218 THE FOSSIL ECHINOIDEA 



which is formed by the odd posterior interradium, is convex, well arched, curving with 

 a well-rounded bend to the summit of the high, truncate posterior extremity. The 

 actinal surface is slightly convex, caused by a faint tumidity in the actinal plastron ; 

 and there is a slight depression in front of the peristome. The margins are full 

 and tumid. 



The apical disk is slightly excentric in front, and has four generative and five 

 ocular plates ; the anterior pair of generative plates are separated by the odd anterior 

 ocular plate ; the posterior pair of generative plates and pores are slightly larger and 

 wider apart than the anterior pair, to which they are very closely placed ; the posterior 

 pair of ocular plates stand immediately beneath, and are separated by the posterioT 

 prolongation of the central madreporiform body. 



The odd anterior ambulacrum is placed in a wide, deep, and well-defined groove, 

 which extends from the apex, deeply indents the margin, and is continued below the 

 ambitus as a shallow channel reaching up to the peristome, the anterior interradial 

 areas in the actinal surface being somewhat tumid in relation to this groove of the odd 

 anterior ambulacrum. On the abactinal surface this groove is of very uniform width 

 from the apex to the margin, and its floor has a comparatively flat appearance. The 

 poriferous zones are straight and parallel, except near the apex, where they converge. 

 The pores are very minute, those of a pair being placed in a small oval cavity, the 

 series of these giving the zone the appearance of being composed of large single pores. 

 The interporiferous area is wide, and is ornamented with a few very minute and errati- 

 cally placed granules, which are of two sizes, the smaller being more numerous and more 

 crowded. The petaloid ambulacra are comparatively long and straight, placed in deep, 

 sharply-defined grooves, which contract rather rapidly at the apical extremity, and also, 

 but in a less degree, at the outer extremity, where the groove shallows gradually. The 

 anterior pair of petals are slightly longer than the posterior pair, and extend straight 

 from the apex nearly to the margin ; they are widely divergent, and subtend an angle 

 of about 135°. The poriferous zones are broad, and occupy the fianks of the grooves. 

 The pores are subequal, and transversely oval, rather wide apart, and united by a very 

 broad conjugating groove, adjacent pairs being separated with ridge-like divisional 

 costse, which bear a single series of small, ill-defined granules. The pairs of pores are 

 comparatively rather widely spaced. The pores of a pair diminish in size and distance 

 apart as they approach the apex, and also, but to a less extent, at the outer extremity. 

 The interporiferous area is a shade wider than the poriferous zone, and is only about 

 half the width of the same area in the odd anterior ambulacrum ; it appears to have 

 been smooth, as no trace of miliary granulation is to be found in the specimens at our 

 disposal. The posterior pair of petals are similar to the anterior pair, but are rather 

 shorter and rather more contracted towards the outer extremity ; their course from the 

 apex towards the margin is straight, and they subtend an angle of about 54°. There 

 are 24 or 25 pairs of pores in a zone of the po§tero-lateral petals, and 29 or 30 pairs in 

 a zone of the antero-lateral petals. 



All the interradia have a flatly tumid appearance, and become subcarinate near the 

 apical disk ; the odd posterior interradium is more tumid than the others along the 



