FROM THE INFERIOR OOLITE. 379 



narrow, transverse pores, connected with the inner row by conjugate sulci ; the septa 

 between the transverse pores support, on their upper surface, a regular row of small 

 tubercles (PI. XXXII, fig. 1, c). Near the marginal fold the pores of the outer row 

 are round, like those of the inner row, and form narrow zones of close -set pores, only 

 apparent by faint depressed lines, which mark the track of the zones from the border 

 to the mouth ; near the peristome the pores become more numerous, and here form a 

 series of triple oblique rows of pairs. PI. XXXII, fig. 1, d, represents one of the basal 

 ambulacral areas, highly magnified (PI. XXXI, fig. \,b). 



The inter-ambulacral areas are of unequal width at the margin ; the anterior pair are 

 the narrowest, the posterior pair are wider, and the single area is the widest ; they are 

 uniformly convex on the upper surface, and are formed of very large pentagonal plates, 

 bent in the middle ; each plate supports four horizontal rows of small tubercles, surrounded 

 by sunken areolas, and having the inter-tubercular surface covered with fine, microscopic, 

 homogeneous granules. PI. XXXII, fig. 1 c, exhibits the ambulacral plates, poriferous 

 zones, and two inter-ambulacral plates magnified three diameters, and shows that there are 

 seven pairs of pores opposite each large plate. 



The under surface is flat, and the basal portions of the inter-ambulacra are only slightly 

 cushioned ; the tubercles in this region are larger, and not so regularly arranged in rows 

 as on the upper surface (PI. XXXI, fig. 1 h), but are scattered more promiscuously over the 

 surface. 



The height of the test, and the excentral position of the apical disc, inclined backwards 

 behind the vertex, occasions the curvature in the lateral ambulacra already described 

 (PI. XXXII, fig. 1 a). The disc is small, in proportion to the size of the test (PI. 

 XXXI, fig. 1 «), and is composed of two anterior, and two posterior perforated ovarial 

 plates, and a single imperforate plate. The madreporiform body covers all the centre of 

 the disc, and the five ocular plates are only indicated by their marginal orbits at the apices 

 of the ambulacra ; the elements of the disc are closely soldered together, and only 

 occasionally seen distinct in some decomposed specimens. 



The dorsal portion of the single inter-ambulacrum, when viewed posteriorly (PI. XXXII, 

 fig. 1 h), presents a triangular figure; the border forms the base, and the two postero- 

 lateral- ambulacra the sides of the triangle ; near the middle of this space is placed the 

 oval vent, which opened near the surface (fig. 1 b) ; the anal valley, is a very shallow 

 depression, from its sides, two nodulated ridges descend downwards and outwards 

 towards the border, the plane of the intermediate space being a little beneath the general 

 plane of the test (fig. 1 ^). In some smaller specimens there is sometimes a 

 narrow depression which passes upwards from the summit of the arch above the anal 

 valley towards the disc, which is effaced in the large type specimen (fig. 1 h) ; 

 in this urchin the space between the upper border of the opening and the disc is occupied 

 to the extent of one inch by a smooth, slightly depressed portion of test ; which, added 

 to the oval vent, shallow valley, and superficial depression beneath, form a group of 



