PEEIGNATUIC GIRDLE OF THE ECHINOIDEA. 201 



plate 1 is followed by plates 2 and 3 ; and in zone h there is the 

 same succession, but the plates of zone & are the largest. 



In the other condition (fig. 34) there is almost a complete re- 

 semblance to one of the combinations seen in Strpngylocentrotus, 

 where one of the first plates is very small and much of its normal 

 position is occupied by the second plate of the same zone. Were 

 the plates of this combination whijh come to the edge of the ridge 

 taller and all combined into one, there would be the counterpart 

 of the single edge-plate of the Echinidse and Temnopleuridae ; in 

 other words, the large plate 1 of zone h and the small plate 1 

 of the opposite zone combined with plate 2 of zone a would 

 form a single plate on the plan of Echinus. 



In concluding this notice of the girdle of the Echinometradse, 

 it is necessary to remind naturalists that the most extraordinary 

 processes of EcMnometra suiangularishsire a tall rectangular cap. 

 It looks very much like an addition to the processes ; and indeed 

 it cannot but be a subsequent development induced by the large 

 retractor muscles which this species requires. 



The Diadematid^. Diadema setosum, Gray. — The great width 

 of the ridges, their small height, the slender sloping and con- 

 nected processes, and the extension of the ambulacral area inwards 

 towards the peristome and beyond the bases of the processes 

 characterize this genus. There is nothing to notice in the pro- 

 cesses of unusual nature ; but they are readily separated from the 

 ridges ; and indeed the specimens sometimes fall to pieces, and 

 show stirrup-shaped pieces which are the arches of processes and 

 the ambulacral bases from which they sprang. 



The ridges are interesting ; and there is always a low median 

 suture to be distinguished with benzene ; but the arrangement of 

 the plates at the free upper edge is very varied. 



The diversity is due to the crowding of the plates during the 

 growth of the ridges, and to the consequent absorption and alte- 

 ration of shape of some of the implicated plates. In one specimen 

 (fig. 41) the plate 1 of zone i occupies all its half of the edge ; 

 but the corresponding plate of the other zone is small, and so the 

 plate 2 comes in at that half also. These plates are succeeded in 

 the normal manner by others of different dimensions in the two 

 zones. In another specimen (fig. 42) the plate 2 of zone b has 

 pushed up and inwards the first plate, which occupies only a small 

 space at the edge near the median line and suture ; whilst in 

 zone a the plate 2 is very close to the edge, being removed 

 only by a very low plate 1. 



