EOCENE ECHINODEKMATA. 137 



Mortonia rogersi Dana, 1895, Manual of geology. 4th ed., p. S98, fig. 1491. 



Clypeaster douvillei Stefanini, 1911, Soc. geol. italiana Boll., vol. 30, p. 682, PI. XXII, figs, la-c; p. 698. 



Clypeaster jonesi Stefanini, 1911, Soc. geol. italiana Boll., vol. 30, p. 705. 



Clypeaster turgidus Stefanini, 1911, Soc. geol. italiana Boll., vol. 30, p. 698. 



Determinative characters. — Test medium in size; subpentagonal to suboval, anterior end 

 rounded, posterior end truncated at the corners and centrally, longer than broad, usually 

 widest opposite ends of anterior petals ; much depressed, upper surface rising more or less gradu- 

 ally from the rather thick margin to the low central apex, sometimes regularly arched or of 

 convex outline from margin to margin, sometimes subcorneal centrally; under surface flat around 

 the margin, concave centrally. Ambulacral petals large, tumid, elliptical, anterior pah shorter 

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

 the margin, wide open at the ends; poriferous zones wide, outer rows of pores depressed below 

 the general surface. Apical system central; genital pores five. Peristome medium in size, 

 central; ambulacral furrows simple, straight, well defined, reaching the margin. Periproct 

 small, inframarginal; situated from one-fifth to one-sixth the distance from the margin to the 

 peristome. 



Dimensions. — Specimen A: Length 70 millimeters; width 63.5 millimeters; height 16.5 

 millimeters. SpecimenB: Length 45 millimeters; width 43.5 millimeters; height 17 mhlimeters. 



Description. — The species, to judge from the number of specimens reported, is the most 

 abundant of American clypeasters, being rather common in the Jackson and Vicksburg forma- 

 tions of Mississippi, Alabama, and Georgia. It is also one of the first species of echinoids dis- 

 covered in American Cenozoic deposits, having been figured and described by Morton in 1S34. 

 This species has been frequently confused with Mortonella quinquefaria (Say), partly because of a 

 resemblance in general appearance, but more because of certain mistakes in identification made 

 byL. Agassiz and byDesor. In 1S41 Agassiz described and figured a specimen of Mortonella quin- 

 quefaria (Say) under the impression that he was dealing with one of Clypeaster rogersi (Morton). 

 In 1S58 Desor made matters worse by describing Say's form and founding irpon it the new genus 

 Mortonia; but, instead of figuring it, citing as a good illustration Morton's figure of C. rogersi. 

 As a result of these mistakes many specimens were found mislabelled in the various collections 

 studied and the synonymy of the two forms was found to be in a tangle requiring considerable 

 labor to unravel. M. quinquefaria can easily be separated from the present species by its more 

 circular outline and discoidal form, less tumid petals, less concave under surface, branching 

 ambulacral furrows, and the more distant position of its periproct from the posterior border. 

 About ten years after Morton's description of the present species was published, Lyell collected 

 in Georgia some specimens which were described and figured by Forbes and given the new spe- 

 cific name of Scutella jonesii, but a careful study of the figures and descriptions, aided by an 

 extensive series of specimens, although without the types, fails to reveal any essential differences 

 between the two forms. Conrad's Mortonia turgida, which he appears to have also called 

 Mortonia tumida and still later Clypeaster tumidus was probably a specimen of C. rogersi, which 

 was tumid centrally. 



The test of Clypeaster rogersi is of medium size, ranging from about 1 \ to about 3 inches in 

 length. In marginal outline it is somewhat variable, being subpentagonal in most specimens 

 though nearly oval in others ; the anterior end is usually rounded; the posterior end is truncated 

 centrally, with corners either truncated or rounded; the sides are either straight, slightty convex, 

 or slightly concave; the longitudinal diameter exceeds the transverse; most specimens are 

 broadest opposite ends of anterior jDetals, but some are broadest centrally. The whole form is 

 greatly depressed, the upper surface either rising gradually and gently from the margin to the 

 apex and showing an almost regularly arched or convex outline from margin to margin when 

 viewed from the side or end, or rising centrally in a subcorneal mound involving the petaloidol 

 area; apex central, low, its height rarely exceeding one-fourth the width of the test; margin 

 thick and rounded anteriorly, becoming thinner and more wedge-shaped posteriorly. The 

 under surface flat, or nearly so, around a wide marginal area, concave centrally; the concavity 

 increasing more rapidly as the peristome is approached, the depth of the concavity being about 

 one-third the height of the test. 



