28 PEOF. p. M. DUNCAN AND ME. W. P. SLAUEN ON 



cene and Miocene of Sind and Kaclih are characterized by the 

 great development of the ambulacra, their tumidity, the presence 

 of great primary tubercles in them, decreasing in size towards 

 the peristome and the apical region, and the presence of triple 

 pairs of pores around the great tubercles. 



The arrangement of the three plates which form a compound 

 and tubercle-bearing plate, iu so many genera, has been described 

 in many works on the Ecbinoidea, and illustrated by almost 

 every writer on the subject. Take, for example, a plate with 

 tbree pairs of pores in the young form of a species of Strongylo- 

 centrotus, as drawn by Loven * (PI. I. fig. 1). 



The plate is seen to be traversed by the imperfectly united 

 sutural edges of three plates, of which the middle one is small 

 and does not reach far into the united plate. The two others are 

 large, and they both and their uniting suture reach the inner or 

 median line of the ambulacrum. 



The two large plates, a & a" , are called primary, because they 

 reach from the outer to the inner edge of the multiple plate, and 

 thus are comparable with true primary plates which are separate, 

 and reach from the interradium to the median line of the ambu- 

 lacrum. 



The small plate, 5, which fits in between the two primaries close 

 to the interradial edge of the compound plate is called a demi- 

 plate. The junction of the edges of the large plates, beyond the 

 inner angle of the little demi-plate, is long, simple, and directed 

 inwards towards the inner margin of the multiple plate and also 

 upwards. 



This line of united suture-edges is oblique in its direction, 

 and therefore one of the large or primary plates must be larger 

 than the other. In the example of the young Strongylocentrotus 

 it is, as is usual, the lower or adoral primary which is the 

 largest, and because the direction of the upper edge of it is 

 inwards and upwards. When there is a tubercle on the multiple 

 plate, the line of the junction of the upper and lower primary 

 plates crosses it. 



The line may cross the boss, or the mamelon, or the space 

 between the top of the boss and the mamelon, and usually the 

 inner end of the demi-plate separates the long line of junction 

 of the others, on the shoulder of the tubercle. 



The study of the ambulacral plates close to the radial plate 

 shows that, however complicated they may be lower down in the 

 * ' Etudes,' Plate xvii. fig. 140. 



