112 PEOr. p. M. DUNCAN ON THE ANATOMY OP THE 



The ambiilacral plates are, on the whole, low and broad, and, 

 except close to the apex and peristome, are compound plates, 

 each being made up of three plates, of which the aboral and 

 adoral, or JSTos. 1 and 3, are small, low, and demi plates ; and the 

 central, or No. 2, is a primary plate expanded towards the median 

 line, supporting the tubercle or the granules, and yery low and 

 sometimes almost linear, towards the interradial or poriferous 

 extremity. 



Tbe drawing on Plate V. fig. 11, is slightly diagrammatic, 

 and it explains the relative position and the shapes of the 

 triplets of three consecutive compound plates. The upper 

 compound plate has two pairs of the outer row of pores, 

 and they correspond with the plates 1 and 3, which are demi 

 plates, not reaching the median line of the ambulacrum. It 

 has also one pair of the inner row of pores on plate 2, which 

 is almost linear in form externally where it is crowded in between 

 plates 1 and 2, and large towards the vertical suture, which 

 limits it entirely at the median line of the ambulacrum. This 

 plate is then a primary. The plate 1 becomes a demi plate, 

 because the suture at its adoral edge, after passing actiually to 

 the outer pore of the pair, comes in contact with the adoral pore, 

 and then, after a course directly towards the median line, 

 suddenly turns abactinally on the aboral shoulder of the tubercle 

 to reach with a curve the transverse siiture of the compound 

 plate next in abactinal succession. The third plate of the triplet 

 is a demi plate because its aboral suture, after passing actiually 

 to the outer pore of the pair belonging to plate 2, touches the 

 adoral pore of that pair, and thence the suture, after a short 

 course towards the median line, turns actinally, with a curve, and 

 reaches the transverse suture, which is placed actinally to this 

 plate 3. The adoral and aboral shoulders of the tubercle on the 

 compound plate are just touched by the curved sutures just 

 mentioned. 



This position of the triplet of pairs of pores is quite excep- 

 tional in the Diadematidse. 



The arrangement of the pairs and the dimensions of the triplet 

 in the next or actinally situated compound plate areas follows: — 

 The compound plate has no large tubercle, and it is lower than 

 the plate above. It has two pairs of the inner row of pores, 

 connected with its demi plates, and one pair of the outer row in 

 association with the central or second plate, which is a primary. 



