76 PROF. p. M. dujStcan's eeviston of the 



interradia and tlieir numerous tubercles allied the forms suffi- 

 ciently with Diadematidae and removed them from Cidaridse. , 



I would point out that in Humbert's figure (Cotteau, Ech. 

 de la Sarthe, Suppl. p. 338, pi. Ivi. figs. 1-4) the outline of the 

 margin of the peristome is decagonal, the ambulacra! borders being 

 very narrow and it is clear that there is a decided brancliial cut on 

 either side of the peristomial end of the best preserved ambulacrum. 

 Moreover, there are indications, in the drawing of the peristome, 

 that tke perignathie girdle is not after the simple Cidaroid type. 

 The statement of M. Cotteau that the pairs of pores form arcs 

 near the peristome is strictly true, and the figure given by Hum- 

 bert of the component plates of three compound poriferous plates 

 and parts of an upper and lower one on fig. 4 are conclusive 

 in respect of Cotteau'a exactitude. If the upper pair of pores 

 of the fig. 4, pi. Ivi. (Cott. op. cit.), be removed and the lowest 

 also (they belong to defective compound plates), it will be evi- 

 dent that three sets of triplets remain, and that each set is in 

 a compound plate made up of three smaller plates, and that 

 the middle one of each carries the tubercle. The thin lines 

 indicate the sutures between the component plates of each com- 

 pound plate ; and it will be observed that tlie middle plate of 

 each is the largest, occupies much of the plate at the tubercle 

 end, which is near the median suture of the ambulacrum, and 

 that its pair of pores is nearer the ambulacro-interradial suture 

 than the other pairs of pores. Then it can be noticed that 

 the adoral suture of the upper pore-bearing plate, in every 

 compound plate, is slightly curved convexity adorally, and that 

 it reaches the median ambulacral sutural line just above the 

 base of the tubercle. The component plate is thus a low pri- 

 mary, and its pair of pores is placed remote from the ambulacro- 

 interradial suture. On the other hand, the aboral suture of 

 the lowest of the pore-bearing plates of each series has its con- 

 vexity directed aborally, and it may noticed that the end of the 

 suture is either at the median line or falls short. In this last 

 instance the plate is a demi-plate ; but in either case the pair of 

 pores is situated normally, that is, remote from the ambulacro- 

 interradial suture. Now this arrangement of the plates com- 

 posing a component ambulacral tubercle-bearing plate is essen- 

 tially " diadematoid," and is most distinct from that of any 

 Cidaroid. (Duncan, Anat. Amb. Eecent Diadejiiatidae, Journ. 

 Linn. Soc. (ZooL), vol. xix. p. 96, 1885.) 



