PLATYCRINID^. 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 leno-th 

 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 occui^ying 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 imiserial, 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 j^lates 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. 



MemarJcs. — Meek and Worthen have referred this form to P. tenui- 

 hracMatus, as their figure 46 on Plate 3, Vol. V. of the Illinois Eeport 

 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, &, on Plate LXVI., repre- 

 sent a variety from the Lower Burlington, which we cannot distinguish from 

 this species. 



