194 PJBOF. p. MAETIK DUNCAN ON THE 



IV. The Echinid^. 



Ecldnus norvegims, Ditb. et Kor. — The perignathic girdle is 

 very delicate and incomplete in the young forms, but becomes 

 strong and well developed in adults. The processes of adults 

 are high, and rather broad from side to side above the margin of 

 the upper edge of the ridges. Tbey are united as a rule along the 

 median line above, and the included space is of moderate size ; 

 it is broad below and more or less angular above, and the sides 

 of the space are curved inferiorly and slant to the upper angle. 

 The base of a process slants in the direction of the median line 

 of an ambulacrum ; and it is limited, on the interradial side, by a 

 line of suture, which slopes from the curved edge, where the ridge 

 merges above into the process, to the actinal surface of the test, 

 just on the ambulacral side of the slight cut and groove for the 

 branchiae. The direction is in a curve downwards and towards 

 the median line of the ambulacrum. The result is to increase 

 the width of the part of the peristome which is in relation with 

 the ridge, and to diminish the width of the base of a process. 

 The upper part of a process has a well-marked upper edge with 

 depressions below it for muscular attachments, and the area 

 of these is also increased by the expansion of the process on 

 either side above. The ascending part (fig. 17) is obscurely tri- 

 angular in transverse outline when fractured across, and there is 

 a projecting line which passes along the peristomial side, that is 

 the true inner face of the jjrocess, so as to cross the base obliquely. 

 This line separates the pores and grooves of the first ambulacral 

 plates, which are at the very edge of the peristome, from the 

 three plates which are seen at the back part of the base of a pro- 

 cess (fig. 17). Taking the basal part of the first process that 

 comes to hand, it will be noticed that the peristomial edge is 

 marked by three grooves (6g. 16) — the one nearest the median 

 line of the ambulacrum being small and without a corresponding- 

 pore, whilst the others have each a pore corresponding with them, 

 that of the groove furthest from the median line being high up 

 on the base, the other pore being lower down. 



On the back part of a process three minute pores may be seen, 

 forming a curve, the third pore being further from the ambula- 

 cral median line than the others. These pores, which are oval 

 with a minute angularity below, are the outer pores of so many 

 plates, the inner pores of which are larger and much nearer the 



