30 PEOF. P. M. DTINCAIf AND MTL. W. P. SLADEN ON 



The demi -plates are small and the primary is very large. 



The demi-plates form a small part of the aboral and adoral 

 mass of the boss, and the primary plate forms part of the outer 

 or interradial side of the boss, the whole of the mamelon, and 

 the whole of the inner half of the tubercle towards the median 

 line of the ambulacrum (PI. I. figs. 2 & 3). 



The part of the tubercle-bearing plate between the mamelon and 

 the median or vertical suture is free from any suture or sutures, 

 and none cross it after the fashion of typical Triplechinidse. 

 The sutures of the demi-plates never come in contact, and 

 therefore the inner part of the compound plate is not divided 

 into two portions, and only one plate, the central of the triplet, 

 reaches the line of the median suture of the ambulacrum. 



The Aboral Demi-plate. — This is rather large for a demi-plate, 

 and is about one third of the whole height of the compound plate 

 in vertical measurement, that is to say near the interradial edge ; 

 further in, and on the shoulder of the boss of the tubercle, the 

 height is about one sixth more. The breadth of the plate is 

 about one half of that of the entire compound plate, but it may 

 be a little more or less (PI. I. figs. 2, 3). The edge of the demi- 

 plate in contact with the interradium is low and curved, with the 

 convexity outwards, and the opposite or inner edge is the longest 

 in vertical measurement, and is situated upon the aboral slope of 

 the boss. It there forms a rounded angle, usually rather acute 

 in large tubercles. The angle is situated at the groove on the 

 summit of the boss, and below the mamelon. 



The direction and relative position of the edges or sutures of 

 this plate are very singular. The adoral suture is entirely in 

 contact with the abactinal edge of the outer or poriferous part 

 of the large primary plate. This adoral suture commences at 

 the interradial edge of its plate, and passes inwards and slightly 

 adorally, to the adoral pore. Thence it is directed still slightly 

 adorally and inwards up the tubercle towards the groove at the 

 base of the mamelon ; it then becomes curved, turns at an angle 

 directly towards the apical or aboral transverse suture of the 

 compound plate, and reaches it in one of three places, according 

 to the position of the compound plate in the ambulacrum. The 

 line of this ascending suture of the aboral demi-plate may be 

 ■upwards and outwards, so as to reach the transverse suture, 

 just alluded to, at the position of the adoral pore of the pair of 

 pores which belong to the compound plate immediately above 



