302 THE FOSSIL ECHINOIDEA 



three plates, seen on an ambulacrum is fifteen, there being eight on one zone and seven 

 on the other. 



Above the highest large tubercle there are some small and rudimentary ones on 

 the larger of the ambulacral plates within the poriferous zones. Quite at the peri- 

 stome the last three triplets have their pairs of pores remote from the outer suture of 

 their plates, and the outer region is a part of the tag-like surface vphich is continued 

 from the fairly well-marked cut on either side of each atnbulacrum. 



The interradia are deprived of large tubercles above the commencement of tbe 

 upward-sloping part of the test, and the highest of the large tubercles is at a lower 

 level than the corresponding one of the ambulacrum. In an interradium which is well 

 preserved (No. II.), the plates are broader than high, and their sutural lines are grooved ; 

 the transverse direction of the plates is decidedly oblique, and rather more so than 

 the direction of the line of the pores close by. The smallest plate at the edge of the 

 apicaldisk is plate 1 of zone " a," and the first rather large tubercle is on plate 6 ; on 

 zone " b " the similar tubercle is on plate 5. Both much smaller than a typical 

 great tubercle, do not cover much of their plate, and have a smaller tubercle close to 

 their bases and nearer the median line. The rest of the plates is well granulate, and 

 the granules of the plate of " b " are rather disposed to be in transverse lines. On the 

 outer edge of the plates, and where there is a continuation of the line of structures that 

 bounds the poriferous zones of the ambulacra, there is a number of small tubercles 

 arranged in three vertical rows. These small tubercles are not very close and are 

 mammillate, one being, moreover, larger than the others. It is this part of the plate 

 which is decidedly oblique. The plate "a" 5 from the apex, has the relic of a small 

 tubercle near the median and adoral angle ; the granulation is small and linear and the 

 direction is oblique to the line of the edges of the plate, and somewhat continuous with 

 that of the plate just described from zone " b." There are two vertical rows of small 

 tubercles close to the outer margin, and one tubercle is slightly larger than the others, 

 which are also circumferential to it. Plate 4 of zone " b " resembles the last plate in 

 the arrangement of its lines of granules in ridges, and in the existence of a degraded 

 small tubercle on the inner and adoral angle ; moreover the arrangement of the small 

 tubercles on the outer edge of the plates is the same. 



The other plates are free from vestiges of tubercles, and the inner region of each 

 has two irregular rows of very small tubercles. 



The aboral edges of the larger primary tubercles of the interradia above the 

 ambitus are ornamented with a row of distinct granules, and there are similar rows 

 adorally. There is much granulation in the median line, and finally the granulation 

 amounts to small tuberculation on the outer edge of each plate, where there are two 

 and in some plates three little tubercles in a vertical or very oblique row. There are 

 four very large and four rather large primary tubercles in an interradium. 



Illustration of the Form in Plate XL VI. 

 Fig. 6. The apical disk: magnified. 



