210 PEOP. p. MAETIN DUNCAN ON THE 



the peristome, and the curved dark hne is the free upper edge of 

 the ridge. Zone " a " has one plate and " b " has two plates in the ridge. 



Fig. 3. A diagram of the side view of the ridge ; the oblique line of suture "a" 

 is between the two ridge-plates of zone " 6," or rather between plates 

 ^ & 2 of that zone. The first plate carries a small tubercle. The 

 horizontal dotted line is the upper limit of the ambulacral sutural 

 face. 



Fig. 4. View of the ridge from behind. Zone " b " has two plates; a* is the 

 level of the lower part of the ridge. 



Fig. 5. Interradial ridge of the girdle of Phyllacanthus im2:)erialis, Lmk. 



Fig. 6. Another form, showing ambulacral plates also. 



Fig. 7. Showing arching of ridges over an ambulacrum, and junction in one 

 instance. 



Fig. 8. Goniocidaris geranioides. A ridge and the peristomial end of an ambu- 

 lacrum ; part of a second ridge. 



Fig. 9. Salmacis bicolor, Agass. The processes forming an arch, the ridge joined 

 to the processes by a suture "s;" "j8"the groove of one side of a 

 ridge leading to a branchial cut. The grooves and pores are shown 

 at the peristomial edge of the ambulacrum and in relation to the pro- 

 cesses. 



Fig. 10. Side view of ridge (diagram) ; the connection of a tubercle with the 

 plate is shown. 



Fig. 11. Back view of the interradium, its plates and ridge, and of more or less 

 complete processes of the girdle. There is no median siiture to the ridge- 

 plate which reaches the free edge, and the base of the ridge is seen to 

 be formed by two plates, by nearly the whole of a low plate in zone "a," 

 and a part of a large plate in zone " b." The pores and their relation 

 to the suture between the process and the ridge are figured. 



Fig. 12. Oblique view of a ridge separated from the adjoining process of the 

 ambulacrum (diagram) ; the sutural face is covered with minute 

 sockets, and there are traces of a suture which indicate that the low 

 plate is really not plate 2, but that the ridge originally consisted of 

 more than one plate towards its free edge. There is no median suture 

 visible. The figures refer to plates, and the letters to zones of the 

 interradium. 



Fig. 13. The sutural face of the process which corresponds to the ridge fig. 12. 

 The surface is covered with knobs and the relics of three plates. The 

 part above the sutural face is the side of the process towards the top. 



Fig. 14. Side view of the base of a process from the median ambulacral suture. 

 Plates 5 and 6 have their sutures passing from the median line to the 

 line of ambulacro-interradial suture, but the other plates have not. 



Fig. 15. Microcgpkus zigzag, Agass. The peristomial face of a ridge, showing 

 the tuberosities ; there is no visible median suture. 



Fig. 16. Echinus norvcgicus, Diib. & Koren. A pair of processes and a ridge. 



Fig. 17. A process cut short, the ambulacj-al median line is indicated, and on the 

 other side of the pairs of pores is the more or less cm-ved ambu- 

 lacro-interradial sutural line, which is not reached by the plates close 

 to the base of the process. 



