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DISCOIDEA. 



British Museum, and was obtained from the Upper Greensand. Professor Desor's type 

 was collected by Professor Favre from the same stage at Saxonnet. My late colleague, 

 Professor Forbes, stated in his note on allied British species of D. cylindrica : " I think 

 it not improbable that in the end we shall have to adopt the specific appellation 

 Favrina for the Greensand species ; and that the Chalk specimens alluded to will prove 

 varieties of cylindrica ; but a comparison of the types themselves only can settle the 

 matter." c Memoirs of the Geological Survey, British Organic Remains,' Decade i ; note 

 to plate viii, Discoidea cylindrica. 



Discoidea Dixoni, Forhes, 1850. PI. XLVIII, fig. 2 a— d, fig. 3 a, b. 



Discoidea Dixoni, Forbes, in Dixon's Geology of Sussex, pi. xxiv, figs. 13, 14, 1850. 

 — — Forbes, in Morris's Catal. of British Fossils, 2nd ed., p. 77, 1854. 



Diagnosis. — Test small, circular ; upper surface elevated, dorsum convex, sides 

 inflated ; base slightly convex ; mouth-opening and vent very small ; inter-ambulacra 

 wide ; two rows of primary tubercles more prominent than the others ; surface of the 

 plates finely granulated ; basal tubercles larger ; apical disc small ; the five genital plates 

 all perforated. 



Dimensions. — Height five twentieths of an inch ; latitude three tenths of an inch. 



Description. — In describing this Urchin, which he found in Mr. Frederick Dixon's 

 ' Cretaceous Fossils from Sussex,' my late colleague, Professor Edward Forbes, observes : 

 "Inform and size this species resembles Discoidea subuculus ; also in the proportional 

 number of ambulacral as compared with the inter-ambulacral plates, and the granulation 

 of their surfaces. It is distinguished, however, by the proportions and dimensions of 

 mouth and anus as compared with the whole ventral surface. The mouth, instead of 

 being (as in subuculus) nearly equal in diameter to the distances between its sides and 

 the margin of the inferior surface, is scarcely half that size, and the anus, instead of 

 occupying the greater part of the space between the mouth and the margin, fills less than 

 half of it/' 1 



This elegant little Urchin has likewise the upper surface elevated, the sides inflated, 

 and the base slightly convex, a careful comparison of type-specimens discloses the affinities 

 and differences subsisting between these congeneric forms. 



The ambulacral areas, half the width of the inter-ambulacral, have a row of small 

 tubercles on the zonal side of the plates, one tubercle on every other plate in the column, 

 and a second row more irregular than the former, filling in only the wider part of the area 



1 1 Dixon's Geology and Fossils of Sussex,' p. 34 1. 



