90 PSEUDODIADEMA 



Neocomian ; and at Laiuleron, Saiiite-Croix, Ilauterive in the Middle Neocomian, so that 

 it forms a leading fossil of the Neocomian formations. 



History. — This Urchin was at first referred by Professor Forbes to the Diadema dubium 

 of Albin Gras, but a careful comparison of specimens proved this to be an error. It 

 appeared under that name in Mr. Sharpe's list of Echinodermata from the sands and 

 gravels of Farringdon, and in the second edition of the ' Catalogue of British Fossils.' 



PsEUDODiADEMA FiTTONi, Wright. PI. XV, Figs. 1, a — g. 



Diadema Autissiodokense, Wright. Ann. and Mag. of Nat. History, New Series, 



vol. X, p. !)1, 1852. 



Test pentagonal, depressed ; inter-ambulacral areas with two rows of primary tubercles 

 and two incomplete series of small secondary tubercles, which disappear on the upper 

 surface ; ambulacral areas prominent, with two rows of primary tubercles much diminished 

 in size at the upper surface; poriferous zones narrow, subflexuous. Pores bigeminal 

 near the ovarial disc, and at the circumference of the mouth. 



Dimensions. — Height four tenths of an inch ; transverse diameter nineteen twentieths 

 of an inch. 



Description. — In its general outline this beautiful Urchin resembles P. depressum of 

 the Inferior Oolite ; in the details of structure, however, it is very distinct from that form. 

 The circumference is pentagonal, from the convexity of the ambulacral areas, and the upper 

 and under surfaces are much depressed (PI. XV, fig. 1 a, b, c, d). 



The inter-ambulacral areas are one third broader than the ambulacral ; two rows of 

 primary tubercles occupy the centre of the plates ; there are about ten pairs of tubercles 

 in each area, which are of a moderate magnitude, and gradually diminish in size from 

 the ambitus to the base and summit ; the mammillary eminences are small, their summits 

 sharply crenulated, and the tubercles, of proportional size, are deeply perforated (fig. 1 (/) ; 

 at the ambitus six rows of granules separate the tubercles from each other (fig. 1 e) ; 

 towards the upper part of the mihary zone the four central rows are absent, leaving a naked 

 space in the middle of the area ; three rows of granules in like manner separate the tubercles 

 from the poriferous zones ; at the base of the area, and extending as far as the ambitus, 

 there are incomplete rows of small secondary tubercles ; these gradually diminish in size, 

 and disappear at the upper surface, which is occupied with an unequal close-set granulation 

 about three rows deep (fig. 1 b) ; the ambulacral areas, one third narrower than 

 the inter-ambulacral, are very prominent and convex, and occupied by two rows of 

 primary tubercles about ten in a row ; the lower six pairs of tubercles are nearly 

 as large as the corresponding tubercles in the inter-ambulacral areas, but the upper 



