EOCENE ECHINODEKMATA. 129 



Say's Scutella quinquefaria he applied to the form Morton's name Scutella rogersi. The forms 

 themselves are perfectly distinct, even belonging to different families, Say's form being one of 

 the Scutillidas and Morton's one of the Clypeastridse ; but, owing to the mistakes mentioned 

 great confusion exists in regard to them in the various collections and in the literature. The 

 separation of the two forms is not at all difficult, Chjpeaster rogersi (Morton) being easily distin- 

 guished by its straight unbranched ambulacral furrows, its more concave under surface, its more 

 tumid ambulacral petals, its less circular and more pentagonal marginal outline, and the location 

 of its periproct nearer the posterior border. 



This species is also interesting because on it Desor founded his genus Mortonia, based chiefly 

 upon the unusually thick margin for a Scutella-like form. The name Mortonia was changed by 

 Pomel to Mortonella because Gray had applied the name Mortonia to a different genus prior to 

 Desor's use of it for the present genus. 



The test of the present species is notably discoidal in form. It is of medium size, the dimen- 

 sions varying but slightly from those already given. In most specimens it is usually circular in 

 marginal outline, though in some it is broader posteriorly than anteriorly. The whole form is 

 much depressed, the upper surface being either nearly flat or having a low, gently rounded cen- 

 tral mound involving only about half of the region covered by the ambulacral petals. The vari- 

 ations in marginal outline and tumidity of the upper surface seem almost of specific value, 

 but intermediate forms show a gradation from one to the other, and the variation in 

 the character of the upper surface occurs in both the forms which are circular in marginal 

 outline and those which are broader posteriorly. The region involving the outer part of each' 

 petal is faintly concave. The margin and submarginal area are thick, usually almost as thick as 

 the average thickness of the test; the margin being slightly thinner and less rounded posteriorly 

 than anteriorly, faintly undulating. The under surface is flat or broadly and faintly concave. 

 The apex is subcentral, or slightly excentric anteriorly, at the summit of the more or less tumid 

 central area. 



The ambulacra are relatively wide in the petaloidal region, broader in the marginal region 

 where they about ecmal the interambulacral areas. The dorsal portions are petaloid; the petals 

 large, broad, subecmal in length, the odd petal being slightly longer than the rest, spatulate in 

 form, open at the ends and extending nearly two-thirds the distance from the center to margin. 

 The poriferous zones are very wide, wider than the interporiferous areas; the inner row of pores 

 nearly straight and composed of oval openings, the outer row slitlike; the pairs of pores 

 conjugated. 



The interambulacra are relatively narrow in the petaloidal region, wide in the marginal 

 region, where about equal to the ambulacral areas, composed of large polygonal plates. The 

 whole surface of the test, including the margins, is covered with small uniform tubercles which 

 are slightly larger on the under side. 



The apical system is subcentral or slightly excentric anteriorly at the summit of the test. 

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

 and flush with the surface. There are five large genital pores near the edge of the madreporite 

 and five small radial plates each perforated by a small pore. 



The peristome is small, circular, central, the ambulacral furrows simple and straight nearly 

 half the distance to the margin, then forking symmetrically, each pair of branches continuing 

 almost to the margin. Each branch gives off on the outside one side branch at right angles about 

 a fourth of the radius from the margin, which side branches soon bend and in turn approach the 

 margin. A small ridge extends along the middle of each ambulacral furrow from the peristome 

 to the fork in the furrow. 



The periproct is very smaU, circular, situated on the under surface about half the way from 

 the peristome to the posterior margin. 



Related forms. — The thick margin and discoidal form of Mortonella quinquefaria afford a ready 

 means of separating it from most American echinoids. Dendraster perrini is similar in having 

 a thick margin but differs in having its apical system posteriorly excentric, its ambulacral 



