PERIGNATHIC GIRDLE OF THE ECHINOIBEA. 205 



which is towards the ambulacrum (fig. 55). The process is then 

 a part of an ambulacral plate, and as there are two plates in 

 each ambulacrum at the peristomial edge, so there are ten pro- 

 cesses as jaw-supports. 



Each process is tall, and has a narrow but long base and flanks ; 

 the top is small and more or less oval or circular in outline and 

 is smooth. The general direction of a process is upwards and 

 outwards (towards the circumference), and slightly on one side 

 towards the process on the other plate of the ambulacrum. But 

 there is a bending forward towards the peristome in the direction 

 of the upper third of a process, and the slope of it is much sharper 

 in that direction than in the opposite (figs. 54, 55). The processes 

 which look stout, when seen from their sides, are slender and 

 narrow when seen from the front or peristomiallj, and in that 

 view their divergence over the narrow interradium is evident. 



It would thus appear that the processes of Glypeaster are the 

 homologues of the processes of the Grlyphostomes, and that the 

 function is not the same. In the Clypeastroids the processes 

 are more or less pivots and underneath supports to the jaws, 

 and the duty of the muscle said to be attached is not apparent, 

 but it may be a retractor. 



Glypeaster humilis. — The study of a test of this species at the 

 British Museum proved that there is a close resemblance be- 

 tween the processes and those of Glypeaster rosaceus. 



The interradial plate at the peristome is, however, better de- 

 fined than in the instance of G. rosaceus, and it projects back- 

 wards, so that the posterior edge is seen to be thick and curved, 

 and projecting beyond the first ambulacral plates. In front or 

 towards the peristome the interradial plate is low and narrow, 

 and conforms to the general shape of the peHstomial margin 

 (fig. 56). 



The processes are not connected with this plate, and it has no 

 growth whatever upon it. The processes are similar to those of 

 G. rosaceus in shape and in position ; they are growths of the 

 ambulacral plate near the peristome, and arise close to the 

 ambulacro-interradial suture. (Probably the first ambulacral 

 plate of an ambulacrum in the Glypeasters is a compound one, 

 but I have not proved it to be so.) It appears that the small 

 pairs of pores which are to be seen on the ambulacral side of a 

 process close to its base in Glypeaster rosaceus are not visible in 

 G. humilis ; but I have not been able to examine a suificient number 



