OF WESTERN SIND. 311 



plate it will be noticed that the median plate of this triplet is the highest (^. e. forms 

 the broadest band) ; this is the most adoral of the two demi poriferous plates, and 

 the one which is immediately superposed on the primary poriferous plate ; its pair of 

 pores, which are borne close to the outer end, go to form the outermost of the three 

 vertical rows in the poriferous zone. The second demi poriferous plate is the most 

 aboral of the triplet, and bears near its median region the pair of pores which fall in 

 the median or sporadic vertical series. The primary poriferous plate (the most adoral 

 plate of the triplet) bears the pair of pores which stand in the inner of the three 

 vertical series of the poriferous zone, and are situated rather nearer the centre of the 

 plate than one third of the distance from the inner to the outer end. On carefully 

 examining an ambulacral area it will be observed that not infrequently one ambulacral 

 plate (^. e. a composite triplet, as above described) simulates the character of a demi 

 plate in relation to the next adorally adjacent ambulacral plate, in so far that the 

 breadth of the " simulating " triplet is reduced from the adoral margin on the inner 

 part of the plate, the part thus apparently cut away obliquely being occupied by 

 the increased primary poriferous plate of the subjacent triplet ; the whole of 

 the upper of these two ambulacral plates thus simulating the character of a simple 

 demi poriferous plate in an Echinoid triplet, relatively to the underlying (^. e. more 

 adoral) ambulacral plate. Occasionally the increase of the one primary poriferous 

 plate to the sacrifice of the primary of the next aboral plate {i. e. triplet) is so great ' 

 that this latter is excluded altogether from reaching to the median suture of the inter- 

 poriferous area. At first sight these plate arrangements are very puzzling, and need 

 careful study before they can be traced. The various features mentioned above are 

 represented in Fig. 4. 



The primary tubercles which ornament the plates are small, subequal, imperforate, 

 and non-crenulate ; and those of the ambulacral regions are very little smaller than 

 those of the interradial areas. On the interporiferous areas there are six vertical rows 

 of primary tubercles, which also form regular horizontal rows of three ; these are equi- 

 distantly spaced and disposed in the following noteworthy manner : — The two tubercles 

 nearest the poriferous zone stand on the same primary poriferous plate, whilst the third 

 tubercle, which is next to the median suture, is placed on the next aboral primary 

 poriferous plate. The succeeding aboral primary poriferous plate bears no tubercles ; 

 and this disposition is maintained very prevalently throughout the right-hand column 

 of the ambulacral area. In the left-hand column the three tubercles often form an 

 oblique series, with a tubercle on each succeeding plate ; and very frequently the 

 posture of the respective tubercles seems singularly independent of the plates, in being 

 placed either midway on the horizontal suture, or with the boss only encroaching more 

 or less on the neighbouring plate. This slight but remarkable diversity in the 

 arrangement of the tubercles of the interporiferous area in the right-hand columns of 

 the ambulacral area is to be seen in all the ambulacra which we have been able to 

 examine in the example under notice. In the poriferous zones there are three vertical 

 rows of tubercles of the same size as those in the interporiferous area. Two of these 

 are located between the outermost and the median vertical rows of pores, whilst the 



