PLATYCRINIDZ. rs) 
Platycrinus cavus HAtt. 
Plate LX VII. Figs. Sa, 6. 
1858. Hat; Geol. Rep. Iowa, Vol. I., Part IL., p. 527, Plate 8, Figs. la, 6b. 
1881. W. and Sp.; Revision, Part IL, p. 71 (Proceed. Acad. Nat. Sci. Phila., p. 245). 
Of medium size. Dorsal cup saucer-shaped; the basal disk slightly pro- 
jecting below the lower margins of the radials; the latter curving gently 
upwards. The facets face somewhat outward, and have a slight depres- 
sion around their lower margins. Surface marked by concentric lines of 
nodes or granules, which are sometimes confluent. There are two or three 
ranges of these nodes parallel to the margins of the basal disk, and similar 
ranges of nodes surround the radials, leaving a plane space equal to the 
granulate portions between the latter and the edges of the facets. Basi- 
radial and interradial sutures channeled, and the sides of the plates slightly 
beveled. Ventral disk high, pyramidal ; the plates moderately convex. 
Basal disk rather large, concave in the middle to the width of the stem ; 
the interbasal suture lines grooved, but the sutures invisible. Width and 
length of the projecting radials as four to three. Facets projecting, semt- 
circular; slightly notched at the upper end; the surface concave. Costals 
subtrigonal, moderately large, occupying the full width of the facets; much 
wider than high. Distichals rather short; placed obliquely against the cos- 
tals, and directed outward. The higher orders of brachials are not preserved 
in the specimen, but portions of the arms indicate that the species had from 
six to eight arms to the ray, which were comparatively delicate. Ventral 
disk twice as high as the dorsal cup; the interambulacral spaces depressed, 
the ambulacra rising abruptly above the general surface of the disk. Orals 
incompletely visible in the specimen, so that their arrangement could not be 
ascertained, but the plates are large. Covering pieces alternately arranged 
in two rows; all equal in size. Interambulacral plates 3 and 2; the two 
outer ones of the first row curving outward, and the middle one larger than 
the others. The middle plate of the anal side somewhat the widest and 
shortest, and its upper margin slightly furrowed by the anus, which is placed 
low down upon the disk and directed laterally. 
Horizon and Locality. — Lower part of Upper Burlington limestone ; 
Burlington, Lowa. 
Type in the (Worthen) Illinois State collection at Springfield. 
