376 THE FOSSIL ECHINOIDEA 



OrAer ECHINOIDEA EXOCYCLICA. 



.Suborder GNATHOSTOMATA. 

 Family CLYPEASTBIBjE. 



Subfamily EUCLYPEASTBINJE. 



Genus CLYPEASTER, Lamarck, 1801. 



1. Clypeaster suffakcinatus, Duncan & Sladen. Plate LVIII, Figs. 1-5. 



Marginal contour subpentagonal, with the angles full and well rounded, their 

 prominence being emphasized by a well-defined reentering curvature in the paired 

 interradia, the lateral being the deepest ; the odd posterior interradium is usually 

 truncate, though in some examples slightly reentering.- The contour is thus remark- 

 ably undulating, and is subject to considerable variation. The length is greater than 

 the breadth in the proportion of 1 : 0"872 ; and the greatest breadth is opposite the 

 extremity of the antero-lateral petals; the breadth across the test opposite the 

 extremity of the postero-lateral petals is distinctly smaller, and the intervening area of 

 the margin is deeply incurved. The margin is thin and rounded, thicker anteriorly than 

 elsewhere, but shows scarcely any undulation, in consequence of the generally level 

 character of the abactinal surface between the margin and the area of the petals. The 

 abactinal surface passes from the margin with a gentle slope, having a very small angle 

 of declivity until it reaches the area of the petals, where it rises rather abruptly at a 

 steeper angle to form a convex dome, which comprises the whole region of the petals. 

 Seen in longitudinal profile the posterior declivity of the dome is more rapid than the 

 anterior, and whilst a portion of the test from the dome to the margin forms a gentle 

 slope posteriorly, it is nearly level and subtumid anteriorly. The summit of the dome 

 appears in this view slightly truncate, and this truncate area slopes slightly to the front, 

 the greatest height of the test being excentral and situated at a little distance behind 

 the apical disk. 



The apical disk is subcentral. The ambulacral petals are moderately long, wide, 

 petaloid, elliptically ovate in outline, completely closed at the outer extremity, and 

 more or less tumid, sometimes prominently so. The odd anterior and the postero- 

 lateral ambulacra are of equal length, and longer than the antero-lateral pair. All are 

 subequal in width. The greatest width of the paired petals lies a little further 

 outward than midway between the extremities of the petal, and is very nearly half the 

 length, the actual dimensions of the postero-lateral petal in the specimen under de- 

 scription being 30 millim. long, and its greatest breadth 14-5 millim. The poriferous 

 zones are wide, increasing gradually from the apex up to the commencement of the 

 outer third, where the maximum breadth is reached ; they then decrease slowly and 

 gradually to the extremity, where they are still wide ; the two zones of a petal trending 

 towards one another with a well-rounded curvature, until they meet and completely 



