PEBIGNATHIC QIEDIjE OF THE ECHTNOIDEA. 209 



line, and cruslies uj) a small first plate in tlie other zone. These 

 plates are followed in their respective zones by the plates noticed 

 in the Echinidae and Temuopleuridse. It is evident that the single 

 plate of Ecliimis is composed of fused first plates and probably 

 of the low second plate of one zone ; so that if the ridge-plates of 

 Cidaris were united without any relics of sutures, the solitary 

 plate of the ridge of the Echinidse and Temnopleuridse would be 

 exemplified. 



The ridges of the Glyphostomes are the homologues of the 

 so-called auricles of the Cidaridse ; but their "processes," which 

 are not in existence in Cidaris, give attachment to important 

 retractor muscles which are not much required in this last genus. 



In the Clypeastridse there are disconnected growths which 

 carry the jaws and have slight muscular attachments. In 

 Clypeaster there are ten processes, and each one arises from 

 an anibidacral plate (or plates), and one process leans somewhat 

 towards the other of its pair over the ambulacrum. There are 

 no interradial structures like ridges. The processes are the 

 homologues of those of the regular Grlyphostomes. 



In Laganum there are five grow^ths, and each arises from 

 a single first interradial plate ; so they are " ridges," and the 

 homologues of those interradial structures of the Regular 

 Echinoidea. 



The Clypeastridge may be divided into two groups, on account of 

 the presence of processes in one, and of the homologues of ridges 

 in the other. The relation of the single interradial plate at the 

 peristome of many edentulous (or presumedly so) Regular Echi- 

 noidea to the siugle plate of the edge of the ridge in Echinus is 

 evident ; and this is the result of junction of the two plates 1 

 with or without the addition of plate 2 of one zone. The 

 distinction between the Cidaridse and the Glyphostomata is well 

 defined by the perignathic girdles. 



DESCRIPTION OF THE PLATES. 



All the figures in both Plates are more or less magnified. 



Plate XXX. 



Fig. 1. Borocidaris papillatayljes^B. Interradial ridge and the peristomial face 

 of the first ambulacral plates. Part of a ridge beyond an ambu- 

 lacrum. 

 Fig. 2. The ridge of interradium 5, seen from above. The arrow points to 

 LINN. JOUBN. — ZOOLOGr, VOL. XIX. 17 



