108 PSEUDODIADEMA 



DiPLOPODiA sUBXUDUM, Coquand. Ibid. 



— VARioLAiiis, Dvjardin et Hupe. Hist. Nat. des Zoopli. Echinoderraes, 



p. 501, 1862. 



— SUBNUDA, Dvjardin et Hupe. Ibid. 



— STRIATULUM, Bujardin et H^ipe. Ibid. 



PsEUDODiADEMA VARiOLARE, Cotteau. Palcontologie Fran9aise, Terrain Cretace, t. vii, 



p. 488, pis. HI 7, 1 1 18, 1 1 19, and 11 20, figs. 1—3, 1864. 



Test large, subcircular, slightly subpentagonal, nearly equally depressed on the upper 

 and under surfaces. Ambulacral areas narrow and contracted at the upper part by the 

 poriferous zones ; two rows of tubercles, 1 5 — 1 7 in each series, separated by a single line 

 of granules ; pores round, in oblique single pairs in the middle, widely bigeminal on the 

 upper surface, and trigeminal near the peristome. Inter-ambulacral areas wide, with 

 four rows of primary tubercles and two short rows of small secondary tubercles. The 

 middle of the upper surface of the area nude and often depressed. Under surface convex, 

 mouth-opening small j peristome with feeble entailles. Discal opening large, sharply 

 angular, pentagonal. 



Dimensions. — a. — Height half an inch ; transverse diameter one inch. b. — Height 

 half an inch ; transverse diameter one inch and a half. 



Description. — The identity of this species has long been uncertain from the impos- 

 sibility of ascertaining the Urchin intended by the figure and description given by 

 Brongniart, without reference to the type specimen ; this comparison has now fortunately 

 been made by M. Cotteau, who has given admirable figures and most ample descriptions 

 of the various forms D. variolare exhibits under different conditions of age and habitat. I 

 shall first describe the general characters of the species, and secondly point out the three 

 chief varieties it assumes. 



The test is of medium size, subcircular, sometimes lightly pentagonal, and equally 

 depressed at both poles. The ambulacral areas are narrow, and contracted at their upper 

 part by the excessive development of the poriferous zones in this region ; they possess two rows 

 of large tubercles, 1 5 — 17 in each series, according to size and age, which gradually diminish 

 in size from the equator to the peristome, and become small and rudimentary on the upper 

 surface (PI. XVII, fig. 3 a). The rows are closely approximated, there being only a single 

 series of small granules, of unequal size, forming a zigzag line, between them ; the 

 tubercles have narrow areolas, prominent bosses, with sharply crenulated summits, and 

 deeply perforated mammillons (fig. 4). The poriferous zones are narrow, the pores 

 round, in single oblique pairs on the sides, in triple oblique pairs near the peristome, and 

 they are widely bigeminal in all the upper fourth of the zones (fig. 5 a). 



The interambulacral areas are nearly four times as wide as the ambulacral in the spe- 

 cimen figured in PI. XVII, figs. 3 a, h, c, rather more than one inch in diameter. There 

 are four rows of large tubercles, and two secondary rows, at the ambitus ; the two inner 

 primary rows, with fourteen tubercles in each series, extend from the peristome to the apical 



