20 



THE TERTIARY FOSSIL ECHINOIDEA 



is produced with a decided latitudinal curvature to form a well-developed projection 

 nipped-in on either side. The ambulacra, flush with the test near the margin, are 

 slightly sunken above in consequence of the tumidity of the interradia. 



The apical system is on the anterior slope of the test, is in a very slight depression, 

 is circular in outline, and is flat button-shaped. There are four large generative pores ; 

 the front pair are closer than the others, and all are at the edge of the disk, which is 

 granular and mainly consists of madreporiform body. The ocular pores are small and 

 are covered by the edge of the disk. 



The ambulacra are long, reaching close to the margin, widely open, unequal ; 

 and whilst the odd and antero-lateral are straight, the postero-lateral have a slight 



curvature. 



The poriferous zones are well developed and are very slightly sunken below the 

 level of the interporiferous areas ; the inner lines of pores are in long, straight, diverging 

 lines, so that the interporiferous areas show no tendency to close, and they gradually 

 increase in width from the apex to close to the margin. The poriferous zones are 

 broadest from a third to about halfway down, and they diminish in breadth graduaUy 

 to a point, and are of unequal length— the right zone of the odd ambulacrum, the 

 anterior zone of the antero-lateral, and the posterior zone of the postero-lateral ambu- 

 lacra being shorter than their fellows by a very few pores. Moreover the antero-lateral 

 poriferous zones slope slightly backwards near the margin. The pores are conjugate, 

 the groove being well developed ; and the inner pores are elliptical and the outer elongate 

 in outline. There is often a faint curvature of the conjugating grooves. There is a 

 well-deVeloped costal ridge between each pair of grooves, and it is ornamented with a 

 single row of large granules, which are sometimes separated by much smaller ones or by 

 a space. Towards the margin the poriferous zones become narrower and the pores closer 

 together, until at last a simple narrow groove only contains one pore. In the postero- 

 lateral ambulacra, about halfway up the test, there are nine ambulacral plates to one 

 interradial plate. The ambulacra do not come to a point at the apex, and there is a 

 little breadth there. 



The interporiferous zones, narrow at the apex, increase very gradually in breadth 

 to their termination near the margin. Slightly above the level of the poriferous zones 

 they are nearly flat, and are ornamented with small sunken tubercles which are mam- 

 millate, crenulate, and perforate, and which are separated by narrow tumid ridges 

 minutely but sharply granular. Below the margin of the test the poriferous zone is 

 continued over the actinal surface, one row of pores existing for about two thirds of the 

 distance to the peristome, and the pores are close, denoting a considerable number of 

 ambulacral plates. The rows of pores are in exceedingly faint grooves, which gradually 

 approximate until the open and somewhat rudimentary phyllode is reached. The 

 interporiferous zones become narrow from the margin to the peristome, are rather 

 tumid from side to side, and are decidedly below the level (in the normal position of 

 the test) of the pores in their grooves. Towards the peristome the pores, which are 

 small and surrounded by minute cavities, and are elongate (their long axis being on a 



