OF WESTERN SIND. 301 



The following is the order of the succession of the plates in ambulacrum III. : — 



Zone "a" — 1, small primary. Zone " b " — 1, small primary. 



2, small „ (2, small demi. 





3, small „ (3, large primary. 



4, small „ 1 4, small „ 



5, small „ 1 5, large „ 



6, large „ [6, small „ 



7, large „ 7, small „ 



8, small „ r 8, small „ 



9, small demi. < 9, large „ 

 riO, small „ (lO, small demi, 



11, large primary. Ql, small „ 



[l2, small demi. <^ 12, large primary. 



[l3, small demi. 



Taking row " «," the small tubercle-bearing plates are 6 and 7, and the first large 

 tubercle is composed, really, by plates 10, 11, and 12, these being a large primary 

 between two demi plates. 



The plates 6, 7, and 8 are very instructingly placed, for they form a triplet of 

 three primaries without any demi plates, as in the instance of the combinations of two 

 demi plates and a middle primary to form the great tubercles lower down. It is 

 evident that the crowding and resulting pressure produce the demi plates, and that the 

 fusion more or less of the ornamentation of the triplet develops the great tubercle. 

 The original tubercle-bearing plate, although enlarged by the accession of the demi 

 plates, retains the greater part of the boss and all the mamelon. The tubercles of the 

 plates marked " small " are little more than large granules, and hence the non-tuber- 

 culate nature of this part of the ambulacrum of the species. 



On zone " b" the first large tubercle is on a triplet of ISos. 11, 12, and 13, and 12 

 is the large primary, the others being demi plates. But 8, 9, and 10 form a triplet, 

 and it will be noticed that the highest plate is not a demi but a small primary. Thus 

 growth and pressure afiected the lower member of the combination and not the upper 

 one, and the lower became a demi plate, being cut off from the median line. In the 

 triplet of plates 4, 5, 6, there is no demi plate, but a central large one and an upper 

 and a lower small plate, all being primaries, however. But the crowding-in of the two 

 upper plates, or 1, 2, after a large plate, for that part of the ambulacrum, caused 

 plate 2 to become a very small demi. 



With regard to the ornamentation of the ambulacra at the ambitus, it may be 

 noticed that the great tubercles have a scanty line of large granules along the line of 

 the median suture. These granules become rarer higher towards the apex. 



The granules are very scanty on the highest large tubercular plate, and a few are 

 seen on the aboral margin of the plate. 



The usual number of large primary tubercles, the result of the combination of 



