FROM THE STONESFIELD SLATE. 115 



When collecting materials for my ' Memoir on the Cidarida),' this species engaged my 

 attention, and I endeavoured, in vain, to find the original of Parkinson's figure, as I was 

 under the impression that it was a juvenile form of Pseudodiadema versipora, from the 

 Coral Rag of Wiltshire, very much resembling a specimen in the cabinet of Mr. Mackniel, 

 of Trowbridge ; for this reason it was not included in my c Memoir.' As M. Desor has 

 now, however, named the specimen, and entered it in his ' Synopsis/ I have considered 

 it desirable to have an accurate copy of Parkinson's figure transferred to my work. 



The ambulacral areas are moderately wide, and carry two rows of tubercles nearly as 

 large as those of the inter-ambulacral areas; the pores are arranged in a sino-le file 

 throughout; the inter-ambulacral areas are about once and a half as wide as the 

 ambulacral, and have two rows of primary tubercles ; nor is there any evidence of the 

 existence of a secondary range ; the spines are thick and subulate, slightly bent in the 

 middle, and marked with longitudinal lines ; they are nearly as long as the diameter of 

 the shell and are very robust in proportion to its dimensions. 



Affinities and differences. — This species resembles Pseudodiadema depression in the 

 absence of secondary tubercles ; but that species is distinguished from it by the test being 

 more circular, and its spines being thicker and bent. It may, however, be a juvenile 

 form of Pseudodiadema versipora, from the Coral Rag, the artist having overlooked the 

 crowding together and doubling of the pairs of pores in the upper part of the poriferous 

 zones ; not having the advantage of examining the specimen, I make these remarks with 

 much hesitation, well knowing from experience how readily we may be deceived by figures, 

 where no details are added, to indicate the specialities of the organism. 



Locality and Stratigrapliical position. — Mr. Parkinson says it was collected from the 

 Stonesfield Slate at Stonesfield, Oxon, where it must be exceedingly rare. I have collected 

 fragments of a small Pseudodiadema from the Yellow Clays of the Stonesfield Slate at 

 Sevenhampton, Gloucestershire, which may belong to this species. My specimens are 

 mislaid, but they were not in a good state of conservation. 



D. Species from the Great Oolite. 

 Pseudodiadema pentagonum, M'Coy. PL VI, fig. 3 a, b, c, d. 



Diplopodia pentagona. M'Coy, Annals and Magazine of Natural History, 2d series, 



vol. ii, p. 412. 

 Diadema pentagonum. Morris, Catalogue of British Fossils, 2d edition, 18.J-4, p. 77. 

 — — Woodward, Memoirs of the Geological Survey, Decade V. Notes 



on Brit. Foss. Diademas. 



Test small, pentagonal, depressed ; ambulacral areas with two marginal rows of primary 



