E0CE2STE ECHIXODERMATA. 131 



mound is greater than in most forms likely to be confused with it, being more than one-fifth of 

 the diameter of the test. The margin of the sides and posterior end is thin. The under surface 

 is flat, or faintly concave between the margin and the region around the peristome. The apex 

 is noticeably excentric anteriorly at the summit of the tumid area. 



The ambulacral areas are relatively wide in the petaloid region though wider in the mar- 

 ginal region, in both cases being wider than the interambulacral areas. The dorsal portions of 

 the ambulacra are petaloid. The petals are relatively large and broad; subequal ha length, the 

 posterior pair being slightly shorter than the rest; subelliptical in form; slightly open at the 

 ends; extending slightly more than halfway to the margin. The poriferous zones are rather 

 wide, but less so than the wide interporif erous areas ; the inner row of pores oval, outer row slit- 

 like; pairs of pores conjugated. A few pahs of pores occur beyond the ends of the petals. 



The interambulacral areas are relatively small between the petals and though wide near the 

 margin in both cases are smaller than the ambulacral areas; 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, located well back of the apex on a flat- 

 tened area which extends obliquely downward and backward from the apex and is about three 

 times the diameter of the system. The madreporite is relatively large, occupying the larger 

 part of the area covered by the system, subpentagonal, flush with the surface. There are five 

 large circular genital pores perforating the basal plates at the points of the madreporie pentagon, 

 and five small radial plates, each porf orated by a minute pore. 



The peristome is small, circular, slightlj T excentric posteriorly; the ambulacral furrows 

 simple and straight for slightly more than half the distance to the margin, then forking symmet- 

 rically, and at rather a wide angle, each pah of branches continuing nearly to the margin. A 

 small ridge, extends out for a short distance from the peristome along the middle of each ambula- 

 cral furrow. 



The periproct is very small, subcircular, situated on the under surface about two-fifths 

 of the way from the peristome to the posterior border. It is nearer the peristome than are 

 the forks in the ambulacral furrows. 



Related forms. — The species is most closely related to P. lyeUi, from which, howeyer, it is 

 easily distinguished by its anteriorly excentric apex, by the noncoincidence of its apical system 

 and apex, by the fact that the central tumid mound involves nearly the whole upj>er surface 

 instead of being confined to the petaloid region, and by its broader petals. No foreign forms 

 appear to be closely related to this species. 



Locality. — Core Creek, Craven County, N. C. 



Geologic horizon. — Trent marl, middle Eocene. 



Collection. — Williams College. 



Periarchtts lyelli Conrad. 



Plate LXI, figures 2a-f ; Plate LXII, figures la-e, 2a-d. 



Scutella lyelli Conrad, 1834, Acad. Nat. Sei. Philadelphia Jour., 1st ser., vol. 7, p. 152, no figure. 



IScutella lyelli Morton, 1834, Synopsis organic remains Cretaceous, p. 77, PI. X, fig. 8. Figure shows upper surface 



only; no description. 

 Scutella lyelli Conrad, 1842, Nat. Inst. Promotion Sci. Proc, 2d Bull., p. 175. 



IScutella lyelli Bronn, 1848, Index palseontologicus, vol. 1, p. 1126; vol. 2, p. 196. Based on Morton. 

 ISismondia lyelli Desor, 185S, Synopsis des echinides, p. 227. Based on Morton. 



non Scutella lyelli Emmons, 1858, Agriculture Eastern Counties, North Carolina Geol. Survey Rept., p. 30S, text fig. 246. 

 ISculella lyelli Gabb, 1359, Cat. invert, fossils Cretaceous, p. 19. Based on Morton. 

 tScutella lyelli Dujardin and Hupe, 1862, Hist. nat. zoophytes echinodermes, p. 559. Based on Morton. 

 Sismondia lyelli Conrad, 1865, Acad. Nat. Sci. Philadelphia Proc, p. 74. 

 Mortonia (Periarchm) lyelli Conrad, 1866, Smithsonian Misc. Coll., vol. 7 (200), p. 21. 

 IScutella lyelli Agassiz, 1883, Mus. Comp. Zoology Mem., vol. 10, No. 1, p. 89. Based on Morton, 

 non Scutella lyelli Heilprin, 1SS4, Contrib. tertiary geology U. S. p. 17. 

 Scutella lyelli Smith and Johnson, 1S87, U. S. Geol. Survey Bull. 43, p. 21. 

 tSismondia lyelli Cotteau, 1889-1S94, Paleontologie francaise, Echinides eocenes, vol. 2, p. 301. Based on Morton. 





