PERrGIfATHIC GIRDLE OF THE ECltrNOTDEA. l93 



suture between one side of a ridge and a process is tallj 

 nearly vertical, slants but slightly, and it traverses the outer 

 edge of the branchial groove. The peristomial swelling over the 

 actinal tubercles is usually tall, and not united in one mass, but 

 more or less separated along the median line as well as trans- 

 versely. Immediately under the somewhat overhanging upper 

 edge of a ridge are two distinct depressions, one on either side 

 of the median line, and the base of each depression is curved 

 downwards, and there is a blunt projection between them at the 

 median line. The height of the ridges is remarkable. 



The direction of the girdle is as in the other forms, and is 

 upwards and slightly outwards, that is towards the circumference. 



Amhlypneustes ovum, Agass.— ^The perignathic girdle is stout, 

 high, and oblique. 



The ridges are broadest below, and the whole of the groove on 

 either side of one comes within its area, so that the suture between 

 the ridge and the process is oblique from the upward slope of the 

 ridge downwards and towards the median line of the ambulacrum. 



The free edge of the ridges is curved, and there is a median 

 projection. 



The processes slant gradually from the upper edge of the 

 ridge and are rather narrow, they join above by median suture ; 

 and each is expanded laterally there. The space included is tall, 

 triangular, and rounded slightly at the angles. 



It is quite evident that the Temnopleuridas have the peri- 

 gnathic girdle made after a different plan to the Cidarid», and it 

 is proved that the processes which enclose the opening over the 

 peristomial part of the ambulacra are parts of the ambulacra. 

 The processes are made up at their bases and to a certain height 

 by combined and deformed and, to a certain extent, displaced 

 ambulacral plates, and especially of their poriferous areas. A 

 process is united to the ridge on the interradial area by suture. A 

 ridge consists of interradial plates and there is a single plate which 

 forms the greater part of the ridge at and below its free upper 

 edge. There is no separation of the interradial ridge of the peri- 

 gnathic girdle into two parts by a median suture as in Gidaris. 



There are some points about the origin and structure of the 

 perignathic girdle which are not quite clearly made out in the 

 Temnopleuridae, and it is therefore necessary to consider the 

 girdle iu the Echiuidas, and in some of the other Triplechiuida) 

 and Polypores also. 



LINN. JOUR>r. — ZOOLOGY, YOL. XIX. 16 



