PERIGNATHrC GIEDT.E OP THE ECHINOIDEA.. 197 



and still disunited (fig. 20) ; and the suture between the processes 

 and the ridges could be separated, and then it was seen that there 

 were at least two plates forming a ridge (fig. 22) : nothing could 

 be seen with any reagents which would prove, that the single 

 plate was divided in the youngest and smallest forms. The 

 largest of the specimens showed that the processes unite above 

 very soon, and that they grow upwards with the general growth of 

 the test (fig. 19). 



EeJiintos esculentus, Linn. — There is an excellent preparation of 

 a large test of this species in the British Museum, and the girdle 

 is well shown. The processes are large, broad, and rounded 

 'above, and the position of their vertical suture, which was high, 

 cannot be seen even with benzene, for perfect union has occurred. 

 The space included by the arch of the processes is large and the 

 ridges are well developed, and they have the usual number of 

 plates. 



PsammecJdnus miliaris. — If the characters of the structures of 

 the perignathic girdle of Echinus be remembered, it will only be 

 necessary to treat of those of the corresponding parts of this 

 species briefly. The processes of the girdle are rather tall, 

 and are rather narrow superiorly, where there is normally a 

 slight bending forward of the upper edge, and a corresponding 

 convexity of the outer or circumferential part of the summit. 

 Usually the vertical height of the suture which unites the pro- 

 cesses above is small (fig. 25) ; and it sometimes happens that 

 they are not attached by a suture, and there is not a com- 

 pletion of the arch over the included space. This disconnection 

 is not by any means uncommon. The included space is ovoid or 

 obscurely triangular in outline, or it may be decidedly triangular. 

 When there is no arch, the processes are less aslant (fig. 26), and 

 may approach the upright in position. Often the tops of the 

 jn'ocesses only just touch. The sutures between the processes 

 and the perignathic ridges are very distinct, and often so without 

 benzene (figs. 25 & 26) ; and each commences at the upward slope 

 of the process from the free edge of the ridge, and passes down- 

 wards with a curve which has its convexity towards the base 

 of the process. The suture just comes withiu the edge of the 

 branchial groove on each side of the ridge. 



The peristomial margin within these sutures, which belongs to 

 the ambulacra, is marked by the notches and is perforated by 

 the corresponding pores of the first series of ambulacral plates ; 



