PEBIGNATHIC GIRDLE OF THE ECHINOIDEA. 187 



III. The Temk^opleueid^. 



It was thougbt best to take a species of the Tenmopleuridae 

 as an example of the characteristic perignathic girdle of the 

 Grlyphostomata of the Regular Echinoidea, on account of the 

 readiness with which the sutures of the plates separate. 



Salmacis bicolor, Agass. — The large specimen of this species 

 which was examined in the first instance has a large and fully 

 developed perignathic girdle, which is continuous, and consists 

 of five ridges and five arches, each of these last being made up 

 of two processes united above (fig. 9). The whole girdle is stout, 

 tall, and slopes obliquely upwards and outwards. 



The ridges of the girdle are rather tall, and have a sharp upper 

 free edge, with a projection at the spot above the median line, 

 and there is a slight concave or downward bend of the edge on 

 either side of the median process. The upper edge bends inwards 

 very slightly, and immediately below it, on the inner or peristomial 

 surface of the ridge, is a slight hollowing on both sides of an 

 imaginary median line, for the attachment of a muscle ; and below 

 these hollows is a decided transverse concavity, which is placed 

 immediately above the inward projection of the base of the 

 ridge, which corresponds to the bases of the first two tubercles 

 seen on the actinal surface. This transverse hollowing is not 

 very broad, for there is a more or less vertical groove on either 

 side of the same surface of the ridge, which is pronounced below 

 on either side of the basal projection, and which becomes shallow 

 towards the top of the ridge, where it is lost. These lateral 

 grooves (/3) are continuous with the " cuts " for the branchiae, 

 and they nearly entirely belong to the ridge ; but a small part 

 and that forming the side of a groove towards the ambulacra, is 

 on a girdle-process. The line of suture which passes obliquely 

 from above downwards (s), and which indicates the union of the 

 ridge and a perignathic process, marks the outer part of the 

 branchial groove. This suture commences above at the free 

 edge of the ridge, where the upward slojDe of a neighbouring 

 process begins, and it has a direction obliquely downwards and 

 sideways, so that the base of a ridge is broader than its free 

 upper edge (fig. 9). 



The ridges are thin from without inwards at their tops, and 

 they become thicker towards the base, and this corresponds at 



