190 THE FOSSIL ECHINOIDEA 



interradial (genital) plates are elongately hexagonal in shape and subequal in size, all 

 touching one another equally. Three only are perforated by the genital foramina, which 

 are large, circular, and occupy the greater portion of the plate. The fourth, or 

 unpunctured, plate is the madreporite, and is perhaps a shade smaller than the other 

 three, no extra development of this plate centrally taking place. The madreporiform 

 body is small, compact, slightly convex, and confined to a limited oval area in the 

 median portion of the plate. The ocular plates are comparatively large and subpent- 

 agonal in shape, and the posterior part of the plate forms a depressed cavity, vi^ithin 

 which the ocular pore is situated. The plates corresponding to the three anterior 

 ambulacra each fill the angle between two of the facets of neighbouring genital plates. 

 The two posterior ocular plates are juxtaposed; the right-hand plate is somewhat 

 larger than its companion, and touches both of the posterior genital plates, the left 

 posterior ocular plate filling in the angle formed by the sides of the left posterior genital 

 plate and the right posterior ocular plate. 



The ambulacra are short, broad, and petaloid, situated in broad but shallow 

 depressions of the test, which thin out gradually at the distal extremity. The odd 

 anterior ambulacrum is narrower than the paired petals, and is situated in a shallower 

 groove, the depression being scarcely perceptible at the ambitus. After the petal has 

 attained its normal width, at a short distance from the apical disk, the poriferous zones 

 run parallel, and do not diverge apart. The pores are round, equal, and the pairs .are 

 placed oblique, each in a very faint depression. There are 14 or 15 pairs in each zone, 

 extending to the ambitus, and the outermost three or four pairs are very widely spaced, 

 and much smaller than the others. The interporiferous area is nearly twice the width 

 of the same region in the paired petals, and its surface is covered with small, uniform, 

 miliary granules. 



The lateral or paired petals are subequal, the anterior pair being a shade longer 

 than the posterior pair. The petals proceed direct from the apex outwards, and have 

 no flexure ; the anterior pair are widely divergent, the enclosed angle being about 140°. 

 The poriferous zones increase in breadth gradually as they proceed from the apical 

 extremity until past the middle of the petal, and then contract again slightly towards 

 the distal extremity, which is rather remote from the margin. The interporiferous area 

 is straight and nearly uniformly broad, excepting the normal constriction at the apical 

 extremity ; also a slight contraction at the distal extremity, which is remarkable. The 

 surface of the area is smooth and devoid of granules. The poriferous zone is wider at 

 its broadest part than the interporiferous area. The pores are wide apart, elongately 

 pyriform, both inner and outer, the latter being slightly larger ; and they are united by 

 a shallow furrow, are slightly obliqu^ in position, and are separated from adjacent pairs 

 by narrow costse, which are ornamented with a single line of six or seven small, uniform, 

 miliary granules. The granules are best defined on the outer portion of the costal 

 ridge, and tend to disappear before reaching the interporiferous area. Consequent on 

 the gradual increase in the breadth of the poriferous zones in proceeding outwards, and 

 the succeeding diminution towards the distal extremity, the outline of the ambulacra is 

 markedly petaloid. Beyond the petaloid portion of the ambulacra there are about three 



