294 THE FOSSIL ECHINOIDEA 



In another specimen the following is the order of the succession of the plates : — 

 Zone ««," 1, 2, 3, U, 5, 6, 7, M, 9, x, lid. 



Zone " h," 1, 2, 3, 4, 5, 6, Id, 8, 9, 10, lid, lid, xiii, lid. 



The last-noticed three plates of zone " b " form the first triple combination. 



Small tubercles are on " a," 5 and on " b," 8 and 10. 



The interradia are very slightly broader than the ambulacra, and are depressed 

 and even slightly caved in above the ambitus, but at the ambitus and below to the 

 peristomial margin there is hardly any difference between the circumferential dimen- 

 sions of the two areas. The plates are high except at the peristome, and are nearly 

 square, or rather are nearly as broad as high when they carry the largest tubercles ; 

 but all the plates are remarkable for the obliquity of the direction of their aboral and 

 adoral sutures near the poriferous zones of the neighbouring ambulacra. Near the 

 peristome the whole plate is oblique and slants down from the median line of the 

 interradium. This general obliquity is in relation, above the ambitus, with the 

 obliquity of the pores, and it also relates to the special ornamentation of the outer 

 part of every coronal plate above the ambitus, and to the position of the characteristic 

 row of well-developed secondaries lower down. 



The outer part of each coronal plate, for about one third of its breadth, is orna- 

 mented with small secondaries, large and small rounded granules, and with irregulaiv 

 shaped narrow ridges and furrows. 



The second, large plate from the apex has a small secondary on the oblique part 

 close to the adoral outer angle ; plates 3 and 4 have each two of these tubercles and in 

 the same relative position. 



Plate 5 has three small secondaries on its extremely oblique adoral and outer part, 

 and the next plate has four. There may be a repetition of the four tubercles on the 

 plate below, and they slope round the flank of the tubercle more or less vertically at 

 last, and this appearance is intensified by the occurrence of a secondary on the outer 

 and upper part of the plate below. Near the peristome the number of these side 

 tubercles diminishes from three to two, and then, before the plates become very small, 

 to one. 



These details are usually very constant, and in full-grown specimens the rows of 

 small secondaries close to the large tubercles are very striking. 



There is great diversity in the arrangement of the granules and ridges, however. 

 There are often rows of large granules placed between the small primaries, and inter- 

 vening are rows of small granules. The direction of the rows is often the same as that 

 of the oblique secondaries ; but the granules are placed higher up the plates than the 

 secondaries. In those upper plates which have not tubercles there is usually a bunch 

 of close granules halfway up the outer third, and elegant ridges and furrows radiate 

 from the group and pass over the breadth of the plate to the median line, and then on 

 to the opposite pkte, ornamenting the adoral or the aboral part of its surface as the 

 case may be. A zigzag arrangement is thus produced, and the ridges and furrows are 

 long and very variable in size. Usually the outer third of the plates above the ambitus 



