102 PROF. P. M. DFlSrCAlSr ON THE ANATOMY OF THE 



one tubercle upon it, for study, besides a number of unseparated 

 compound plates from the ambitus to tlie apex. 



Tlie Ambulacral Compoimd Tuhercle-h earing Plate, from the 

 inside of tlie Test after the application of Benzole. — Each plate 

 is much broader than high, and is composed of three plates, 

 the triplet of pairs of pores being arranged as in Diadema. The 

 sutures of the poriferous part of the plates are neaiiy parallel, but 

 those between the first and second and the second and third plates 

 approach just internally to their adoral pores, but do not unite. 

 Thence the sutures diverge and reach the median line, or the 

 vertical suture of the compound plate close to the actinal and abac- 

 tinal angle of that part of the plate, respectively. Hence the first 

 and third plates, although primaries, are often low, close to the 

 median line, whilst the second plate is large and high there. The 

 reverse of this is seen internally to the adoral pores of the triple 

 plates, for there the second plate is low and thie first and the 

 third are correspondingly high. (Plate V. figs. 6, 6 a, and 7.) 



It is difiieult to trace the line of the sutures on the outside of 

 the test, and over the tubercles ; but as the benzole dries, the 

 sutures may be seen crossing over or rather passing along the 

 breadth of the plate, approaching one another on the tubercle, 

 having the mamelon between them, and then one passes towards 

 the abactinal and the other towards the actinal angle of the 

 median suture of the compound plate. (Plate V. fig. 6.) 



The regular distribution of the pairs of pores, from close to the 

 apex to near the peristome, is very striking when viewed from 

 within the test. The lines of the pairs are very oblique, and the 

 third pair of pores of the triplets is larger than the others, and 

 the adoral pore of this pair is always very large. It is nearer 

 the median line of the ambulacrum than the other pores. As 

 the series of these third pairs are numerous, the large pores just 

 mentioned form definite lines on either side of the median 

 suture and at some distance from it. (Plate Y. fig. 7.) 



The plates forming the compound plates close to the radial 

 plate are of course small and low, but they are nearly or quite of 

 equal size. The sutural lines pass directly towards the median 

 line, and without any bending, so that the three plates are almost 

 rectangular in shape and all reach the median line. This is the 

 simplest possible arrangement of triplets in a compound plate. 

 A little way down the ambulacrum, the pressure from the coming 

 in of new primaries at the radial plate and the expansive growth 



