22 THE MESOZOIC EOHINODERMATA OF UNITED STATES. [bull. 97. 



and iuterradially until true pinnules are developed (5u the free brachials. 

 The brachials are united either by articulation or sizygial suture, in 

 the latter case the line of union often becoming obliterated. The in- 

 terradial plates, eig'ht or nine in number, form a rounded, slightly 

 elevated shield-like area, surrounded above by the alternating rows of 

 plates that diverge from the second distichal. 



Dimensions. — Calyx: height, 2 inches; breadth, 2J inches. Arms: 

 length, 20 inches ( ? ). 



Description. — The first specimen of this species was collected by 

 Prof, O. C. Marsh in 1870 from the Cretaceous of the Uinta Mountains 

 of Utah and by him compared with Marsupites of the English chalk.' 

 Subsequently other specimens were obtained from Kansas, and in 1876 

 were described by Mr. G. B, Grinnell, an assistant to Prof. Marsh at 

 tliat time, under the name of Uintacrinus sociaJis. Prof. F. B. Meek, 

 who had already in his possession similar material, added, in the Bulle- 

 tin of the U. S. Geological and Geographical Survey of the Terrtories, 

 descriptions of certain points in the structure not mentioned by Grinnell. 



All the specimens hitherto collected are much compressed, so that 

 the original form is with difficulty determined. In general, on any 

 single specimen only the plates of one side are preserved sufficiently 

 well for identification. 



The calyx is subglobose, with ten long, simple arms extending from 

 the upper side (PI. i. Fig. 16). The plates are thin, slightly convex, 

 and joined together by simple, though well marked, sutures, that are 

 generally slightly channeled. The calyx is without column or stalk, 

 and belongs to the class of "free" forms. 



The basis is composed of a centro-dorsal plate, surrounded by five 

 basals. The centro-dorsal plate is doubtfully visible ui^on only a 

 single specimen. Judging from the problematical portion of the plate 

 exposed, and the shape of the lower edges of the basals, it must be 

 small and pentagonal in form. Its presence was not noted by either 

 Grinnell or Meek, upon the specimens examined by them. The five 

 basals, also undescribed by previous writers, encircle the centro-dorsal 

 plate (PI, IT, Figs, la, 16). They rest with one side against the latter 

 and terminate in a sharp angle above. The whole basis forms a nearly 

 perfect pentagon, which is slightly depressed where the line of junc- 

 ture of two adjacent basals reaches the edge. Five large primary 

 radials surround the basals and alternate with them. They are six 

 or seven sided, the latter occurring when the edge toward the basis 

 is broken by an angle into two portions. Tbese plates are wider than 

 high, and come in contact with one another only in the lower i)ortion 

 of each side. The uj)per j)arts are separated by the interradial area, 

 as are the secondary and tertiary radials that succeed the primary. 

 The secondary radials are smaller than the primary, and are separated 

 by horizontal suture lines from them. The tertiary radials are axil- 



• Amer. Jour. Sci., 3d ser., vol. 1, 1871, p. 195. 



