THE AMBTJLACRA OF FOSSIL ECHINOIDEA. 431 



which is placed immediately actinally. The tubercle of the com- 

 pound plate is large, and there is some space between it and the 

 median suture. 



Fig. 8 (see p. 452). 



Taking the compound plate above the ambitus, first of all, for 

 descriptive purposes, there are to be noted four plates and four pairs 

 of pores in peripodia. The actinal or fourth plate (no. 4) is a 

 low and broad primary having a convexity directed aborally, so that 

 the plate is low in the poriferous zone and at the median line 

 of the ambulacrum, and much higher midway where it reaches 

 across the adoral part of the base of the boss. The next plate 

 situated abactinally (no. 3) is the largest of all in the compound 

 plate, and assimilates in shape to the middle plate in the triplet 

 of a Diadema ; it is largest near the median suture of the ambu- 

 lacrum, is nipped in on the tubercle, and is somewhat higher at 

 the poriferous zone. The adoral pore of its pair is in relation with 

 the suture between its adoral edge and the aboral edge of the plate 

 just described. The abactinal edge of the plate now under descrip- 

 tion crosses the boss and the centre of the mamelon, and then passes 

 towards the median line, with an abactinal and inward path, so as 

 to give a curved appearance to the suture which joins this plate to 

 the one immediately above. The third plate from the adoral edge, 

 or no. 2 of the compound one, is a long or rather broad, low 

 primary, the actinal edge of which corresponds with the abactinal 

 curved edge of the plate just described. So this third plate has a 

 bent actinal edge, and this is indicated by the suture. The abactinal 

 edge of the plate is also curved, and with the convexity directed 

 actinally, and the height of the plate at the median line of the 

 ambulacrum is small and less than at the poriferous zone. 



The most apically placed of the plates, or the first (no. 1), is a low 

 and broad primary, lowest at the vertical suture, and with the adoral 

 edge curved adorally, the abactinal edge being straight and trans- 

 verse. The adoral edge of this plate crosses the tubercle not very 

 far from the mamelon. The transverse aboral edge is in contact 

 with the actinal plate of the compound plate situated immediately 

 abactinally. 



All four plates combine to form a solid compound plate, and they 

 are to be recognized by the direction of their sutures. The angle 



