113 PSEUDODIADEMA. 



specimens have been compared with typical examples from the Upper Greensand of 

 the Perte du Rhone, and identified as the true forms of P. Brongniarti, Agas. 



The test is moderately large, sub-circular, and elevated ; the sides are tumid, and the 

 upper surface is flat ; the base is convex, contracted at the circumference, and having the 

 peristome sunk in a considerable depression (PL XXI a, fig. 2 c). The ambulacral areas are 

 narrow, and sharply lanceolate ; they have two rows of primary tubercles, from sixteen to 

 seventeen in each row, which diminish gradually from the ambitus to the peristome and 

 the disc j they are placed closely together, and have narrow ring-like areolas ; the bosses 

 are stout, with sharply crenulated summits, and the mammillons are large and perforated 

 (fig. 2 e) ; a row of granules, of unequal sizes, sometimes mammillated, separates the 

 tubercles at the middle and base of the area, whilst in the upper part branches of fine 

 granules pass off horizontally, forming circlets around the tubercles (fig. 2 e). The 

 poriferous zones are narrow ; the pores are in single pairs in the middle and infra-marginal 

 region, near the peristome they lie in triple oblique pairs, above the ambitus they fall out of 

 their regular ranks, and at the upper surface are distinctly bigeminal (PI. XXI a, fig. 2 h). 



The inter-ambulacral areas are more than twice the width of the ambulacral ; they 

 have four rows of primary tubercles nearly identical with those in the ambulacra. The 

 two inner rows are best developed, and extend from the peristome to the disc, whilst the 

 external rows are absent from the three uppermost plates ; in the large specimen there 

 mustha\'e been sixteen to eighteen tubercles in each internal row (PI. XX, figs. 2 a and h) ; a 

 series of small unequal secondary tubercles ascends from the peristome to above the ambitus, 

 situated between the primary tubercles and the zones (PI. XXI a, figs. 2 d, e) ; they are 

 altogether absent from the upper surface, but constant in the region I have described. 

 The miliary zone is narrow in the infra-marginal region, wider in the middle, and 

 expands at the upper surface ; it is filled with four rows of small irregular granules, among 

 which a number of small mammillated tubercles as large as the secondaries are placed (PI. 

 XXI A, fig. 2 e) ; above the ambitus horizontal branches of granules extend from the median 

 rows, separating the areolas, and forming hexagonal divisions between them (fig. 2 e) ; 

 at the upper fourth of the area the granules disappear from the middle of the zone, and a 

 triangular nude space is exposed, having its base at the disc and its apex at the fifth 

 plate ; the circlets of granules are absent from the areolas of these plates. 



The upper surface is flat, and the opening for the apical disc large and pentagonal, the 

 angles extending into the inter-ambulacra (fig. 2 6.) 



The base is convex and contracted at the side ; the centre is concave, about one third 

 the width of the shell, and the mouth-opening lies in a considerable depression ; the 

 peristome is small, about one third the width of the shell ; in a fine test showing the base 

 one and a half inch in diameter ; that of the peristome is half an inch (fig. 2 c). 



Affinities and Differences. — P. Brongniarti is distinguished from its congeners by 

 its elevated test, with tumid sides, flat upper surface, and contracted, convex under 

 surface. 



