PLATYCRINID^. 715 



Platycrinus cavus Hall. 

 Plate LXVII. Figs. 8a, h. 



1858. Hall ; Geol. Eep. Iowa, Vol. I., Part II., p. 527, Plate 8, Pigs, la, b. 



1881. W. and Sp. ; Revision, Pari II., p. 71 (Proceed. Acid. Nat. Sci. Pliila., 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 grannies, 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 fonr to three. Facets projecting, semi- 

 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. 



Horison and Locality. — Lower part of Upper Burlington limestone ; 

 Burlington, Iowa. 



Ti/pe in the (Worthen) Illinois State collection at Springfield. 



