ANATOMY OF THE TEST OF DISCOIDEA CTLINDRICA. 



57 



of tlie edge of the girdle which correspond with the iuterradia 

 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 " 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 Biscoidea 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. JOUEN. — ZOOLOGY, VOL. XX. 5 



