EOCENE ECHINODERMATA. 125 



varying somewhat both among the petals on a single specimen or on different specimens, being 

 slightly wider, equal to, or slightly narrower than the interporiferous areas; the inner row of 

 pores oval, outer row slitlike, pairs of pores conjugated. A few pairs of pores occur beyond the 

 ends of the petals. 



The interambulacral areas are large though somewhat smaller both between the petals and 

 at the margin than the ambulacral areas. The whole surface of the test is covered with small 

 uniform tubercles which are slightly larger on the under side. 



The apical system is subcentral, at the summit of the central tumid area. The madre- 

 porite is relatively large, occupying the larger part of the area covered by the system, and flush 

 with the surface. There are five genital pores near the edge of the madreporite. One specimen 

 studied possessed the abnormal feature of a double posterior genital pore. There are five small 

 radial plates, each perforated by a minute pore. 



The peristome is very small, not over double the size of the periproct, circular, central or 

 slightly excentric posteriorly; the ambulacral furrows simple and straight for two-thirds or 

 more of the distance from peristome to margin, then apparently forking symmetrically, each 

 pair of branches continuing almost to the margin. The under side is revealed on few specimens, 

 and on few can the furrows be made out. 



The periproct is very small, circular, almost at the. ambitus though slightly inframargmal, 

 situated at the under and inner edge of the notch in the middle of the posterior margin. 



Related forms. — S. mississippiensis does not appear to be closely related to any other 

 American form. It resembles Periarchus lyetti in general appearance, and specimens have 

 doubtless been erroneously labeled S. lyelli in some collections; S. mississippiensis, however, is 

 readily distinguished by its submarginal periproct, greater width, the notch in its posterior edge, 

 and the fact that its tumid central area involves nearly the whole ujDper surface. It bears an 

 interesting resemblance to the recent species Scutella parma, but differs from it in having less 

 widely open petals, much narrower interporiferous areas, a less circular and more anteriorly 

 pointed form, and in not having its periproct exactly marginal or at the ambitus. 



Of foreign forms S. patagonensis (Desor) Ortman, especially the "alate" form described by 

 Ortman and formerly known as Ecliinarachnius juliensis Desor from the Neocene of Patagonia, 

 appears to be most closely related to S. mississippiensis. It is very similar in marginal outline, 

 location of periproct, etc., but differs in having a less central apical system, longer petals, nar- 

 rower interambulacral areas, and in having its ambulacral furrows beginning simply and then 

 forking in symmetrical curving branches almost immediately. S. suitetragona De Grateloup and 

 S. striatula M. de Serres, both from the upper Eocene of France, also present certain points of 

 identity with $. mississippiensis. Each, however, differs from the American form in having 

 four genital pores instead of five and in having the ambulacral furrows fork near the peristome. 



Localities. — -Cemetery Branch (type), Radiate Banks, Spillman's Clam bed and other 

 localities in and around Enterprise, Clarke County, Miss.; San Augustin, and 1 mile southwest 

 of Palestine, Tex. 



Geologic horizon. — Claiborne group, middle Eocene. In Mississippi, at the base of the 

 Lisbon marl. In Texas, in the marine substage of the Claiborne. 



Collections. — TJ. S. National Museum (137655, A; 137655a, B); Wagner Free Institute of 

 Science. 



Scutella tuojieti Twitchell, n. sp. 

 Plate LX, figures la-d. 



Determinative characters. — -Test medium in size, subovate in marginal outline, somewhat 

 pointed anteriorly and truncated posteriorly; the whole form greatly depressed, being thin and 

 nearly flat around a narrow marginal area, and only slightly tumid within the petaloidal area; 

 under surface flat. Ambulacral petals narrowly elliptical, the posterior pair extending about 

 half, the others decidedly more than half way to the margin. Apex and apical system central, 

 at the summit of the tumid area. Peristome, small, circular; ambulacral furrows simple and 

 39S00 — 15 



