EOCENE ECHIXODEBMATA. 135 



Peeiaechl-s pileus-sixexsis (Ravenel). 



Plate LXIII, figures la-e, 2a-d. 



Scutella pileus-sinensis Ravenel, 1844, Acad. Nat. Sci. Philadelpliia Proc, vol . 2. p. 97-98, no figure. 

 Scutella pileus-sinensis Ravenel, 1850, Am. Assoc. Adv. Sci. Proc, vol. 3, p. 160. 

 Sismondia pileus-sinensis Conrad, 1865, Acad. Nat. Sci. Philadelphia Proc, p. 74. 

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

 Periarchus pileus-sinensis Gregory, 1891, Geol. Soc America Bull., vol. 3, p. 105. 

 IScutella caput-linensis Kennedy, 1891, Texas Geol. Survey Third Ann. Rept., pp. 55, 56. 

 ^Scutella caput-sinensis Kennedy, 1895, Acad. Nat. Sci. Philadelpliia Proc, pp. 113, 114, 115. 

 Periarchus pileus-sinensis Stefanini, 1911, Soc. geol. italiana Boll., vol. 30, p. 688. 



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

 depressed, less so centrally where it rises in a high well-defined cone with straight or concave 

 sides; margin and submarginal area thin; under surface flat. Ambulacra! petals elongate- 

 elliptical, extending more than halfway to the margin. Apical system, central or shghtly excen- 

 tric anteriorly, f orming the summit of the central conical area. Peristome small, circular, ambu- 

 lacral furrows simple and straight for more than one-third of the distance to the margin then 

 forking symmetrically. Periproct very small, subcircular, shghtly more than one-third the way 

 from peristome to posterior border. 



Dimensions. — Specimen A: Length 79 milhmeters; width 76 milhmeters; height 16 milli- 

 meters. Specimen B: Length 65i milhmeters; width 67 millimeters; height 14 millimeters. 



Description. — This species was first described by Ravenel from the Eocene deposits of South 

 Carolina in 1844. His name, pileus-sinensis or "Mandarins hat," is most appropriate for the 

 form, as may readily be seen by a glance at the side view. The form is rather abundant in certain 

 localities, sometimes occurring in association with Periarchus lyelli and Periarchus protuberans. 

 The test is of medium size, ranging from 2\ to 3| inches in diameter. In marginal outline it is 

 almost circular, though shghtly broader posteriorly than anteriorly. All around the area outside 

 the ambulacra! petals it is greatly depressed and nearly flat ; but within the petaloidal area it rises 

 in a high well-defined cone with straight or concave sides. The height of the cone is somewhat 

 variable but in the larger forms it usually rises to over half an inch. The margin of the test is 

 thin and slightly undulating, both characters being most pronounced posteriorly. In some 

 specimens the submarginal area is thinner than the edge. There is usually a more pronounced 

 notch in the margin opposite the ends of the posterior paired ambulacra! petals. The under sur- 

 face is flat, even faintly concave between the margin and the region around the peristome. The 

 apex is shghtly excentric anteriorly, at the summit of the central conical area. 



The ambulacra! areas are narrow in the petaloicl region, wider in the marginal region where 

 they equal or shghtly exceed the width of the interambulacral areas. The dorsal portions are 

 petaloid. The petals are subequal in length, the anterior paired petals being shghtly shorter 

 than the other three; elongate elliptical in form; extending more than halfway to the margin, 

 the odd petal usually approaching the margin more nearly than the rest; shghtly open at the 

 ends. The poriferous zones are rather wide, about equal to the interporiferous areas; the inner 

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

 beyond the ends of the petals. 



The interambulacral areas are large, nearly tmiform, and composed of large polygonal plates. 

 The whole surface of the test is covered with small uniform tubercles, which are shghtly larger 

 on the under surface. 



The apical system is subcentral or shghtly excentric anteriorly at the summit of the conical 

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

 system and is somewhat tumid. 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 small, circular, central or slightly excentric anteriorly; ambulacra! furrows 

 simple and straight for more than one-third the distance from peristome to margin, then forking 

 symmetrically, each pah - of branches continuing nearly to the margin, each branch giving off 

 one important side branch at right angles at a point from one-third to one-fourth the radius of 



