108 PEOF. P. M. DUNCATSr ON THE ANATOMY OE THE 



lu tlie illustration (fig. 10), tlie first great plate, A, consists of 

 two sets of triplets. It carries a large tubercle, the concavity of 

 which is seen on the plate, from within the test. The pairs of 

 pores are in a group of three as in Diadema ; the pores of the third 

 plate are larger than those of the others, and the adoral pore is 

 nearer the vertical suture than any of the others. The arrangement 

 and the shape of the sutures between the triplets are on the type 

 of Diadema and JEcliinotlirix ; but there is this difference, that 

 instead of the first and the third plates suffering from the pres- 

 sure of growth, the second plate is affected, and so much so that 

 the other plates nearly meet at about the concavity for the 

 tubercle, before their sutures diverge to reach the vertical suture. 

 In this instance there is clearly a very narrow space between the 

 adoral suture of the single first plate and the aboral suture of 

 the third plate of the combination. The three plates are there- 

 fore primaries, and there is no demi plate in the combination. 

 The adoral pore of plate 3 is on the transverse line of suture 

 between the first and second sets of triplets of the great plate A. 

 This line reaches the vertical suture adorally to the salient angle. 

 The first plate of the next triplet (2) is large, and it stretches 

 over to the median line, which it reaches by a narrow termination ; 

 for the second plate, also a primary, expands there as well as the 

 third plate. The expansion of the third plate, towards the median 

 line, is considerable, and the result is to push the other plates of 

 the triplet abactinally. There is no tubercle on this part of the 

 great plate A, but the sutures which limit the first and second 

 plates of the triplet nearly come in contact at the spot where 

 a tubercle might have been. The suture placed adorally to the 

 third plate limits the great plate A actinally ; and its direction, 

 in the main transverse, is slightly abactinally and towards the 

 median line, for the expausion of the base of the tubercle on the 

 next set of triplets pushes the suture upwards and towards the 

 median line. 



The great plate B is made up exactly after the plan of 

 plate A, with this exception. The first plate of its second set 

 of triplets (4, 1) does not reach the median line, being crowded 

 out by the increased size of the second and third plates ; so that 

 the second plate is a primary much nipped-in at the position of 

 the defective tubercle, and sufficiently expanded at the vertical 

 suture to form the aboral half of the surface there, and also to 

 reach the transverse suture between the two sets of triplets of 



