324 THE FOSSIL ECHINOIDEA 



3, Clypeaster pelvipobmis, Duncan & Sladen. Plate L, Figs. 7-9. 



Marginal contour suboval, or with a slight pentagonal tendency in large specimens. 

 The posterior angles and the posterior margin are rather flatly truncate, whilst the 

 remainder of the contour is well rounded. The length is greater than the breadth in 

 the proportion of 1 : 0'9, and the greatest breadth is opposite the termination of the 

 antero-lateral petals. The general form is low and subdepressed, but the margins are 

 thick and tumidly rounded ; and the abactinal surface rises from thence to the apex, 

 forming a slightly conical convexity of low elevation aind with gentle declivities. 



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

 petaloid, suboval in contour, and rather widely open at the outer extremity. There is 

 a slight depression in the interradial areas, which gives a faint suggestion of tumidity 

 to the petals; although strictly these cannot be described as tumid. The odd 

 anterior petal and the postero-lateral pair are subequal in length, and the antero- 

 lateral pair are slightly shorter. The greatest width of the paired petals is about 

 midway between the apex and the extremities, and is proportional to the length as 

 11:20, the actual dimensions of the postero-lateral petal in the type specimen being 

 20 millim. long and its greatest breadth 11 millim. The poriferous zones are very 

 wide, and the breadth increases continuously from the apex up to within 9 or 10 

 pairs of pores from the extremity ; 5 or 6 pairs of pores are then of uniform width, 

 and the zone is terminated by only 3 or 4 much narrower ; there is also sometimes 

 a little group of 3-4 sporadic spores immediately at the termination of one of the 

 zones. The inner pores are round, and the outer ones slightly larger and transversely 

 oval, their elongation being greatest on the outer part of the zone. The pores of a pair 

 are united by a distinct conjugating furrow, the intermediate costse are rather broad, 

 and their ornamentation consists of two or three widely-spaced scrobiculate tubercles 

 with granules between. The broadest part of the poriferous zone is 3-5 millim., and is 

 nearly two thirds of the width of the interporiferous area at that place. The widest 

 part of the interporiferous area is about midway between the extremities, or perhaps 

 slightly more outward, where it measures 6 millim. The odd anterior petal is similar 

 in all respects to the paired petals above described, but is perhaps a shade narrower. 

 The distance of the petals from the margin is subequal, and is approximately about 

 half the length of the petal, although it appears much less on account of the rounding 

 of the margin. 



The interradial areas are very contracted on the inner half of the abactinal surface, 

 the length of the eight uppermost plates in each column being greater than their 

 breadth. The ornamentation consists of small scrobiculate tubercles which are much 

 more widely spaced than usual, the feature being at once conspicuous. Very frequently 

 the granular interspaces are twice the width of the diameter of the scrobicules and 

 sometimes more ; the scrobicules are less regular in their spacing, and have somewhat 

 of a sporadic disposition. The scrobicules of the interporiferous areas are more 

 closely placed than in the interradial areas, but are less numerous than usual. 



The actinal surface is deeply concave and widely basin-formed; the concavity 



