FROM THE CORALLINE OOLITE. 125 



however, it inclines to a pentagonal form (fig. 4 a), in consequence of the prominence of 

 the ambulacral areas, which are finely and regularly formed (fig. 4 a, f) ; they have two 

 rows of primary tubercles on the margin of the areas, and in each row there are from 

 twelve to fourteen tubercles, which are nearly as large as those in the inter-ambulacra ; 

 their summits are not deeply perforated, and they are raised on bosses with finely crenulated 

 summits; in the middle third of the area there is a narrow miliary zone (fig. 4/). 



The poriferous zones, from the peristome to the equator, are slightly undulated (fig. 4 b, e) ; 

 near the mouth they are arranged in triple oblique pairs (fig. 4 e), but above that point they 

 are in single pairs (fig. 4/) ; just above the equator, however, they form double pairs in 

 the rest of the upper part of the zone (fig. 4 a, d) ; this diplopodous character is very 

 constant in this species, and may be detected even in very young specimens. 



The inter-ambulacral areas are nearly twice as wide as the ambulacral (fig. 4 a, c) ; 

 they have two rows of primary tubercles, from twelve to thirteen in each row, and two 

 rows of secondary tubercles on the ambulacral side of them (fig. 4 c, e). The primary 

 tubercles are very uniform in size, and arranged with great regularity, those on the upper 

 part being only a little less than those in the middle of the area (fig. 4 d) ; the tubercle 

 is large (fig. 4g), and only slightly perforated, so much so, that a little friction effaces the 

 aperture ; the bosses are small, and their summits are finely crenulated (fig. 4 </) ; the 

 areolas are circular, or slightly inclined to an oval form ; they are all confluent above and 

 below (fig. 4/), and the miliary zones form crescents of minute granules on their sides, 

 so that the scrobicular circles are incomplete throughout. The miliary zone down the 

 middle of the inter-ambulacra is broad in the middle, contracted below, and preserves its 

 width above : it is composed of numerous minute granules, closely set together, among 

 which an irregular row of larger granules extends up the middle thereof (fig. 4 d,f) ; 

 about the upper third of the area the granules are absent from the middle of the zone, 

 which is consequently naked (fig. 4 d) ; between the primary tubercles and the poriferous 

 zones there is another miliary zone, about half as wide as that which occupies the centro- 

 sutural region ; it is composed of like minute granules, among which a row of secondary 

 tubercles are disposed in the basal region ; five or six of these tubercles are about half the 

 size of the primary tubercles in the same region, but from the equator upwards they are 

 very small, and dwindle into granules (fig. 4 e, f). 



The apical disc is absent in all the specimens I have examined ; the opening is large 

 and pentagonal, and the terminations of the areas where the disc was inserted are in many 

 shells curved outwards (fig. 4 a). This peculiarity in the structure of the terminal plates 

 may have rendered the union of the discal elements with them less firm than in other 

 Pscudodiademas. " There appear to be two varieties of the species ; one with the upper 

 surface evenly inclined all round ; the other tumid at the angles, and depressed in the 

 centre above." — Woodward ; a remark in which I entirely concur. 



The mouth opening is nearly one half the diameter of the base (fig. 4 b) ; the peristome 

 is decagonal and unequally lobed, the ambulacral lobes being one third larger than those 



