ANATOMY OF THE TEST OF DISCOIDEA CYLINDRICA. 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 the raised ambulacra are larger than the 
inner series ; and when this is the case, the alternations of the 
paii’s 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 Plates. — 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 plate; and although 
we are not quite satisfied on the point, it appears that there are 
single plates at the peristomial margin of the interradia Nos. 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 JEchinoneus and Amblypygus f. Loven has figured 
a solitary plate iu the peristome of Discoideci conica in inter- 
radium 5 J. 
The Peristome. — The actinal aperture is very small in relation 
to the dimensions of the test ; it leads up through a sort of funnel- 
shaped 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 slightest 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 — rive, broad 
and decided, are interradial, and five, narrower and less formed, 
are ambulacra! 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 interambulacral plate {op. cit. p. 206) ; 
but in the plate drawn by Bone, pi. xlvi. iig. 1 d, nine pairs of pores correspond 
to two interambulacral plates ; and that is correct. 
t Duncan and Sladen, Pal. Inch, ser. xiv., Tert. Echin. of Kachli, p. 17. 
\ Loven, ‘ Etudes,’ pi. xiv. fig. 125. 
