THE FAMILY AEBACIADJi:. 31 



(PL I. fig. 4) ; or it may be directly upwards, so as to make a 

 right angle with the transverse suture at its junction (fig. 7) ; 

 or, as is the most usual in the very large plates, the line is up- 

 wards and slightly outwards so as to reach the transverse suture 

 nearer the pore than in the last instance. 



The Adoral Demi-plate. — This plate resembles the aboral one 

 reversed. It bounds the central primary plate ad orally, as far as 

 the lower side of the mamelon, and this structure separates it from 

 the aboral demi-plate. The upper suture of the adoral demi- 

 plate commences at the interradial edge, and passes inwards and 

 slightly aborally to reach the adoral pore of the pair belonging 

 to the primary just above ; thence the line of the suture is still 

 inwards and abactinally until the groove of the mamelon is 

 reached immediately actinally to the mamelon. From that spot 

 the inner suture of the demi-plate is bent and directed actinally, 

 and either directly so as to meet the transverse suture of the 

 compound plate (the adoral transverse suture), or obliquely so 

 as to pass to the adoral pore of the demi-plate. In some tubercles 

 an intermediate path is taken, and the line of suture reaches 

 the transverse suture of the compound plate between the adoral 

 pore and the spot situated vertically below the mamelon. The 

 adoral suture of the demi-plate forms the outer or interradial 

 half of the actinal transverse suture of the compound or tubercle- 

 bearing plate. 



The Central or Primary Plate. — This plate is the only one of 

 the triple combination whicli reaches the median ambulacral 

 suture. In shape and size this primary differs entirely from the 

 demi-plate above and below it. Eor the purpose of facilitating 

 the description of the plate, it may be divided into a low, broad, 

 pore-bearing, outer part, and a very large inner and expanded 

 portion. The first-mentioned part has a curved edge in contact 

 with the interradium, and it is highest there ; but further inwards 

 and up the slope of the tubercle, the plate becomes low from 

 above downwards, in consequence of the approach of the two 

 demi-plates to the adoral and the aboral base of the mamelon. 

 Nipped in as is the primary at this spot, still it embraces the 

 whole of the mamelon and also much of the groove around it. 

 This part of the plate forms the most outwardly projecting parb 

 of the compound plate, and its pair of pores is placed slightly 

 externally to those of the demi-plates. The remaining part of 

 the plate is the inner or expanded portion which spreads out 



