634 GEOLOGICAL SURVEY OF IOWA. 



This species differs from the preceding, both in form and proportionate size of the 

 plates, as well as in the number of plates in each division of the ray. It is a much 

 more delicate species than either of the others. 



Fig. 2. View of the anal and postero-lateral side of specimen. 



Fig. 4. Diagram of plates enlarged, showing their form and arrangement; those of the 

 anal area being determinable only on one side. 



Geological formation and locality. This specimen was given me by Mr. 

 GIDDINGE, of Boonville, Missouri, and supposed to be from the Keokuk 

 limestone. 



Platycrinus safforcli. 



PLATE XVIII. FIG. 5 & 6. 

 Platycrinus saffordi : TROOST in MS. 



CALYX large, urnshaped or subturbinate : base moderately 

 depressed for the insertion of the column. Basal plates large, 

 from three-fifths to five-eighths as long as the radial plates. 

 Radial plates about as long as wide ; the articulating surface 

 narrow, occupying little more than one-third- the width of 

 the plate at the summit, and margined by a moderately 

 thickened angular border. Surface of basal plates ornamented 

 by lines of radiating nodes, which extend from the base to 

 the upper margins of the plates. Radial plates marked by a 

 prominent line of nodes- from the two outer basal angles 

 diagonally to the base of the brachial articulating scar, and, 

 between these and outside of them, by concentric lines of 

 nodes parallel to the margins of the plates. 



The specimen figured is much smaller than the authentic specimens of Dr. TROOST, 

 on which the species was founded. The proportions of the plates and of the cup, as 

 well as the exterior markings, correspond so closely as to leave no doubt of the 

 identity of the specimens when compared with several in Dr. TROOST'S collection. 

 We may presume, moreover, from the occurrence of this species and of Jlctinocrinus 

 nashvilloR, Jlgaricocrinus tuberosus and Cyathocrinus stellatus in the same association 

 in Iowa and Illinois, that the Keokuk limestone is the horizon from which these 

 species are obtained in Tennessee. 



Fig. 5. Lateral view, showing the base and three radial plates. 



Fig. 6. Basal view of an imperfect specimen. 



Geological formation and locality. In the Keokuk limestone : Warsaw, 

 Illinois. 



