anatomy of the test of discoidea cylindrica. 57 
of the edge of the girdle which correspond with the interradia 
are boldly curved outwards and are larger than the others. 
The outer wavy free edge of the whole girdle overhangs the 
inner surface of the base of the test, and the ribs just men- 
tioned arise from the underpart of the overhanging structure. 
The upper surface of the girdle is of course between the edge 
just alluded to and the peristome ; and its slanting surface is not 
a simple plane one, for on each of the broader interradial portions 
there is a slight depression on either side of the centre and an 
elevation close to the ambulacral curves. This depression is 
probably the relic of a muscular origin, one on each side of the 
median line of a plate. No sutures occur in the interradial parts 
of the girdle ; but it is not satisfactorily shown that there are not 
sutures between the ambulacral and interradial portions along 
the line of the slight groovings which are on either side of an 
ambulacrum high up in the peristome, and at the lower edge of 
the inner surface of the girdle — that is to say, in the usual 
position of sutures in relation with branchial grooves or cuts 
There is a distinct median and more or less vertical suture in 
every ambulacral part of the girdle, and there are pairs of pores 
on the sides of it (fig. 8). 
In ambulacrum III., zone “ b,” there are two pairs of pores 
which are placed obliquely, as is the case with the single pairs of 
the other ambulacra, and the pairs of the other ambulacra are at 
different distances from the free edge of the ambulacral parts of 
the girdle ; and these different positions are exactly those which 
occur in the particular plates around the peristomes of other 
regular Echinoidea according to Loven. The lower part of the 
girdle is continuous with the ambulacral and interradial plates of 
the actinal part of the test around the peristome. The inner 
part of the girdle is the upper continuation of the peristomial 
tube. 
The anatomy of the perignathic girdle in the Echinoidea was 
described by one of us in this Journal (Journ. Linn. Soc., Zool. 
vol. xix. p. 179, 1885) ; and that of Discoidea cylindrica was noticed 
as follows (p. 182) : — “ In Discoidea there is a continuous girdle 
without arches, although the homologues of the processes exist.” 
Reference must be made to the above-mentioned communi- 
cation in order to comprehend the terminology. It is evident 
that there are no arched parts in Discoidea cylindrica ; but it is 
* Duncan, Journ. Linn. Soc., Zool. 1885, pi. xxx. fig. 9, letter s. 
LINN. JOURN. — ZOOLOGY, VOL. XX. 5 
