ANATOMY OF THE TEST OF DISCOIDEA CYLmDUICA. 55 



and then the three plates were joined into a triple compound one. 

 It appears that in some specimens the outer set of pairs of 

 pores on the flank of tlie raised ambulacra are larger than the 

 inner series ; and when this is the case, the alternations of the 

 pairs are very well seen. 



The width of the ambulacra, compared with that of the inter- 

 radia, is from one fourth to one third ; and above the ambitus 

 there are nine pairs of pores opposed to two interradial plates*. 



The Interradial JBlates. — These gradually diminish in size 

 towards the peristome, and still more so as they pass up the 

 funnel of the peristome to the perignathic girdle. At the girdle 

 the odd interradium (No. 5) has a single piate ; and although 

 we are not quite satisfied on the point, it appears that there are 

 single plates at the peristomial margin of the interradia JSTos. 1 

 and 3. Double plates are seen at the margin of the peristome 

 in interradia Nos. 2 and 4. 



This is the arrangement of plates noticed in the funnel-shaped 

 peristomes of Ecliinoneus and Amhlypygus -f. Loven has figured 

 a solitary plate in the peristome of Discoidea conica in inter- 

 radium 5 %. 



TJie Peristome. — The actinal aperture is very small in relation 

 to the dimensions of the test ; it leads up through a sort offunnel- 

 sliaped tube to the peristomial margin, which is therefore well 

 within the test, and not flush with its actinal surface. The peri- 

 stomial tube becomes narrower upwards, and its upper margin 

 merges into a remarkable perignathic girdle. The opening of the 

 peristome actinally is very nearly circular, and in no instance is 

 it worthy of the term decagonal ; there is the sligiitest departure 

 only from a continuous curvature. But when the upper margin 

 of the peristome is seen from below, the outline departs more 

 from that of a circle, and ten slight curves are seen — five, broad 

 and decided, are interradial, and five, narrower and less formed, 

 are ambulacral. There are no notches actinally ; but at the 

 margin of the peristome and at the edge of the perignathic 

 girdle there are faint groove-like notches between the interradia 

 and the ambulacra. 



* In describing the ambulacra, the late Dr. Wright stated that about three 

 of its plates correspond vertically to one interanibulacral plate {op. cit. p. 200) ; 

 but in the plate drawn by Bone, pi. xlvi. %. 1 d, nine pairs of pores correspond 

 to two interanibulacral plates ; and that is correct. 



t Duncan and Sladeu, Tal. Ind., ser. xiv., Tert. Echin. of Kachh, p. 17. 



\ Loven, * iitudes,' pi. xiv. fig. 125. 



