EOCENE ECHXNODERMATA. 127 



Sismondia conradi Cotteau, 1891, Paleontologie francaise, Echinides Eocenes, vol. 2, p. 300. 

 non Sismondia marginalis Cotteau, 1891, Paleontologie francaise, Echinides Eocenes, vol. 2, p. 267. 

 Monostychia marginalis Gregory, 1S91, Geol. Soc. America Bull., vol. 3, p. 105. 

 Pcriarchus (?) marginalis Stefanini, 1911, Soc. geol. italiana Boll., vol. 30, p. 697. 



Determinative characters. — Test small in size; marginal outline subquadrate to suboval, the 

 sides and posterior margin being truncated so that the posterior border is semiquadrate and the 

 anterior border is semicircular ; the whole form much depressed, though centrally it rises in the 

 form of a low gently rounded mound; margin thin, with a small notch in the middle of the pos- 

 terior edge; under surface flat. Ambulacral petals elliptical, extending about halfway to the 

 margin. Apex and apical system central, or slightly excentric anteriorly. Peristome small, 

 subcircular ; ambulacral furrows simple and straight for over halfway to the margin, then becom- 

 ing obscure. Periproct very small, subpentagonal or subcircular, inframarginal. 



Dimensions. — Length 21 millimeters; width 21 millimeters; height 3 millimeters. This is 

 Conrad's type, here figured for the first time. 



Description. — The species, though described by Conrad in 1865, has never been figured and 

 is not very well known. There is some doubt in regard to its generic position. Gregory's remark 

 that it should be transferred to the genus Monostychia is not agreed to, for the reason that the 

 type reveals five genital pores and fails to show any continuation of the actinal grooves abac- 

 tinally. The test is small in size, the type being about ^-f inch in diameter and about \ inch 

 in height. In marginal outline it is subquadrate to suboval, with the sides and posterior margin 

 truncated so that the posterior half of the border is semiquadrate and the anterior half semi- 

 circular. The upper surface is so much depressed all around the marginal area as to be quite 

 flat, or even slightly concave, but within the area covered by the ambulacral petals it rises in the 

 form of a low, gently rounded mound with a rather broad, flattened summit. The margin is thin, 

 though slightly thicker than the submarginal area, with a small notch in the middle of the pos- 

 terior side. The under surface is flat. The apex is central or slightly excentric anteriorly, at 

 the summit of the tumid central area. 



The ambulacral areas are narrow in the petaloid region, wider in the marginal region; but 

 the condition of the specimen does not permit the making out of many details. The dorsal por- 

 tions of the ambulacra are petaloid. The petals are subequal in length, elliptical in form, slightly 

 open at the ends and extend about halfway to the margin. The poriferous zones are rather wide, 

 but slightly less so than the interporif erous 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 relatively large. 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 mound. The madreporite is 

 relatively large, occupying the larger part of the area covered by the system, and faintly tumid. 

 There are five genital pores near the edge of the madreporite and five small radial plates each 

 perforated by a minute pore. 



The peristome is small, subcircular, central; the ambulacral furrows simple, straight, and 

 quite distinct for slightly more than half the distance to the margin, then becoming obscure. A 

 small ridge extends from the peristome along the middle of each ambulacral furrow for a greater 

 distance than the diameter of the peristome. 



The periproct is very small, considerably smaller than the peristome, subpentagonal or sub- 

 circular, situated on the under surface near the posterior margin, being about one-fifth the radius 

 from the border. 



Related forms. — Of American forms S. (?) conradi most closely resembles S. tuomeyi and some 

 of the smaller specimens of Periarchus lyelli, but is easdy separated from these species by its 

 subquadrate marginal outline and the inframarginal position of its periproct. It also resembles 

 S. mississippiensis, but its periproct is less nearly marginal. It does not appear closely related 

 to any of the known foreign forms. 



Locality. — Conrad said his type came from near Charleston, S. C. 



Geologic horizon. — Probably Cooper marl, upper Eocene. 



Collection. — Academy of Natural Sciences of Philadelphia (1081). 



