PEEIGKATHIC GIRDLE OP THE ECHINOIDEA. 189 



of interradial plates, and that there has been union of the ter- 

 minal, or rather primary, plates so as to obliterate the median 

 and other sutures. The ridges are united by separable sutures 

 with the processes, are grooved for the branchige, and are marked 

 by muscular impressions. Two sets of muscles are attached on 

 each side of the inner face of a ridge— the thread-like radiales, 

 probably ligamentous, and the large and broad protractores. 



Variation. — The size of the median projection on the upper 

 free edge varies, and is often absent in young specimens. 



The processes of the girdle enclose an ovoid and rather pointed 

 opening over the peristomial part of each ambulacrumj and 

 the tall, broad processes, upwardly curved at the top, contrast 

 with the comparatively small openings. The processes of each 

 ambulacrum are joined by a vertical suture in the median line 

 above, are broad from side to side and thin from before back- 

 wards, and slightly bent inwards superiorly, although the general 

 direction of the processes is that of the whole girdle, namely 

 upwards and slightly towards the circumference of the test. 

 The slope of the side margins of the processes to reach the tops 

 of the ridges is abrupt, and the suture which unites them with 

 the ridges is long vertically and rather narrow from within out- 

 wards. When this suture is separated, the articulating surface 

 of the process being exposed, it is found to present opposite 

 characters to those of the corresponding part of the ridge. 

 There are numerous knobs (fig. 13) placed in a space close to 

 the base, and above they are limited by a set of lines of knobs 

 and elevations more or less oblique in direction. Above are some 

 more knobs and linear ridges, and quite at the top of the surface 

 there are other knobs. The impression given is the same as that 

 noticed in describing the corresponding surface of the ridge, 

 except that in the process the markings are all convexities, and 

 in the ridge they are all receptive concavities. It is evident 

 that the relics of the borders of three plates exist. 



At the peristome the edge of an ambulacrum, which is bounded 

 by the origin of the process on either side, is low and is marked by 

 the ambulacral median suture, and by grooves and pores on either 

 side of it, for the passage of structures which come from the inside 

 of the test to reach the bases of the actiual pedicels (figs. 9, 11, 

 14). The pores on either side of the median line are placed on 

 the ascending base of a process and on the peristomial face. On 

 looking at the process at its back and ambulacral side, at least 



