PLATYCRINIDZ®. 699 
nent and larger, forming a sort of rim around the stem. Suture lines slightly 
grooved, except the interbasal ones, which are invisible. 
Basal cup moderately deep, its height equal to almost one half the length 
of the radials; the bottom slightly truncated, and having a circular depres- 
sion wide enough for the reception of the proximal stem joint. Radials a 
little wider than long, slightly spreading upwards, the outer ends of the 
upper face distinctly truncated; the facet semicircular, deep but rather 
narrow, directed obliquely upwards, slightly notched at the top. Costals 
small, sometimes not occupying the full width of the facet. Distichals twice 
as wide as long, free above the costals. Palmars as long as, but narrower 
than, the distichals. Arms six to eight to the ray, rather long, the three or 
four proximal plates above the axillary cuneate and uniserial, those beyond 
gradually passing into biserial. Ventral disk one third shorter than the 
dorsal cup, sub-pyramidal, the plates convex. Posterior oral larger and 
more prominent than the others, the latter twice as large as the adjoin- 
ing covering pieces. Ambulacra almost on a level with the general surface, 
dividing upon the disk. Interambulacral plates 3 and 2, those of the first 
row in part interbrachial and almost flat; the middle one hexagonal and 
longer than wide; the two at the sides scarcely bending outward. The 
middle plate of the anal side projecting and forming with the plates above, 
which are quite small and very numerous, a low ridge following the median 
line. Anus low down, directed almost horizontally. Column facet circular ; 
the stem joints elliptic and twisted, those close to the root nearly three times 
as wide as the proximal ones. 
Horizon and Locality. — Upper Burlington limestone ; Burlington, Towa. 
Types in the collection of Wachsmuth and Springer. 
Remarks. — Meek and Worthen have referred this form to P. tenwi- 
brachiatus, as their figure 4b on Plate 3, Vol. V. of the Illinois Report 
evidently represents this species, the ornamentation not being correctly 
given in the figure. It differs from that species in the more elongate 
form of the dorsal cup, the much deeper base, and the surface markings. 
Specimens of this type exhibit much variation in number of arms, nine 
arms to the ray being sometimes found. Figs. 5a, b, on Plate LXVL, repre- 
sent a variety from the Lower Burlington, which we cannot distinguish from 
this species. 
