FROM THE GREAT OOLITE. 91 



which attain their greatest development at the equator of the test, and diminish in size 

 near the anal and buccal openings. The mammillary eminences supporting the tubercles 

 are large, prominent, and surrounded by confluent areolas. The tubercles are small and 

 perforated ; one row of granules separates the large tubercles from the poriferous zones, 

 and a double row occupies the middle of the area. The lateral boundaries of the areolas 

 are surrounded by semicircles of granules, whilst the upper and lower boundaries of the 

 same blend into each other. 



The ambulacral areas are narrow, slightly undulated, and furnished through nearly 

 all their extent with a double roAv of small tubercles, which are not very apparent, but are 

 larger on the sides than at the apex of the area ; between the size of these and the three 

 pairs of semi-tubercles at the base, a sensible difference exists. The mouth opening is large, 

 and is one half the diameter of the test ; it is of a decagonal form, with the peristome 

 deeply notched. The apical disc is not preserved, and the spines are unknown. 



Ajfmities and differences. — Hemicidaris icaunensis, in its general form and charac- 

 ters, closely resembles Hemicidaris intermedia. It is distinguished from the latter by 

 having the primary tubercles of the inter-ambulacral areas less prominent, by the ambulacral 

 areas being less waved, and in having the semi-tubercles much smaller. This character 

 assimilates Hemicidaris icaunensis with Hemicidaris Thurmanni, but it is sufficiently distin- 

 guished from that urchin by its greater height, less undulated ambulacra, and the greater 

 number of tubercular plates in the inter-ambulacral areas. 



Locality and StratigrapMcal position. — This rare species was obtained by Mr. Lycett 

 from the Great Oolite of Minchinhampton. M. Cotteau collected it in France from the 

 superior beds of the Bathonian stage at Chatel-Censoir ; and M. Bathier found it in the 

 Forest Marble of Chatel- Gerard, where it is likewise rare. 



History. — This species was first figured and described by M. Cotteau,* and was 

 provisionally identified by Professor Forbes. f It is figured in PI. a, fig. 9, of the 

 ' Monograph of Great Oolite Fossils,' to be published by the Palasontographical Society. 

 The specimen before me, which belongs to Mr. Lycett's collection, is only a cast ; the 

 determination, therefore, is doubtful, but as it was made by my lamented friend, after a 

 careful consideration of the facts, I have adopted his views without vouching for their 

 accuracy. The specimen is so imperfect, that I have followed M. Cotteau's description. 



* Ecliinides Foss. du Departement de I'Yonne, t. 3, p. 56. 



f Memoirs of the Geological Survey; Brit, Organic Remains, Decade III, description of pi. 5. 



