PERIGNATHIC GIRDLE OP THE ECniNOlDEA. 183 



gii'dle whicli was so well drawn by J. Miiller, and figured, with 

 the aid of ])hotography, by A.. Agassiz in the ' Revision.' 



The processes do not exist, and the tall, stout, notched, and 

 sideway-s I anting interradial ridges form the whole of the dis- 

 continuous girdle. An interradial ridge is high and broad, and 

 notched, on its upper edge at the median line, so as to give the 

 appearance of being formed by two halves placed side by side, 

 and a vertical suture passes down from the notch to the peri- 

 stomial margin *. 



The upper edge of each half of the ridge is convex and thin, 

 and is produced sideways so as to partly overhang an ambulacrum, 

 (Plate XXX. figs. 1 & 2). The outer, that is the circumferential, 

 surface of the ridge is slightly convex from side to side and from 

 above downwards; and internally the surface is slightly concave 

 above and rather tumid below. The upper edge overhangs the 

 concave surface at the upper part of the ridge on either side of 

 the median line, and this surface is the point of attachment of 

 muscles. In the midst of this concave surface, and about half- 

 way down the ridge, there is a small depression on either side of 

 the median line and midway between the outer side of the ridge 

 and the median line. The sides of the ridge are curved, concavity 

 towards the ambulacra, and the suture between the ambulacral 

 plates and the ridge on either side is nearly vertical at the 

 peristomial edge (fig. 1). 



The general direction of the ridges is upwards and slightly 

 outwards, so that they are not vertical, and the lower part of 

 each ridge projects more towards the axis of the animal than the 

 upper and free part (fig. 3). The lower part has tubercles in 

 relation to it actinally, aud the characteristic ununited plates of 

 the Family are attached to the lower and inner part of the 

 ridge where it merges into the peristome, and the union is mem- 

 branous. Circumferentially the ridge slopes rather sharply at 

 first from above downwards, and then more gradually so as to 

 merge into the upper surface of the coronal plates near the 

 peristome. The ridge is thin above, and stout lower down where 

 it joins the general surface of the plates just mentioned (figs. 2 

 &3). 



Seen from without, the outer (circumferential) surface of the 

 ridge has the median suture of the interradium coming in a 



* Benzene was used when requisite to render the sutures distinct. 



