S AND P.LASTOTDS. 187 



in a marked degree with P. truncntiilus Hall, from tlie Lower 

 Burlington, i Inscribed in the Report on theGeology of Iowa, 1858, 

 Vol. I. Pt. '2. p. .~>:ix. and we should regard the two as identical, if 

 it were not for the fact that Hall's species has three primary 

 radials instead of two. as all other known species of the genus 

 with the exception of one from the Devonian, of New York. This 

 feature was indicated by Hall Iowa Report, supra who described 

 the second radial as of "quadrangular form, with a single groove 

 on the inside, showing that there was no bifurcation from this 

 plate." This was so great a departure from the normal type of 

 the genus, that we regarded it as an aberrant individual until re- 

 cently, when we obtained two specimens of that species with parts 

 of the arms preserved, which have a quadrangular second and 

 pentagonal third primary radial in every ray. This character 

 being thus apparently constant for the species, the LeGrand form 

 cannot be referred to it, and must be regarded as distinct. 



The color of the specimens is very light, in marked contrast 

 with Rhodocriuus kirbyi and Dichocrinus iuormitns. 



The species is of rather small size, its general form more 

 elongate than discoid ; the rays rising vertically in symmetri- 

 cal clusters, by equal divisions, from the upper portions of the 

 first radials. 



Calyx cup-shaped or subturbinate. spreading uniformly to the 

 top of the cup; truncate below, with a slight elevated rim for 

 the attachment of the column; plates convex; interradial and 

 basi-radial sutures beveled and indented. Basal disk small, con- 

 vex, margins slightly concave, the plates so closely anchylosed, 

 that no traces of sutures can be discovered. 



First radials a little higher than wide, increasing in width 

 upward; articular facets facing upward, and occupying a little 

 more than one-third the width of the plate. Second radials 

 triangular, sometimes pentangular, completely filling the facet. 



Arms given off from the second plate above the second radial, 

 and on the inner branches again from the second plate above, 

 making six arms to the ray in equal divisions, or 30 in all. 

 Two or three joints above the last bifurcation are usually 

 cuiieate, in single series, and above these the arms are biserial, 

 rounded, slender, tapering gradually to the tips. Pinnules 

 strong, not in very close contact. 



