296 



THE FOSSIL ECHINOIDEA 



The ambulacra project more than the interradia, the primary tubercles of which 

 are the smallest at the ambitus. The median interradial areas, above the ambitus, are 

 sunken below the level of a raised line or ridge, composed of a linear series of small 

 secondaries and granules which occupies nearly the outer third of each coronal plate. 

 The plates of this part of the test are higher than broad, and are indistinctly orna- 

 mented with a small secondary tubercle, or there may be two or more of such tubercles, 

 forming a linear series. Sometimes a small pit is seen in the position of a tubercle. 

 The sutural lines between the tubercle-bearing plates are very distinct, slightly excavated, 

 and there is a remarkable denticular interlocking of the edges of the opposed plates. 

 The direction of the upper interradial plates is transverse. Including the small new 

 plates in contact with the basal plates, the first primary tubercles are on the fifth inter- 

 radial plates from the disk. One of these tubercles is larger than the other, and both 

 are of the usual shape of the tubercles of the genus. The plates are, however, not 

 with sutures that are uniformly transverse in direction, for their outer and adoral angle 

 is directed slightly adorally, towards the poriferous zone, close by ; moreover this part 

 of the plate carries at least two small secondaries. 



There are a few granules upon the aboral edges of these plates. The dimensions 

 of the primary plates diminish rapidly towards the peristome, and they become crowded 

 there and the interradium becomes narrower. There are at least four large tubercles, 

 and the couple next to the highest are the largest, namely those of the sixth interradial 

 plates. 



The ambulacra are well developed, and the size of the tubercles at and above the 

 ambitus is considerable. The pairs of pores are in arcs at the edges of the great 

 tubercles, and the sutures are in the same direction as in the adult specimens. But 

 the number of small plates in the interporiferous area towards the radial end is small, 

 and the development of the triplets occurs high up. The pairs of pores are large and 

 by no means close, there being two pairs in relation to the first interradial plate and 

 three to the second ; moreover the direction of the pores is decidedly oblique, and from 

 the interporiferous area outwards and upwards. In each poriferous plate there is a 

 broad rather high process, which separates the pores from the aboral edge, or the line 

 of suture that limits the plate from that immediately above. There is a gradual increase 

 in size of the pairs until the seventh, when the triplet arrangement becomes evident, 

 the pairs wider apart, and the pores larger. The following is the disposition of the 

 plates in one of the ambulacra : — 



No. III. Zone " a '■ 



-Plate 1, small primary, 



2, small primary, 



3, small demi, 



4, large primary, 



5, small demi. 



6, small demi. 



7, large primary. 



8, small demi. 



Zone '* h " — 1, small primary. 



2, small „ 



3, small „ 



4, small „ 



5, small „ 



6, large „ 



7, small demi. 



8, small demi. 



