lateral ray, showing the form 

 and proportional size of the plates, 

 c ; l the mode of bifurcation in the supra- 



__, ( ^ radial plates, and the origin of the 



\^\ ( / "^i r^\ arms. The structure of the supra- 



Xxx^v^^^x^,) radial and brachial series is precise- 



I ^ I / 2> / ty ^ e that of A. mvltibrachiatus ; 



^N^^VX^/ but the plates are larger, the inter- 



radial plates are more numerous, and 

 there is an intersupraradial series as 

 well as axillary plates which do not 

 exist in that species. 



The series of plates referred to are numbered in the diagram as follows: 1, Basal 

 plates; 3, Radial plates; 4, Supraradials; 5, Brachial .plates; 6, Arm-plates; 7, 

 Interradials; 8, Intersupraradials; 9, Axillary plates. 



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

 lington, Iowa. 



Actinocrinus proboscidialis ( n. s.). 



PLATE X. FIG. 13. 



BODY below the arms turbinate, above terete conical, 

 gradually contracting into an elongate proboscis : base ex- 

 cavated for the reception of the column. Basal plates short, 

 thick, projecting over the columns in a thickened rim, and 

 deeply grooved along the sutures. First radial plates much 

 wider than long. Second radials quadrangular, nearly twice 

 as wide as long. Third radials somewhat irregularly hexa- 

 gonal or pentagonal, and supporting on each of the upper 



