AMBTJLACEA OF THE EECENT DIADEM ATID^. 103 



of the inner part of the compound plate where the tubercle is 

 going to be, determine the diminution in the height of the first 

 and third plates, close to the median line. 



The Feristomial Begion of the Ambulacra. — The character of the 

 tripletis gradually alters from a little distance from the ambitus 

 actinally. At a distance from the peristome corresponding to 

 the sixth interradial plate, the ambulacral compound plates 

 suffer from slight crowding from above downwards, and begin 

 to widen, so that the pairs of pores gradually become more and 

 more distant from the interradial sutures. (Plate V. fig. 8.) 

 Instead of there being three plates to each compound plate 

 at the ambulacro -interradial suture, there are only two, and they 

 belong to the first and second plates of the triplet, for the exclu- 

 sion of the third plate occurs gradually, its external part becoming 

 smaller and smaller in the few triple combinations which are placed 

 near the peristome. 



Taking the three compound plates which are in relation to the 

 fifth interradial plate from the peristome, as our example, it will be 

 noticed that the peripodium of plate 2 of the abactinal compound 

 plate is so closely placed above that of plate 3, that there can 

 hardly be room for the extension of this last to the interradial 

 suture. On cleaning the test within, and applying benzole, the 

 absence of the third plate at the interradial edge is very evident 

 (fig. 8), and it is seen that the compound plate is made up there of 

 plates 1 and 2 only. The edge of the first plate at the ambulacro- 

 interradial suture is not as high as that of the second plate. On 

 carrying the eye along the transverse suture between the com- 

 pound plate under consideration and that placed adorally, the 

 outer edge of the third plate will be seen as an angular process, 

 which terminates slightly externally to the aboral pore of the 

 third pair. 



The first plate of this triplet (Plate V. fig. 8) is bounded abacti- 

 nally by the transverse suture between it and the plate immedi- 

 ately above, and which is the third plate of the triple combination 

 placed abactinally. The external limit of the plate is a low 

 curved edge at the interradial suture. The actinal boundary is 

 a suture which passes from the adoral part of the curved edge 

 just mentioned, towards the median line of the ambulacrum, 

 adorally to the outer pore of the pair and then into the inner 

 or adoral pore. Thence the suture passes inwards and slightly 

 aborally, to reach the median or vertical suture of the compound 



