134 MES0Z0IC AND CENOZOIC ECHINODERMATA OF THE UNITED STATES. 



symmetrically. Periproct very small, oval or subovate, on the under surface about one-fourth 

 the distance from the peristome to the posterior margin. 



Dimensions. — Length 54 millimeters; width 53 millimeters; height 9 millimeters. 



Description. — This new species is one of the handsomest of American scutellid forms. It 

 is very delicate and fragile and so thin that it is translucent from the edge halfway to the center. 

 It is a rare form, not over half a dozen individuals having been reported, all of which are from 

 the Eocene deposits of Mississippi, where it occurs in association with P. pileus-sinensis and 

 P. lyelli. The test is of medium size, ranging from 1 J to 2£ inches in diameter; and subcixcular 

 in marginal outline. All around the area from the edge nearly to the petals it is greatly depressed 

 and nearly flat; but within and slightly beyond the petaloidal area it rises in the form of a high, 

 slightly unsymmetrical cone which varies in height from f to 4 an inch. The margin of the test 

 is very thin all around, almost like a knife edge, and is decidedly undulating, or zigzag, especially 

 posteriorly, where it is broadly dentate. The under surface, though for the most part flat, is 

 slightly and broadly concave around the margin and the peristome. The apex is subcentral 

 at the summit of the central cone. 



The ambulacral areas are narrow in the petaloid region but wide in the marginal region 

 where they about equal the interambulacral areas. The dorsal portions are petaloid. The 

 petals are subequal in length, the odd petal slightly longer than the rest; relatively quite small, 

 extending only about two-fifths the distance from center to margin, the total area covered by 

 the petals being noticeably less than in most similar forms. The petals are elliptical hi form 

 and nearly closed at the ends ; the poriferous zones rather w T ide though less so than the inter- 

 poriferous areas, widest at then distal ends ; the inner row of pores oval, outer row slitlike ; the 

 pah's of pores conjugated. The anterior interporiferous area is swollen near the apical region 

 so as to form a small, narrow protuberance, confined withm the interporiferous area, largest 

 nearest the apical system and constituting the apex of the test. This protuberance is one of 

 the most striking and most distinctive features of the form ; and suggested the specific name. 



The interambulacral areas are large, nearly uniform, and composed of large polygonal 

 plates. The whole surface of the test is covered with small uniform tubercles which are slightly 

 larger on the under side. 



The apical system is slightly excentric posteriorly, inclined downward and backward from 

 the swollen anterior interporiferous area, on the steep posterior slope of the central subconical 

 mound. The madreporite is relatively large, occupying the larger part of the area covered by 

 the system. There are five genital pores near the edge of the madreporite, and five small radial 

 plates, each perforated by a small elliptical pore. 



The peristome is very small, not much larger than the periproct, circular, central, in a slight 

 concavity; ambulacral furrows simple and straight for about one-half the distance from peris- 

 tome to margin, then forking symmetrically, each pah of branches continuing nearly to the 

 margin and each branch giving off one important side branch at right angles at one-third to one- 

 fourth radius length from the margin; these side branches soon bend and hi turn approach the 

 margin. A small ridge extends for a short distance along the middle of each ambulacral furrow. 



The periproct is very small, oval or subovate in outline, the round end being nearest the 

 peristome, on the under surface from one-fourth to one-third the distance from peristome to the 

 posterior margin. It is much nearer the peristome than are the forks in the ambulacral furrows. 



Related forms. — P. protuberans is very similar in appearance to P. pileus-sinensis (Ravenel), 

 resembling it more closely than it does any other American form. It is, however, readily dis- 

 tinguished by its protuberant anterior interporiferous area, smaller petaloidal area, the position 

 of its apical system to the rear of the apex, its thinner marginal area, smaller peristome, and the 

 nearer approach of its periproct to the peristome. No foreign forms appear to be closely related 

 to this species. 



Localities. — Shubuta Creek, 2h miles north of Shubuta (type), and Chickasawhay River. 

 Shubuta, Miss. 



Geologic horizon. — Jackson formation, upper Eocene. 



Collection.— U. S. National Museum (164685). 



