FROM THE GREAT OOLITE. 117 



having their summits feebly crenulated, and the tubercles not deeply perforated. The 

 areolas are superficial, and confluent above and below (fig. 3 d) ; five or six granules 

 form semicircles on each side of the areola. The miliary zone, which is of moderate 

 width, contains few granules besides those forming the scrobicular semicircles, in the upper 

 part of the test this space is almost naked ; between the peristome and the equator there 

 are five or six secondary tubercles, about one third the size of the primaries, situated between 

 them and the poriferous zones, forming a short, irregular row, which scarcely reaches the 

 middle of the test (fig. 3 b). 



The mouth opening in width is nearly one half the diameter of the test ; the peristome 

 is unequally decagonal, the ambulacral being larger than the inter-ambulacral lobes. 



The disc opening is large, and nearly circular ; but the disc is absent in all the speci- 

 mens I have examined, and in none of them are the spines preserved. 



Affinities and differences. — Pseudodiadema pentagonum very much resembles Pseudo- 

 diadema depressum in its pentagonal and depressed form, in having the primary tubercles 

 of both areas at the equator nearly of equal size, and in having the upper part of the 

 miliary zones almost naked ; but it is distinguished from that species in having a short 

 row of secondary tubercles at the base, between the inter-ambulacral primary tubercles 

 and the poriferous zones ; in having the poriferous zones wide, and the pores bigeminal in 

 all the upper portion of the zones, a character which is at once evident and distinctive 

 between these two nearly allied forms. Pseudodiadema pentagonum resembles Pseudo- 

 diadema versipora in its depressed and pentagonal form, and in the bigeminal character 

 of the pores in the upper parts of the zones ; but it is distinguished from that beautiful 

 Coral Rag urchin in having much smaller primary tubercles, and fewer and larger granules 

 in the miliary zones; the central and lateral zones of the inter-ambulacra in Pseudodiadema 

 versipora being much wider, and filled with several rows of small, close-set granules ; 

 whereas in Pseudodiadema pentagonum there are few granules besides those forming the 

 scrobicular semicircles. The diplopodous character of this species serves to distinguish it 

 from all other British Oolitic Diadems. 



Locality and Stratigraphical position. — This beautiful urchin has been found in the 

 shelly beds of the Great Oolite at Minchinhampton, where it is very rare. Associated 

 with it are the following Echinoderms : Acrosalenia hemicidaroides, Wright ; Polycyphus 

 nodulosus, Miinster ; Pygaster semisulcatus, Phillips ; Ilyboclypus agariciformis, Forbes ; 

 Echinobrissus clunicularis, Llhwyd. 



History. — First described by Professor M'Coy in the ' Annals of Natural History,'* 

 in his paper " On some new Mesozoic Radiata," under the name Diplopodia pentagona, 

 and regarded by him as the type of a new genus of Diademad.e, which he supposed was 



* 'Annals and Magazine of Natural History,' 2d series, vol. ii, p. 412. 



