138 MESOZOIC AND CENOZOIC ECHINODERMATA OF THE UNITED STATES. 



The anibulacral areas are rather wide, both at the margin and across the middle of the 

 petals, in both cases wider than the interambulacral areas; dorsal portions petaloid. The 

 petals are rather large, elliptical to subspatulate in form, anterior pair slightly shorter than the 

 rest which are about equal in length, all extending from about one-half to two-thirds the way 

 to the margin, moderately to widely open at the ends. The poriferous zones are wide, inner 

 rows of pores elevated, nearly straight, either nearly parallel or slightly diverging and consist 

 of round openings; outer row of pores depressed below the general surface, curving and com- 

 posed of slitlike openings; pairs of pores conjugated; interporiferous areas varying in width 

 from only slightly wider than poriferous zones to nearly twice as wide, elevated so that the 

 petals are somewhat tumid. 



The interambulacral areas are wide oj>posite the ends of the petals, rather narrow at the 

 margin. The whole surface of the test, including even the ridges between the pairs of pores of 

 the poriferous zones, is covered with very small, uniform imperforate tubercles, set in rather 

 deep scorbicules. The tubercles are slightly larger on the under surface. 



The apical system is central, coincident with the apex. There are five genital pores at the 

 tips of the points of the almost star-shaped madreporite; and five small ocular plates extending 

 between the points of the star, each perforated by a minute pore. The apical system, together 

 with a very small area immediately surrounding it and involving the inner ends of the petals, is 

 elevated slightly above the rest of the test. 



The peristome is medium in size, central, subcircular to subpentagonal, and situated at the 

 deepest portion of the concavity on the under surface. The anibulacral furrows are simple, 

 straight, well defined, extending from peristome to margin; on well-preserved specimens they can 

 be seen to continue as a faint line over the margin and along the middle of the ambulacral petals 

 nearly to the apical system. 



The periproct is small, subcircular, inframarginal, from one-fourth to one-sixth of the dis- 

 tance from the margin to the peristome. 



Belated forms. — Clypeaster rogersi is not closely related- to— any- American species, but 

 resembles a number of foreign forms in general appearance, and is very similar to several of them. 

 C. biarritzensis Cotteau, from the upper Eocene of France, greatly resembles the present species, 

 but its ambulacral petals are more nearly equal (or, if there is any difference, the odd petal is the 

 longest), its interporiferous areas are relatively wider, and its poriferous zones relatively nar- 

 rower, more divergent, and therefore more widely apart at the ends. C. simplex Duncan and 

 Sladen, from the Ohgocene of Western Sind, India, also closely resembles C. rogersi, but differs 

 in having petals which are longer and tend to close more nearly at the ends. Two species from 

 the Miocene of Cuba, C. concavus Cotteau and C. cotteaui Egozcue, resemble C. rogersi in certain 

 points; but both are relatively higher than the present species and C. cotteaui has straighter and 

 more widely divergent poriferous zones. Michelin 1 stated that he regarded C. jonesii Forbes 

 (which is here regarded as a synonym of C. rogersi) as the young of C. ambigenus De Blainville, a 

 recent West Indian species. The writer can not agree with this view, as C. ambigenus is con- 

 siderably higher, less concave on the under suiface, and has more elliptical interporiferous areas 

 due to the more curving character of the inner rows of pores of the poriferous zones. Stefanini's 

 C. douvillei appears to be a C. rogersi with relatively thin margin and subcorneal upper surface, 

 such as the writer has found to grade into the more typical representatives of the species. 



Localities. — Near Shubuta, 5 miles west of Shubuta, Vicksburg, and Jackson, Miss.; St. 

 Stephens Bluff, 9 miles north of St. Stephens, 5 miles south of Claiborne, Gainestown, near 

 Rescueville, Choctaw County, near Claiborne, and Clarksville, Ala.; Hawkinsville, and 5 miles 

 southwest of Hawkinsville, Ga. 



Geologic horizon. — Jackson formation, upper Eocene; Vicksburg formation, lower Oligocene. 

 Collections. — U. S. National Museum (137801, A and 155377, B); Geological Survey of 

 Alabama; Boston Society of Natural History; Academy of Natural Sciences of Philadelphia; 

 Wagner Free Institute of Science. 



1 Monographic des Clypeastres fossiles: Soc. geol. France Mem., 2d ser., vol. 7, p. 113, 1861. 



