PERIGNATHIC GIRDLE OP THE ECHIN'OIDEA. 185 



of the median suture of tlie ambulacrum, and the vertebral 

 processes as they were termed by Miiller, as well as the spines, 

 are elevations of the interporiferous parts of the plates. Some 

 of the projections unite on the side of the median^ line and others 

 nearly arch over the vessels and nerves in that situation. No 

 muscular fibres are attached to the projecting nodules. 



The peristomial edge of the first ambulacral plates of Qidaris is 

 low, and a pair of pores will be seen, if care is taken, on either 

 side of the median sutural line (fig. 1). 



These pairs are continuous with those of the first pairs seen on 

 the actinal surface of the solid test ; and it is evident, from their 

 position on the ambulacral edge within the peristomial membrane, 

 that no muscular attachments can come in on that inner edge. 



In the Cidaridse all the muscles which protrude and retract 

 the jaws arise from or are attached to the interradial ridges. 



In Gidaris, owing to the particular character of the peristomial 

 membrane which is covered with separate plates, the jaws cannot 

 appear so clearly outside the test and beyond the membrane as 

 in EcTiinus for instance. The retractores of Gidaris, which open 

 the jaws inferiorly, are not so much required as they are in 

 Echinus, which extends and widely separates and gapes the ends 

 of its jaws in an astonishing manner. Hence the processes of the 

 girdle are all important in Echinus and can be done without in 

 Gidaris, the ridges being sufiicient. 



Goniocidaris geranioides, Lrak. — The perignathic girdle re- 

 sembles that of Gidaris {Dorocidaris) : the ridges are well de- 

 veloped; the "vertebral processes " of the interporiferous zones 

 of the ambulacra are small. 



The ambulacra are narrow at the peristome, and no part of them 

 contributes to the girdle. The first two ambulacral plates are 

 rather high, in comparison with those of Dorocidaris, at the 

 peristomial edge, and the upper surface is often notched at the 

 median line. A pair of pores is seen on the inner surface of 

 each of the first ambulacral plates, and these openings are close 

 to the upper edge of the plates ; they correspond with the first 

 pairs of pores visible on the actinal surface of the test. The 

 peristomial membrane and its plates are attached to this inner 

 surface of the ambulacral plates, below the position of the pores, 

 and consequently there are no muscular origins or attachments 

 on the plates. 



A line of suture separates the interradia from the ambulacra 



