PERIGNATHTC GIBDLE OP THE ECHINOTDEA. 199 



plates are wanting, and the plates merge into the mass of the 

 process. The ridges are low and broad, and are well-marked by 

 the attachmeat-liaes of muscles ; and the branchial grooves are 

 well developed. 



The interesting and important points in this species and genus 

 are that the ridge is composed of more than one plate at its free edge, 

 and that two or even three interradial plates mag enter into its 

 composition there (figs. 32, 33, and 34). In fig. 32 it will be ob- 

 served that the plate 1 of zone h has pushed aside plate 1 of zone 

 a ; and this can be well understood if the nature and position of 

 the first plates of both zones in an JEchinometra (fig. 35) are 

 studied. "When there are three plates at the edge (fig. 33), it 

 will be noticed that the growth of plate 2, zone a, has pushed 

 aside the first plate of its zone. Plate 1 of zone 6 is in its normal 

 position. 



It will be observed that in zone a there are two plates follow- 

 ing the first, and that one is low and the other is large. This is 

 the succession as seen in Echinus and the Temuopleurids. 

 Again, in the zone h the plates, both of which are large, are 

 numbered 3 and 4 ; and these are the homologues of the two 

 large plates which succeed to the single plate in Echinus &c. 

 Plate 2 of this zone has no representative in Echinus, unless it is 

 admitted that it is united with the first plates of both zones to 

 form the single large edge-plate in this last-mentioned genus. 

 This must, I think, be admitted. The ridge of Cidaris is com- 

 posed of two plates in one zone, and one in the other ; but 

 there is no fusion as in Echinus. 



As might be expected, there is much crowding of the pairs of 

 pores and of their plates in the base and for some distance up 

 the processes. There are at least three pairs of pores and as 

 many plates to be seen at the back and inner flank of the pro- 

 cesses, and all traces of the ambulacro-interradial sutures are, as 

 is usual in all processes, lost. But the next plate in succession 

 has its outer suture forming a part of the ambulacro-interradial 

 (fig. 38). There are the usual plates seen at th^ peristomial 

 side. Now if the ridge be separated at its junction with the 

 process and ambulacrum, a moderately high face of union is 

 seen (fig. 39). This face is marked by almost vertical lines 

 and with some which are slanting, and each depressed line is 

 the corner between two ambulacral plates ; and the rounded 

 projections on the face, and which are bounded in front and 



