DESCRIPTION OF SPECIES CRINOIDEA 283 



than the radials, followed by two somewhat smaller plates, and these by 

 two other much smaller ones which extend up to the level of the arm 

 openings. Arm openings two in each ray; arms not known. Ventral 

 disk depressed convex, small, its diameter much less than the diameter of 

 the calyx at its mid-height. 



The dimensions of a nearly perfect calyx are : Height, 7.5 millimeters ; 

 diameter, 7.5 millimeters; diameter of ventral disk, 4.5 millimeters. 



Remarks. — This species most closely resembles R. wortheni, from the 

 Burlington limestone, but it has a more deeply excavated base, besides 

 having the surface of the plates marked by the exceedingly fine pits or 

 punctae. It also resembles R. watersianus W. & Sp., from the Kinder- 

 hook at Le Grand, Iowa, but it may be distinguished by its more globular 

 form and its proportionately much smaller ventral disk, the diameter of 

 the circle formed by the arm openings in the Le Grand species being 

 nearly or quite as great as that of the calyx at its mid-height, while in 

 this species the circle has a much smaller diameter. The form of the 

 first costal plates in these two species is also different, in the Fern Glen 

 species this plate usually being quadrangular as it is in R. wortheni, 

 although occasionally an additional face may be developed on one side, 

 while in R. watersianus this plate is commonly hexagonal. 



AGARICOCRIXUS PRECURSOR Rowley 

 Plate 11, figures 7-12 



1902. Agaricocrinus precursor Rowley, American Geologist, volume 29, page 

 303, plate 18, figures 1-5. 



Description. — Calyx subhemispherical or subglobose in form, dorsal 

 cup flat or somewhat concave, ventral disk more or less dome-shaped. 

 Basal plates small, often nearly covered by the column, the column facet 

 either flush with the surface or slightly depressed. Eadial plates smooth, 

 flat or slightly convex, wider than long. First costals quadrangular, very 

 short and broad; second costals much wider than long; in the two pos- 

 terior lateral rays and sometimes in one of the anterior lateral rays this 

 plate is axillary and supports two series of distichals which give origin 

 to the arms ; usually in all three anterior rays the first costal is followed 

 by two other broad and short costals, the last one of which gives origin to 

 an arm. Anal plate about equaling the radials in size, but proportionally 

 longer and narrower, followed by three plates, each of which is nearly as 

 large as the anal itself. The plates of the ventral disk exhibit consider- 

 able variation in the different individuals, but the central plate at the 

 summit is always the largest plate of the entire calyx and is broadly and' 



