188 MESOZOIC AND CENOZOIC ECHIJSTODEKMATA OF THE UNITED STATES. 



given the name of the form as S. alberti instead of S. aberti, as it should be. The test is large to 

 very large in size, ranging from 90 millimeters to about 180 millimeters in diameter. In mar- 

 ginal outline it is subcircular to subquadrate, broader posteriorly than anteriorly, sometimes 

 very noticeably so. The whole form is much depressed although more so around the margin, 

 within the large area covered by the ambulacral petals it rises in the form of a low broad gently 

 rounded mound which varies in height from about \ to about £ inch, and the summit of which is 

 flattened and excentric anteriorly. The margin of the test is relatively thin and undulating; 

 with a faint, broad, shallow curved notch opposite each of the three anterior petals, two large, 

 broad, deep, curved notches opposite the posterior pair of petals and one large, broad, deep, 

 pointed notch in the middle of the posterior edge. Between the large central notch and each 

 of the large curved notches the test extends in the form of a large, broad, blunt projection; the 

 two projections and three large notches together forming the whole posterior border of the test. 

 The under surface is flat. The apex is decidedly excentric anteriorly, at the summit of the 

 tumid central area. 



The ambulacral areas are narrow in the petaloid region, but very broad in the margina 

 region, where they are considerably wider than the interambulacral areas. The dorsal portions 

 of the ambulacra are petaloid. The petals are very large, subequal in length, the posterior pair 

 being somewhat longer than the others; subspatulate to Indian-club shape in form; nearly 

 closed at the ends; extending more than two- thirds the way from the center to the margin. 

 The poriferous zones are wide, wider than the interporif erous areas ; the inner row of pores oval, 

 outer row slitlike, pairs of pores conjugated, the groove lines continuing beyond the inner row of 

 pores to the center of the interporiferous areas, and the spaces between the grooves very narrow. 



The interambulacral areas are broad near the ends of the petals, but become narrower toward 

 both center and margin. The plates are polygonal and are very large, especially those on the 

 under surface. Single plates are often found in a more or less weathered condition showing on 

 both their upper and under surfaces numerous grooves and ridges. The whole surface of the test 

 is covered with small uniform tubercles which are slightly larger on the under surface. 



The apical system is relatively large, slightly excentric anteriorly, on the flattened surface 

 posterior to the apex of the test. The madreporite is relatively large, subpentagonal, occupying 

 the larger part of the area covered by the system and flush with the surface. There are five 

 small genital pores near the points of the madreporic pentagon and five small triangular radial 

 plates each perforated by a very small elliptical pore. 



The peristome is relatively small, subcircular to subpentagonal, subcentral: the ambulacral 

 furrows simple and straight for a short distance from the peristome, then forking and diverging 

 nearly symmetrically, each pah of branches proceeding in a more or less sinuous course and with- 

 out branching for over half the way to the margin, then branching considerably, the branches 

 beino- given off on the inside of the more important furrow, which continues to diverge more and 

 more as the margin is approached. 



The periproct is both relatively and actually very small, subcircular, inframarginal, a short 

 distance in from the point of the large, central notch in the posterior border. 



Related forms. — This species is not closely allied to any other American form. It bears a 

 superficial resemblance to Periarchus lyelli and P. altus but is readily separated from each of 

 them by the character of its ambulacral furrows, its inframarginal periproct, and the large 

 projections and notches in its posterior margin. It is similar in general features to two foreign 

 forms — S. subrotunda Lamarck, 1 from the lower Miocene of Bordeaux, France, and S. tarra- 

 conensis Lambert, from the Burdigalien Miocene of Barcelona, Spain. From each, however, it 

 can be easily distinguished by its much more pronounced posterior notches and projections and 

 by its less central apex. 



Localities. — Jones Wharf, St. Marys County, (figured specimen), Governor Bun, Calvert 

 County, and Dover Bridge, Dorchester County, Md. 



i Agassiz's monograph Des scuteUes, pp. 76-78, PI. XVII, flgs. 1-9. Desor, in Synopsis des echinides, p. 232, says that S. subrotundz LamarcK 

 is from the lower Miocene. 



