FROM THE GREY CHALK. 47 



The test is small and depressed at both poles ; the ambulacral areas are narrow, with six 

 rows of granules at the equator, diminishing to two rows at the discal and oral apertures 

 (PI. Ill a, fig. 2, e). 



The inter-ambulacral areas are wide, and the plates large, from four to five in a column . 

 the areolae are circular and wide apart, the margins prominent, and surrounded by 

 distinct secondary tubercles ; the boss is prominent and slightly granulated ; the tubercle 

 is moderate in size and perforated ; the tubercles near the peristome are small (fig. 2, b), 

 and those nearest the apical disc rudimentary (fig. 2, a), and destitute of true areolae 

 (fig- 2, a). 



The miliary zone is wide and zig-zag, and covered with large prominent granules 

 (fig. 2 c), among which smaller granules are irregularly strewed. 



I have given in fig. 1 a a drawing of the fine specimen of this species from the late 

 Mr. Taylor's collection, now in the British Museum, and in fig. 2, a, b, a larger specimen 

 from the cabinet of the Rev. T. Wiltshire, F.G.S. 



The spines, according to Mr. S. P. Woodward, are of two kinds, " the largest above 

 twenty-four lines in length, slightly swelling above the collar (two and a half lines in 

 diameter), and then tapering to a fine point ; collar short, striated finely ; shaft granulated 

 in lines ; spines of the lower surface very slender, three- to six-sided, serrated at the 

 angles ; spines of the areolar circles two lines in length, compressed and striated." 



Fig. 1, b, is a spine of C. dissimilis belonging to the British Museum. 



Fig. 2, d, e,f, are spines from the collection of the Rev. T. Wiltshire. 



Affinities and Differences. — This urchin so closely resembles C. sceptrifera that it was 

 identified as such by Professor Forbes in Dixon's Geology of Sussex ; in his manuscript 

 notes on more perfect examples he had separated it under the name C. dissimilis. All 

 the best specimens known to me I have figured in PL III a. 



Locality and Stratigraphical Position. — This species was collected by Messrs. Clarke, 

 Dixon, Taylor, and the Rev. T. Wiltshire, from the Gray Chalk at Dover ; the finest of 

 these specimens I have now figured for the first time. Fig. I, a, b, belonged to the 

 late Mr. Taylor's collection, and is now in the British Museum. Fig. 2, a, b,f, and figs. 

 3 to 5, belong to the Rev. T. Wiltshire's cabinet. 



History. — This Cidaris was referred to C. sceptrifera by Professor Forbes in his 

 description of that species in Dixon's Geology of Sussex. The subsequent discovery of 

 more perfect specimens induced him to separate it from that form under the name of 

 C. dissimilis in the MS. notes he left on the Cretaceous urchins. 



