FROM THE INFERIOR OOLITE. 209 



inter-ambulacral plate ; the zones are slightly contracted at the margin, and expand again 

 at the base. 



The inter-ambulacral areas are three times as wide as the ambulacral (fig. 2 c) ; each 

 column consists of about twenty-five elongated, pentagonal plates (fig. 2 d), and a median 

 depression divides the area from the margin to the disc into two lobes ; there are two rows 

 of small primary tubercles in the centre of the plates (fig. 2 c, d), which extend from the 

 peristome to the disc, and two short rows of secondaries on their zonal sides, which dis- 

 appear a little way above the margin ; within the primary rows at the base are six or 

 seven more ; from the circumference to the disc there is a wide miliary zone (fig. 2 a, c), 

 which is filled with small tubercles and minute granules, arranged, however, with con- 

 siderable regularity on the plates (fig. 2 d), and extending even over the median depression, 

 which, when taken in connection with the smallness of the tubercles, about the same size 

 in both areas, imparts a peculiar granular appearance to the test of this species. 



The base is flat, and the tubercles, as usual, are larger in this region ; the mouth 

 opening is eight tenths of an inch, the diameter of the test being one inch and nine tenths ; 

 the peristome is pentagonal, with two notches at each angle, and a small lobe between, 

 the large ambulacral lobes forming the sides of the pentagon (fig. 2 b). 



The apical disc is well developed, and slightly excentral (fig. 2 c) ; the genital plates 

 (fig. 2 a, e), are elongated, and perforated near their outer third, the right antero-lateral 

 being much the largest, and supporting a prominent madreporiform body (fig. 2 <?) ; the 

 ocular plates are small pentagons, with slit-like eye-holes ; the disc is much more developed 

 in this species than it is either in Stomechinus germinans or bigranularis. 



Affinities and differences. — This urchin is distinguished from Stomechinus germinans 

 by the following characters : The test is more regular in its form, less elevated and 

 rounder, and never assumes a conoidal figure ; the tubercles are much smaller, and equal- 

 sized ; the small tubercles and granules on the miliary zone are larger, and have a more 

 regular arrangement, which, with the smallness of the primary tubercles, gives the test a 

 more granular fades. The marginal angle is more obtuse, and the sides are inflated; the 

 apical disc is larger, and its elements are more developed; the mouth opening is pro- 

 portionally smaller, and the notches are neither so wide nor so deeply incised. Sto- 

 mechinus intermedius is considered by MM. Agassiz and Desor to be a variety of St. 

 bigranularis; but, between the structure of the apical disc, the arrangement of the 

 trigeminal ranks in the poriferous zones, and in the size of the mouth opening, we find 

 characters sufficient to show that the affinity between St. germinans and St. intermedius is 

 greater than between St. intermedius and St. bigranularis. 



Locality and Stratigraphical position. — I have collected this species in the upper rag- 

 stones of Shurdington, Rodborough, and Dundry Hills, where it is extremely rare. The 

 Dun dry specimen I formerly described under the name of Echinus granulans. I found a 



