36 THE TEETIARY FOSSIL ECHINOIDEA 



The anterior ambulacrum is broad, lanceolate, with a nearly flat floor, and has 

 between the apical system and the position of the traversing fasciole a tall sharp keel 

 on either side. These keels slope up suddenly from the apex, and then curve forwards 

 at the top, diverging slightly. The groove becomes narrower to the front, and is very 

 slight on the margin, and is still less near the peristome. 



. The floor of the ambulacrum is granular, and the poriferous zones are on the rise 

 of the slope of the keels. The pairs of pores are separated by faint costse and are 

 close together near the apical system, five pairs being very small in front of the very 

 minute ocular pore. Further out the pores increase in size, and the pairs become 

 more distant : twelve or thirteen large pairs are visible within the fasciole ; further 

 out they are invisible. The pores of each pair are separated by a tubercle. 



The antero-lateral ambulacra are slightly sinuous, are deep, short, broad, nearly 

 rounded and slightly curved backwards distally, and shallow, narrow, and curved forwards 

 near the apical system. Their grooves are bounded by the shjpe of the keels of the 

 anterior interradia, and by the less pronounced tumidity and keeling of the postero-lateral 

 interambulacra. The interporiferous areas are well developed, but are narrower than 

 a poriferous zone. The anterior poriferous zone is the longest, and curves outwards 

 and backwards, overlapping the end of the posterior poriferous zone. The large pores 

 of both zones are on the slope of the ambulacral groove, except near the apex, where 

 they are flat and close together and very small. The pores are large in the body of the 

 ambulacrum, broadly comma-shaped, and those of the outer row are equal in size to 

 the others ; they are conjugate and separated by broad costse ornamented with a linear 

 eminence ending in minute granulation. There are 22 pairs of pores in the anterior 

 zone, about seven being small and close near the apex. Ocular plates and foramina 

 small. The ambulacra form an angle of about 75°. 



The postero-lateral ambulacra are small, short, narrow, close, not one half the 

 length of the others, and are placed in pear-shaped grooves which are shallow near the 

 apex and deep distally. There are some pairs of small pores near the ocular plate, 

 and the other pores are larger, and there are about 16 pairs in all. 



The anterior interradia are sharply keeled between the ambulacra, and more in 

 front they are tumid and marked with slightly gibbose plates. The lateral interradia 

 are broadly and lowly keeled near the apex, and further out are precipitous to the 

 margin. The posterior interradium is large, and there is a tall convex keel arising from 

 a broad base and sloping sharply backwards. 



The peristome is rather small, rather crescentic, with broad sides and a very pro- 

 jecting posterior lip, which is considerably below the level of the anterior one. The 

 posterior hp is thick in the centre. The plastron, elongate and rather small, is bounded 

 by a space between it and the mouth, where there is no ornamentation, and at the sides 

 by ambulacra covered with minute granules. The projecting point of the plastron 

 limits a broad keel posteriorly, and the whole is ornamented with low tubercles on 

 slightly raised scrobicules, which are elongate and sometimes imbricating ; the boss is 

 tumid and flat and near one end of the scrobicule ; and the mamelon is small and 



