Anatomy of the Temiioplenridre. 119 



pair in the clemi-plate ; the inner pore of this aboral pair is 

 usually nearly vertical to the outer pore of the adoral pair. 



The pair corresponds to one of the middle vertical series (c) 

 seen on the outside of the test, and is in a low broad demi- 

 plate. 



It is not difficult to make out the sutures of the compound 

 plates with benzule near to the apical system ; but the crowd- 

 ing of the plates lower down and the absorption of a part of 

 the adoral primary, together with the horizontal curvatures of 

 some of the sutures, place considerable difficulties in tiie way. 

 The adoral suture of a compound plate (fig. 15, a) is hori- 

 zontal near the apical system, and usually but not invariably 

 so when the primary is not partly excluded (fig. 1). Tiie 

 adoral or large pore of the primary plate, «, is of coarse tra- 

 versed by the more or less horizontal adoral suture of the 

 geometrical plate ; but the small circular aboral pore of the 

 pair is not remote from the suture, as is usually the case iu 

 Echinoidea, for the suture bends up to it more or less. The 

 direction of the suture from the aboral pore outwards is either 

 straight or in a slight curved convexity directed upwards. 

 In this last instance the suture is in contact with tlie adoral 

 pore of the narrow demi-plate ^, fig. 16. It is anomalous for 

 one line of suture to cross the adoral canals of a lower and 

 upper plate ; but it is well exemplified in the case of this 

 Holopneustes. 



When the primary is not excluded (fig. 15, a) the middle 

 demi-plate of the compound has an adoral suture of its own, 

 and at or close to the adoral pore of the pair it turns upwards 

 with a curved concavity towards the ambulacral median line, 

 to reach the adoral suture of the aboral or upper component of 

 the compound plate. The aboral or outer pore of the pair is 

 not remote from the horizontal suture (tig. 15, Z»), and is often 

 upon it. 



The aboral plate of the compound seen from within (figs. 15, 

 16, c) is a low demi-plate which is longer than the middle 

 demi-plate, and when the primary (a) is not excluded its 

 adoral suture is horizontal and turns up with a curve to reach 

 the upper edge of the compound plate just between the pores 

 of the adoral pair of the next geometrical plate in vertical 

 succession. But when the primary is excluded the adoral 

 suture of this upper component is curved convexly downwards, 

 and it has its curvature continued so as to join the horizontal 

 suture between the compound plate and its fellow above, a 

 little external to the adoial pore of the pair belonging to the 

 adoral primary component of the plate above. The appear- 



