PLATYCKINID^. 655 



tlie pinnules proportionally large and placed wide apart. Hall described a 

 rather young specimen, evidently of this species, as P. mtckiformis, another 

 with somewhat deeper suture lines as P. exsertus, and one in which the arms 

 are as yet in their embryonic state as P. nodobrachiatus ; while S. A. Miller 

 described as P. lautus a specimen showing the anal tube. 



Platycrinus symmetricus W. and Sp. 

 Plate III. Fig. 16; Plate LXIX. Figs, la, b, c. 



1890. W. and Sp. ; Geol. Rep. Illinois, Vol. VIII., p. 186, Plate 15, Fig. 8. 



Below medium size ; the specimens of light color, in marked contrast to 

 Dichocrimis inornatus and other species from the same locality, which are 

 dark. Calyx nearly as wide as high, the ventral disk depressed, and occupy- 

 ing in adult specimens one third of the total height, a little more in the 

 younger ones. Dorsal cup subturbinate, rising gradually from the top of the 

 basals to the arm bases; basal cup small and short. Plates moderately 

 heavy, almost flat except for the general curvature of the cup ; the sur- 

 face without ornamentation ; the basi-radial and interradial sutures dis- 

 tinctly channeled, the interbasal ones invisible. 



Basal cup shallow, rounded ; the column facet proportionally large and 

 circular. Eadials a little longer than wide, increasing in width upwards; 

 the limbs slightly curving inward, their upper faces sloping ; the middle por- 

 tions of the plates somewhat projecting and thickened toward the facets ; 

 the edges rather distinctly beveled. Facets shallow and dii-ected upwards, 

 their width equal to one half the diameter of the radials at the upper end. 

 Costals trigonal, small, but completely filling the facets. Distichals and pal- 

 mars one fourth wider than long, and the lower plates pinnule-bearing. The 

 upper plate of the distichals gives off an arm to the outer side and tAvo pal- 

 mars with two arms to the inner side, making six arms to the ray. The two 

 proximal arm joints following the bifurcations cuneate and singly arranged, 

 the succeeding ones biserial. Ventral disk low-convex, the plates very 

 numerous and small. Orals rather symmetrically arranged, the posterior 

 one a little larger than the others. The covering plates of the ambulacra 

 form narrow ridges composed of two rows of small alternating pieces, which 

 bifurcate upon the disk, and remain closed to the ends of the distichals. The 

 interambulacral spaces are deeply depressed, and at the four regular sides of 

 the disk consist in full grown specimens of fifteen or more pieces, of which 



