PEUIGKATHIC GIRDLE OF THE ECltllSrOIDEV. 191 



Hence tbere is a liollow above this swelling, and between it and 

 the upper edge. There are markings for the insertion of pro- 

 tractor muscles on either side of the median line. The sutures 

 between the sides of a ridge and the corresponding processes 

 are slightly oblique and nearly vertical ; they commence on the 

 upward slope of the ridge, and the direction of the sutural 

 line is sideways and away from the median line of the ridge and 

 downwards. The result is to make the breadth of the base of 

 the ridge broader than the upper edge. 



There is no median suture to be traced in the ridge by means of 

 benzene, and when the structure is examined from within the test 

 (eircumferentially) it appears that the arrangement of the coronal 

 plates in the zones of an interradium near to and in the ridge is 

 very simple. In one zone a coronal plate with distinct sutures 

 forms part of the ordinary plane surface of the test close to the 

 rising up of the ridge, and the rest of the plate contributes to a 

 small portion of the ridge. It is therefore a plate with a curved 

 upper surface, and it is thick from within the test actinally, and 

 carries a tubercle on the actinal surface. In the other zone a 

 coronal plate comes to the edge of the rising part of the ridge, 

 and enters very slightly indeed into the ridge itself It is followed 

 by a low but broad plate, which forms a part of the ridge, and 

 reaches to about the same height in it as the single coronal plate 

 of the opposite zone. 



The whole of the ridge above these plates is composed of a 

 single plate without the trace of a suture in it, median or other- 

 wise, and it is sutured to the plates just noticed inferiorly, and 

 with a process on either side. The arrangement is as in Sahiacis. 



The processes unite above in a broad arch, and they are thin 

 there and have a line of vertical suture. They are stout at their 

 bases, and there is a decided projection passing down the internal 

 surface (that looking towards the jaws), which slopes obliquely 

 towards the ambulacral median line separating the first pair of 

 pores from the second. The side of the base of a process towards 

 the ambulacral median line has four sets of pores on it. The 

 first pair has its pores close and oblique ; and a little care shows 

 that the pair do not belong to one plate, but that the pore 

 nearest the peristomial edge is in relation with a groove in the 

 edge, and that they are the apertures of the first plate. The 

 other pore of the pair is in relation with the groove on the edge 

 nearer the median line than the other, and the groove and pore 



