EOCENE ECHINODEEMATA. 133 



whole surface of the test is covered with small uniform tubercles which are slightly larger od 

 the under side. 



The apical system is central or slightly excentric anteriorly, on the somewhat flattened sum- 

 mit of the central mound. The madreporite 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 rnadreporite and five small radial plates each perforated by a small elliptical pore. 



The peristome is small, subcircular, central or slightly excentric anteriorly; the ambulacra! 

 furrows simple and straight for from one to two-thirds the distance to the margin, then forking 

 symmetrically at an angle usually of about 45°, each pair of branches continuing almost to the 

 margin. On well-preserved specimens, each branch may be seen to give off on the outside one 

 important side branch at right angles about halfway between the fork and the margin, which side 

 branches soon bend and in turn approach the margin. A small ridge extends out from the 

 peristome for a short distance along each ambulacral furrow. 



The periproct is very small, about half the diameter of the peristome, circular or subcircular, 

 situated on the under surface from one-third to slightly more than one-half the way from the 

 peristome to the posterior border. 



Related forms. — This species presents so many variable features that several apparently dis- 

 tinct varieties of it might have been recognized and separately described — such as one with a 

 transversely elongate subtriangular marginal outline, or one with periproct midway between 

 margin and peristome — but in each case intermediate forms show a gradation between the 

 possible variety and typical representatives of the species. P. lyelli Conrad is closely related 

 to the American forms P. pileus-sinensis and P. protuberans, both of which, however, have a 

 relatively higher and more conical central tumid area. P. protuberans is further distinguished 

 by its characteristic swollen anterior interporiferous area and smaller petaloid area. P. lyelli 

 also greatly resembles S. tuomeyi which, however, differs hi being somewhat truncated posteri- 

 orly and in having its periproct nearer the posterior border. In superficial characters P. lyelli 

 greatly resembles S. mississippiensis, but the latter is easily separated by the infrainarginal 

 position of its periproct. It does not appear to be closely related to any of the foreign forms. 



Localities. — Near Claiborne (Conrad's type and our specimen A); Cedar Creek, Clarke 

 County; St. Stephens, near Cocoa, Choctaw County; and Clarksville, Ala.; Shubuta Creek, 

 2\ miles north of Shubuta ; Chickasawhay River, near Shubuta; and Shell Prairie, near Pachuta, 

 Miss.; 14 miles below Macon, Twiggs County, Ga. ; Mount Enterprise, Rusk County, and 2 

 miles east of Alto, Cherokee County, Tex.; Castle Hayne (specimen B); City Rock Quarry, 

 Smith Creek; and Rocky Point, near Wilmington, N. C. 



Geologic •'horizon. — Claiborne group (middle Eocene) and Jackson formation (upper Eocene) . 

 In Texas in the "marine substage" of the Claiborne. In Mississippi probably near the base of 

 the Lisbon marl. In Alabama abundant in the Scutella zone at the base of the Jackson, "just 

 above the Claiborne fossiliferous sand." In North Carolina, hi the Castle Hayne limestone 

 (upper Eocene or Oligocene). 



Collections. — Boston Society of Natural History (larger figured form) (1553, A); U. S. 

 National Museum (smaller figured form); Alabama Geological Survey; American Museum of 

 Natural Historj-; Wagner Free Institute of Science; Williams College; Amherst College ; Johns 

 Hopkins University (T 2003, B; T 2004, C). 



Periarchus protuberans Twitchell, n. sp. 

 Plate LXII, figures 3a-f. 

 Determinative characters. — Test of medium size, subcircular in marginal outline; much 

 depressed, less so centrally where it rises hi a high, slightly asymmetric cone; margin very 

 thin, broadly dentate posteriorly; under sm-face flat. Ambulacral petals elliptical, very small, 

 extending about one-third the distance from center to margin. The anterior interporiferous 

 area swollen near the apical system which is slightly excentric posteriorly and inclined down- 

 ward and backward from the subcentral apex. Peristome very small, circular, in a slight con- 

 cavity; ambulacral furrows simple and straight for about half way tc the margin, then forking 



