662 THE CEINOIDEA CAMERATA OF NORTH AMERICA. 



smaller than the correspondmg plates of the other sides, and placed at 

 a higher level than the middle one. The plates surrounding the anal opening 

 not projecting ; the opening low down, and directed laterally. 



Horizon and Locality. — Upper Burlington limestone ; Burlington, Iowa. 



Tyj^e in the Museum of Comparative Zoology. 



Remarks. — This species differs from all others of this group in the abrupt 

 spreading of the lower brachials, and the quinquelobate outline of the calj-x. 

 The nodes upon the basals, although quite distinct in the type, are in other 

 specimens but faintly represented, and sometimes absent altogether. 



Platycrinus Halli Shum. 



Plate LXXII. Figs. 6a, b, and 7a, b. 



1865. Shumakd; Catal. Palaeoz. Toss. North Amer., p. 388 (Trans. Acad. Sci., St. Louis, Vol. 11.). 

 1873. Meek and Worthen ; Geol. Rep. Illinois, Vol. V., p. 454, Plate 3, Figs. 3a, b, c, d. 



Sjn. P. planus HiH (not 0. and SerJi.) ; Geol. Rep. low.i. Vol. I., Pait II., Plate 8, Pigs. 6a, b. 



Syn. P. olla Hall, 1861 (not De Koninck and Lehon 1853) ; Descr. New Spec. Grin., p. 16. 



A rather large species ; the calyx large in proportion to the length of 

 the arms, one fifth higher than wide; height of the ventral disk, as compared 

 with that of the dorsal cup, as two to three, the former hemispherical, the 

 latter bell-shaped, widest around the fiicets. Plates of the dorsal cup slightly 

 convex, thickened below the facets, and rather heavy throughout ; inter- 

 radial sutures somewhat depressed, giving to the cup a slightly jjentangular 

 outline. 



Height of base- equal to half the length of the radials, the outer surface 

 regularly rounded, except beneath the column, which rests within a circular 

 depre.ssion ; sutures between the plates often visible. Eadials a little longer 

 than wide, slightly spreading, somewhat irregular in form, especially the 

 posterior ones, which are frequently wider than the others and asymmetri- 

 cal, owing to the wider and deeper truncation of the ujDper angles at the 

 anal side. Facets rather wide and deep, semicircular to semiovoid, the upper 

 edges slightly notched. Costals irregularly pentagonal with concave upper 

 faces ; rarely trigonal. Distichals, palmars and post-palmars from once and 

 a half to twice as wide as long ; the distichals of the same ray in sutural 

 contact laterally, but among the palmars and post-palmars only the plates 

 of the same subdivisions. Arms seven to eight to the ray, quite short and 

 moderately heavy, their two or three proximal plates cuneate and alternately 

 arranged, the succeeding ones arranged biserially ; the latter rather long, 



