FROM THE GREAT OOLITE. 87 



ambulacral areas in Hemicidaris piistulosa (PI. Ill, fig. 1) and Ilemicidaris granulosa 

 (PI. Ill, fig. 2) distinguish at a glance these species from Ilemicidaris Bravenderi. 



In its sub-globose form it much resembles Ilemicidaris Davidsoni (PI. IV, fig. 2), but 

 it is distinguished from that Portland species in having wider ambulacral areas, with the 

 minute tubercles thereon both longitudinally and laterally more widely apart ; the semi- 

 tubercles are likewise much smaller, and disposed regularly in pairs, whereas they run (in 

 many specimens) into a single row in Ilemicidaris Davidsoni (PL IV, fig. 2 b, c). The 

 size of the anal opening in the latter is much greater, so much so, that the right postero- 

 lateral and odd genital plates are greatly reduced in size. Ilemicidaris Bravenderi resembles 

 Ilemicidaris Purheckensis (PI. V, fig. 4) in the straightness of the ambulacral areas, but 

 these divisions of the shell are wider, and the minute marginal tubercles are set wider apart 

 in the former species ; the semi-tubercles, likewise, are more regularly disposed in pairs 

 than in the Purbeck form (PI. IV, fig. 4 h), where they assume a linear arrangement. 

 The sudden diminution in the size of the primary tubercles on the upper parts of the 

 inter-ambulacral areas in Ilemicidaris Stokesii (PI. Ill, fig. 3) at once separates that 

 Stonesfield slate species from Ilemicidaris Bravenderi. 



Locality and Stratigraphical position. — This species belongs to the Bathonian Oolitic 

 zone. It has been collected from the Great Oolite of Kill-Devil Hill, near Cirencester, 

 by Mr. J. Brown of that town, but the specimen, formerly in the collection of that gentleman, 

 now presented to the British Museum, is unfortunately not well conserved. Mr. Bravender, 

 E.G.S., of Cirencester, found a fine specimen of this species, with its spines attached, in the 

 Great Oolite at Stratton, near Cirencester, which was figured by Mr.Buckman in the 'Geology 

 of Cheltenham,' Its zoological characters are very well preserved. This species was col- 

 lected by Mr. Bristow, of the Geological Survey, from the Cornbrash, in a lane leading from 

 Stourton Caundle to Lower Woodacre. This fine specimen is in the Museum of Practical 

 Geology, Jermyn Street, and has been admirably figured by Mr. Bone for this Monograph. 

 Ilemicidaris Bravenderi is found, likewise, in the Great Oolite of Langrune, Calvados. I 

 have specimens from that locality kindly sent me by Professor Deslongcharaps and 

 M. Tesson, of Caen. 



History. — Eirst figured, without description, as Ilemicidaris crenularis in the ' Geology 

 of Cheltenham,' and noticed by Professor Forbes in his Note on the species of Hemicidaris 

 found in British strata, ' Memoir of the Geological Survey,' Decade III : " This fine 

 species/' he observes, " is very distinct from any other British one, resembling most nearly 

 Ilemicidaris intermedia, but diifering in having gradually, not suddenly, increasing ambu- 

 lacral areas, with the tubercles upon them set well apart, except below, where the larger 

 ones are closely packed. Until the spines shall have been discovered, I hesitate to 

 give a name to this form, since it so closely agrees with the figure of the Swiss species, 

 Ilemicidaris cremdaris." The detailed diagnosis of the points of difi'erence between 



