44 PEOF. p. M. DUNCAN AND MR. W. P. SLADEN ON 



first and second are farther apart than the second and tMrd, and 

 so on. 



The ambulacra are narrow at this part, and the pores are by no 

 means straight but in slight arcs ; they are large and wide apart, 

 and it is only in relation to the most adorally placed tubercles in 

 this region that tbey assume the form of decided triplets. 



The surface of the ambulacra at this part of the test is very 

 slightly tumid, and it conforms almost to the general curvature 

 of the test ; but it is the sunken median area of the interradia 

 that gives the appearance of unusual tumidity to the rest. 

 The bases of the tubercles resemble those of the others lower 

 down ; the bosses are perhaps more swollen in the smaller tuber- 

 cles, but the mamelon of each has its groove and is the counter- 

 part of the others. The number of very small secondaries is 

 slight amongst the higher tubercles, and it seems to increase 

 suddenly between the third tubercle of one side and the fourth 

 of the other. 



The adoral pores of the pairs cling to the sides of the bases of 

 the tubercles, especially of the larger. Near to the radial plate 

 the pairs are very oblique and close, but the obliquity and pro- 

 pinquity diminish gradually. 



No trace of the structure of the plates of this part of the 

 ambulacra can be distinguished on the outer surface of the test ; 

 but the direction of the sutures can be seen inside, and it is 

 comparable with that observable in medium-sized specimens of 

 Coelopleurus Forbesi, d'Archiac. 



The following is the arrangement of the plates in ambu- 

 lacrum IV. Zone " &" : — 



Plate 1, a small primary. 

 ( 2, a small demi-plate. 

 ( 3, a primary with an expanded inner part. 



4, a small low primary. 



5, a small low primary. 



16, a small demi-plate. 

 7, a large primary with an inner expansion. 

 8, a small low primary. 

 r 9, a small demi-plate. 



J 10, a large primary plate with great inner ex- 

 ] pansion. 



(11, a demi-plate. 

 i 12, a demi-plate. 



< 13, a large primary with expansion. 

 ( 14, a small demi-plate. 



In explanation of these details, it is necessary to state that the 



