438 PEOF. p. M. Dtn!TCAJf OK THE STKUCTTJEE OE 



From the effects of the ohliquity of the adoral edge, the part of 

 the plate near the interradium is greater in vertical measurement 

 than that towards the median line. 



JSTumerous small and usually very low primary plates succeed, and 

 in the ambulacrum under examination, wherever a granule-bearing 

 plate is in contact adorally with a small tubercle-bearing one this 

 last intrudes upon and deforms the other (fig. 11). 



Fig. 11 (see p. 452). 



The plates 15, 16, 17, and 18, of the same ambulacrum and zone, 

 may be taken as typical of the greater part of the area halfway 

 between the great tubercles at the ambitus and the radial plate. 



Plate 15 is a low primary with a very small mamiUated boss and 

 three minute granules. The interporiferous portion is on a slightly 

 higher level than the poriferous area, and the aboral edge of the 

 plate below delate 16) fits into a space which is produced by this 

 want of conformity of level. 



Plate 16 is of the usual height in the poriferous area, but it is 

 forced up aboraUy by the expansion of the tubercle-bearing plate 17, 

 situated immediately actinally to it, so as to conform at its adoral 

 edge with the curve of the base of the tubercle. Hence this plate, 

 which is very sparely ornamented with a granule or two, is almost 

 linear towards the median line. 



Plate 17 has a small tubercle which occupies nearly the whole of 

 a much expanded interporiferous area, the increase in growth being 

 apical. The next plate, 18, is a low primary with granules, and it 

 is not much deformed by the slight adoral extension of the above- 

 mentioned tubercle. All these plates are primaries. 



Js'earer the ambitus the fusion of primaries, forming compound 

 plates with two pairs of pores, becomes evident. Thus at plates 35, 

 36, and 37 (fig. 12), this is weU seen. 



Fig. 12 (see p. 452). 



37 



Plate 35 is a primary, or, rather, was one before its adoral edge 

 united, organically, with the aboral edge of the plate 36, and before 



