FROM THE GREY CHALK. 117 



Glyphocyphus difficilis, Desor. Synops. des fichinides foss., p. 104, 185". 



Cyphosoma difficile, Woodward (pars). Mem. of Geol. Survey, Appendix 



to Decade V, p. 3, 1858. 

 Glyphocyphus difficilis, Dujardin et Hupe. Hist. Nat. des Zooph., Echinid., 



p. 513, 1862. 

 EcHiNOCYPHUS difficilis, Cotteau. Pal. Fran9aise, Ter. Cretace, t. vii, p. 708, 



pi. 1174, 1866. 

 — EOTATUs, CoMeaw (pars). Ibid., t. vii, p. "11, pi. 1174-/5. 



Test small, subcircular, moderately inflated on the upper surface, flattened at the 

 base, concave around the mouth, and rounded at the sides ; poriferous zones narrow, 

 straight, and subflexuous at the ambitus ; pairs of pores in a single series ; ambulacral 

 areas narrow above, enlarged at the ambitus, with two rows of tubercles, one of which is 

 often abortive ; inter-ambulacral areas wide, having two rows of tubercles ; plates pos- 

 sessing small, strongly radiated areolae, and divided by deep sutural impressions ; apical 

 disc equal to the oral opening, flat, and finely granulated. 



Dimensions. — Transverse diameter eight tenths of an inch ; height four tenths of an inch. 



Description. — This Urchin has long been mistaken for a Diadeiiia, and is catalogued in 

 some lists of Upper Greensand fossils as D. rotatmn. Small specimens, measuring from four 

 to five lines in diameter, are abundant in the Upper Greensand of Warminster, and may 

 readily be distinguished from Diademas by the sutural impressions on their plates, the 

 small radiating ribs on many of the large areote, and the irregularity of the two rows of 

 ambulacral tubercles, one of which is often abortive. The larger and taller specimens 

 agree with the published mould of Professor Agassiz's type specimen of Cyphosoma dijficile, 

 to which this Urchin is now referred. 



Almost all the examples I have collected from the Upper Greensand are small and 

 well preserved ; the few I have from the Lower Grey Chalk are larger, and show the 

 sculpturing of the plates better. The test is subcircular, or slightly pentagonal, mode- 

 rately convex on the upper surface, flattened at the base, and rounded at the border. 

 The poriferous zones are narrow, straight above and below, and siibundulated at the 

 ambitus. The pores are small, and form a single series of pairs throughout from the 

 mouth to the disc (fig. 2 d) ; the pairs of pores are separated horizontally by small promi- 

 nent ribs, more or less granular, which correspond to the poriferous plates, and are pro- 

 longed to the base of the tubercles. The ambulacral areas are narrow, and provided with 

 two rows of tubercles ; in some specimens from the Grey Chalk these rows are regular 

 and the tubercles of equal size, but in many specimens from the Upper Greensand, 

 as in figs. 2 a, b, c, d, one row becomes abortive, and the other only is developed ; 

 more than half the specimens that have passed through my hands have been thus 

 formed. The inter-ambulacral areas are wide, with two rows of tubercles a little 

 larger than those occupying the ambulacra ; in large specimens there are ten or twelve 

 tubercles in each row, the number depending on the age of the individual. Between 

 the ambitus and peristome the areolas are well developed, and surrounded by 



