426 PEor. p. M. DmxcA]N- os the stetjcttjee op 



The line is that of the suture between the lower and middle plates 



of the compound plate, and it limits the lower plate aborally and 

 the middle plate adorally. As this suture reaches 'the median line, 

 and as the transverse suture below also does this, the lower or 

 adoral plate of the triplet is a more or less rectangular primary. 



On looking at the apical and inner part of the tubercle a line may 

 be seen passing along the side of the base of the tubercle and going 

 obliquely upwards, or aborally, to the vertical or median suture. 

 This line is to be traced over the boss aborally to the mamelon to 

 the adoral pore of the first pair of the triplet. 



It is the suture which separates the aboral (or first) and the 

 middle plates of the triplet ; and as it reaches the median line, 

 the plate above it, or the first of the series, is a primary, highest 

 at the poriferous zone and low at the median line. This first plate 

 is bounded aborally b)' the transverse suture which adorally limits 

 the plate placed immediately abactinally, and which does not form 

 part of the compound tubercle-bearing plate under consideration. 

 The shape of the middle plate of the combination is determined by 

 the direction of the suture of the edge (adoral) of the first plate 

 and of that (aboral) of the third plate. The plate expands in the 

 direction of the median line aborally, and, moreover, is covered by 

 the mamelon and by much of the boss. 



The structure of the tubercle-bearing compound plate immediately 

 adoral to the last is very simple. The tubercle nearly covers the 

 whole plate, except the narrow poriferous zone, and the peripodia 

 are rather oblique and in an arc, so that the third is nearer the 

 median line than the first of the triplet. 



The course of the sutures from the adoral pores of the peripodia 

 is the same as in the simpler forms of Pscudodiadema and of the 

 modem Diademata. All the plates of the compound one are primaries, 

 and the middle one is the largest : it is covered by the mamelon 

 and by most of the tubercle near the median line, as well as by that 

 portion of the boss which lies on a transverse line with the second 

 peripodium. The suture at the adoral edge of the first plate crosses 

 the boss to the vertical suture with a slight convexity directed 

 actinally, and the suture at the aboral edge of the third plate crosses 

 the boss in the same manner, but the convexity is directed apically. 

 Hence the middle plate is expanded towards the median line, low at 

 the part where the mamelon is, as it were, nipped in between the 

 first and second plates, and not so low at the poriferous part. The 

 first plate is lowest in vertical measurement at the vertical suture, 

 and so is the third plate. The transverse suture which bounds the 

 compound plate adorally is in contact with the adoral pore of the 

 third pair. 



There are no demi-plates in this species, and the compound 

 plates are different in construction from those of Hemicidaris. 



Genus Psettdodiadema, Desor. 



Desor (' Synopsis,' p. 63) gives a short diagnosis of this genus, and 

 classifies it in his group of Oligopores, that is to say in a division of 



