130 MESOZOIC AND CENOZOIC ECHINODERMATA OF THE UNITED STATES. 



petals relatively longer and wider open at the ends, and its periproct inframarginal. Peri- 

 archus lyelli also resembles M. quinquefaria in general but has a thin margin. Of foreign forms 

 Scutella faujasi De France from the middle Tertiary of France is closely related to this species. 

 The specimen S. faujasi figured by Bronn in Lethsea geognostica, Plate XXXVI, figure 8, 

 shows the thick margin, discoidal outline, and similarly brandling ambulacral furrows. 



Localities. — Near Milledgeville (Say's types); 1 mile southwest of Sandersville (figured 

 specimen A); Washington County (figured specimen B); lh miles northeast of Tenville; Nor- 

 ton; and 2h miles south of Warthen, Washington County, Ga. 



Geologic horizon. — Claiborne group, middle Eocene, and Jackson formation, ( ?) upper 

 Eocene. 



Collections. — U. S. National Museum (9500, A); Academy of Natural Sciences of Phila- 

 delphia: Wagner Free Institute of Science; Geological Survey of Georgia; Amherst College (B). 



Genus PERIARCHTXS Conrad. 



Periarchus Conrad, 1866 (subgenus), Smithsonian Misc. Coll., vol. 7 (200), p. 21. 

 Periarckus Grabau and Shinier, 1910, North Am. index fossils, vol. 2, p. 591. 

 Periarchus Stefanini, 1911, Soc. geol. italiana Boll., vol. 30, p. 687. 



The genus Periarchus as used here includes the Scutella- like forms with central or sub- 

 central apical system, periproct actinal, but less than three-fifths of the way from the peristome 

 to the margin, thin margin, five genital pores and more or less abruptly tumid ambulacral area. 

 • The ambulacral furrows are usually simple and straight about halfway to the margin, then 

 bifurcate and proceed to the margin without much further branching. The ambulacral petals 

 are usually short, rarely extending more than halfway to the margin, more or less open at the ends. 



Periarchus altus Conrad. 



Plate LVIII, figures la-e. 



Scutella sp. Emmons, 1858, Agriculture Eastern Counties: North Carolina Geol. Survey Rept., p. 308, figs. 247, 248. 



Sismondia alta Conrad, 1865, Acad. Nat. Sci. Philadelphia Proc, pp. 74, 75, no figure. 



Mortonia (Periarchus) altus Conrad, 1866, Smithsonian Misc. Coll., vol. 7 (200), p. 21. 



Mortonia (Periarchus) altus Heilprin, 1884, Contr. Tertiary geology U. S., p. 17. 



Sismondia alta Cotteau, 18S9-1894, Paleontologie francaise, Echinides eocenes, vol. 2, p. 300. 



Periarchus altus Gregory, 1891, Geol. Soc. America Bull., vol. 3, p. 105. 



Periarchus altus Stefanini, 1911, Soc. geol. italiana Boll., vol. 30, p. 699. 



Periarchus sp. 6 Stefanini, 1911, Soc. geol. italiana Boll., vol. 30, p. 699. 



Determinative characters. — Test of medium size, circular hi marginal outline, much depressed, 

 less so centrally where it rises in a relatively high, broad, obliquely flattened momid, whose apex 

 is noticeably excentric anteriorly; margin of sides and posterior end thin, anteriorly the upper 

 surface declines in a straight line from apex to edge; under surface flat. Ambulacral petals 

 subelliptical, relatively large, extending about halfway to the margin. Apical system slightly ex- 

 centric posteriorly, and well to the rear of the apex. Peristome small, circular, slightly excentric 

 posteriorly; ambulacral furrows simple and straight for slightly more than half the way to the 

 margin, then forking symmetrically. Periproct very small, subcircular, about two-fifths the 

 way from the peristome to the posterior border. 



Dimensions. — Length 57.5 millimeters ; width 59 millimeters ; height 12 millimeters. This 

 is the type, refigured. 



Description. — This species was figured and partly described by Emmons as early as 1858. He 

 placed it in the genus Scutella but did not give the form a specific name. In 1865 Conrad, who had 

 seen Emmons's type, gave a good description of it and applied the ajDpropriate specific name 

 alta but he. did not figure it. It is rare, only the one specimen found by Emmons having yet 

 been reported. The test is of medium size. The marginal outline is circular. The upper sur- 

 face rises centrally in the form of a low broad mound, whose apex is noticeably forward of the 

 center; anteriorly the surface declines in a straight line from the apex to the edge; along the 

 posterior margin it is greatly depressed and nearly flat from the edge to the petaloid region, then 

 rises at first rather steeply then more gently to the apex. The relative height of the central 



