FROM THE CORALLINE OOLITE. 133 



Test circular, depressed ; ambulacral areas wide, with two rows of primary tubercles 

 nearly as large as the inter-ambulacral ; poriferous zones narrow ; pores unigeminal on the 

 sides and upper part of the zones, trigeminal near the mouth ; inter-ambulacral areas 

 with two rows of primary tubercles, and two short rows of secondary tubercles on the zonal 

 sides of the primaries ; the bosses of both areas nearly of the same size, large, prominent, 

 and conical ; spinigerous tubercles of both areas of the same size, large, and widely perfo- 

 rated. Mouth opening large, peristome decagonal, ambulacral lobes twice the width of 

 the inter-ambulacral ; spines long, round, stout, and covered with longitudinal lines. 



Dimensions. — Height, nine twentieths of an inch ; transverse diameter, one inch and 

 three tenths. 



Description. — This Diadem was first discovered in the Coral Rag of Hildesheim, and 

 figured and described by M. Roemer in his work on the * Oolitic Fossils of Northern 

 Germany ;' it has subsequently been found in the Coralline Oolites of France. When 

 I found this species, some years ago, in the Coral Rag of Wiltshire, I was then unable to 

 identify it with any published species, and figured and described it the ' Annals of Natural 

 History ' under the name Diadema Davidsonii, M. Roemer's figure being very indistinct, 

 and without details, did not furnish sufficient data for identification. Since the publication 

 of that paper, M. Desor has compared my figures and descriptions with the moulds in the 

 Museum of Neuchatel, attributed to Diadema mamillanum, and he says, " n'ai pas trouve 

 de raison suffisante pour les distinguer.* Mr. Woodward f states that "the British forms 

 of this species agree perfectly with specimens received from Dr. Roemer, labelled U. Coral 

 Rag, Hildesheim, Hanover." There is, therefore, no doubt that Diadema Davidsonii is 

 identical with Roemer's species. 



This beautiful urchin has a regular cylindrical test, not at all inclined to the pentagonal 

 form of many of its congeners (fig. 2 a, b) ; the ambulacral areas are about three fourths 

 the width of the inter-ambulacral, and nearly of a uniform breadth throughout, taperino- 

 gracefully inwards towards their superior third (fig. 2 a) ; the contraction assumes the 

 form of a gentle curve, slightly inclined towards the centre. The double row of tubercles, 

 which are nearly as large as those of the inter-ambulacra, gradually increase in size from 

 the peristome to the equator, where three pairs are about the same size ; from this point 

 upwards they gradually decrease, and terminate in two pairs of minute rudimentary 

 tubercles near the apical disc. A single row of granules, arranged in a zigzag form, 

 separates the two rows of primary tubercles from each other, a larger granule marking 

 each re-entrant angle (fig. 2 d) ; the areolas abut against the poriferous zones (fig. 2 d), 

 without the intervention of any granules between these eminences and the pores. 



* Desor, ' Synopsis des Echinides Fossiles,' p. 64. 



-j- 'Memoirs of the Geological Survey,' Decade V. Notes on British Fossil Diadcmas. 



