484 PEOF. p. M. DTJ^^CAX 0]S" THE STEUCTUEE OE 



slanting adorally and towards the interradia. But when a series of 

 pairs is considered in relation to a compound plate it appears that 

 there is really great arching, for the second and third pairs of pores 

 in the linear series are the first and second paii'S of the compound 

 plate, whilst the third pair of the plate, or the adoral, belongs to 

 the linear series immediatelj^ actinaUy, or they are much nearer the 

 amhulacral median line. 



The pair of pores which is in the abactinal part of a compound 

 plate is the second pair of a triplet, and of course the third pair 

 belongs to the middle plate of the combination. The second 

 pair of the triplet of pores is nearer the interradium than the 

 first pair, which belongs to the compound plate above ; but the third 

 pair, which is in the middle plate of the compound one, is nearest the 

 interradium. 



The peripodia of the pairs are rather close. The compound 

 plates are low and much crowded, and there are no less than four 

 of them in contact with an iuterradial plate slightly above the 

 ambitus, where the diagrammatic sketch w^as taken. Thus as 

 each compound plate consists of three plates combined, there are 

 twelve in relation to an interradial plate. 



In the specimen in Dr. Wright's collection the sutures of one of 

 the ambulacra can be distinguished in a vertical series of four plates 

 (fig. 9), the most apical of which and the most actinal bear tubercles, 

 the two others being simply granular. The shape of the plates con- 

 stituting the compound plates and the direction of the sutures differs 

 in some of the compound plates ; nevertheless there is no difficulty 

 in seeing that the variation has been due to pressure from growth 

 influencing plates which, under other circumstances, might have 

 remained typical of Diadema. The arrangement of the component 

 plates is not at all like that seen in Echinus, and the genus Pedina 

 does not enter the family of the Echini proper. 



Compound plate I. — The edges of the plate which are in the 

 lines of the sutures between the plate and those immediately apical 

 and adoral are, as is usual, transverse, and reach the median hne 

 at the reentering angle of the median zigzag. 



The aboral plate (a) of the compound is a demi-plate which does 

 not reach far beyond the poriferous half of the combination : and 

 the adoral pore is on the adoral suture, which is curved with the 

 convexity directed actinaUy. 



The middle plate, a primary, carries the bulk of the tubercle and 

 gradually increases in vertical measurement from the poriferous area 

 to the vertical suture at the median line. It is bounded aborally 

 by the demi-plate («') and by part of the transverse suture, and 

 it is bounded actinaUy by the edge and suture of the third plate, a 

 primary (c). 



This suture is curved, with the convexity aboral ; it just touches 

 the adoral part of the tubercle, and reaches the median Une at a 

 slight distance from the actinal and inner angle of the compound 

 plate. The direction of the suture is that seen so commonly in the 

 third plate of a triplet in Hemi_pedina and Diadema. 



