OF WESTERN SIND. 293 



almost a demi-plate. Plate 7 is a low primary with granules. Plate 8 is a primary with 

 an angular interporiferous part : for the expansion of plate 10 extends abactinally so 

 as to crowd out almost the whole of plate 8 and the whole of plate 9, which is a demi- 

 plate. Plate 10 is a large tubercle-bearing plate with an abactinal and actinal expansion. 

 The demi-plate 9 forms the shoulder of the tubercle of plate 10. More abactinally there 

 is a low primary, and then the normal succession occurs at the ambitus of the demi and 

 primary plates in triplet combination. 



As in all species of Ccelopleurus the edges of the large tubercles, at and below the 

 ambitus, are indented by the adoral pore of each pair, and the line of the sutures on 

 the boss of the tubercles is indicated either by a depression or by a slit. 



Finally in very well-preserved specimens, the poriferous part of the plates has a 

 very well-developed arched ridge-like costa situate between the pairs, and it may carry 

 a granule and terminate in one. 



The succession of primary and demi-plates noticed in the last pages, although 

 affording a good example, is not invariable. The following analyses of several ambu- 

 lacral areas is given with the intention of showing that although the succession of the 

 plates near where there are large tubercles is arranged and combined in a geometrical 

 plate — a demi-plate, a large primary with the mamelon, and lastly a demi-plate — still 

 this arrangement is not seen near the top of the ambulacrum. It commences, however, 

 in every ambulacrum not very far down, but the distance relates to the age of the test 

 and in the apparently introductory manner suggested in the long description already 

 given. In that description the small higher and last formed plates were not considered, 

 but they enter into the following descriptions. 



Ambulacra of a rather large specimen of Coelopleurus Forbesi, d'Archiac, the amb. 

 No. III. being examined : — 



Zone " «," pi. 1, 2, 3, 4, '5^7? 7,^8^, lOd, 11, 12. 



Zone " h," pi. 1, 2, 3, 4, 5, 6, 7, 8d, ix, 10, 11, I2d, Ud, xiv, 15d, 16d, xvii, 18d. 



The plates marked " d " are demi-plates, and the others are primary plates ; those 

 with roman numerals are large tubercle-bearing plates. It will be observed that the 

 first plates are all primaries ; they are small, but each reaches the median line of the 

 ambulacrum. 



The first little tubercle-bearing plate is 4 on zone " b," and it has small primary 

 plates above and below. The next is on the zone " a " on plate 6 ; but this tubercle is 

 larger than that of the higher plate " b " 4, and the result is that the preceding plate 

 on zone " a," or no. 5, is a demi-plate. 



The third tubercle-bearing plate is on zone " b " no. ix, and it is the first of rather 

 conspicuous dimensions. The fourth tubercle-bearing plate is xi of zone " a," and it 

 is fairly large. It is preceded by a demi-plate no. 10 ; but the next plate is not a demi, 

 so that the perfect triple combination does not occur so high up. The fifth large 

 tubercle-bearing plate is really of considerable size and is no. xiv on zone " b," 

 and it is preceded and followed by a demi-plate, so that the triple combination is 

 complete. 



