FROM THE PORTLAND OOLITE. 97 



with one collected many years ago from the Portland Sandstone of the Boulonnais by 

 MM. Bouchard, Chantereanx, and Davidson, and beautifully drawn by the latter gentle- 

 man in the first plate of his * Memoir on the Fossils of the Boulonnais,' the publication of 

 which has been delayed until the completion of his great work on British Fossil Brachio- 

 poda for the Palgeontographical Society. Mr. Davidson has kindly sent me his beautiful 

 drawings, to make of them whatever use I may think fit ; and M. Bouchard has most 

 generously forwarded his best specimens of this species to enable me to complete its 

 description. The organic characters of Ilemicidaris Davidsoni are intermediate, like its 

 stratigraphic position, between Hemicidaris intermedia and Hemicidaris Purbeclcensis. 

 The shell has a sub-globose form, slightly flattened at the poles ; it is rather more tumid 

 towards the base than above ; the ambulacral areas are narrow, and sHghtly nndulated ; 

 the base is occupied by large semi-tubercles (PI. IV, fig. 2 b), which are not disposed 

 regularly in pairs, as in Hemicidaris intermedia (PI. IV, fig. 1 c, d) ; but in consequence 

 of the narrowness of the areas in this region, the four upper semi-tubercles extend one 

 above another, forming an irregular continuous line, ascending nearly half-way 

 up the area (fig. 2 b) ; the upper half of which has two rows of minute perforated 

 tubercles, raised on small eminences (fig. 2 c) ; these tubercles, about twelve in each row, 

 alternate with each other, and are separated by a zigzag line of miliary microscopic granules. 

 The poriferous zones form a series of crescentic waves round the ambulacral sides of the 

 tubercular plates (fig. 2 c) ; the pores are in pairs, with a slightly elevated portion of the 

 septum between them, and there are nine pairs of pores opposite each of the large inter- 

 ambulacral plates ; only at the base of the areas, and close to the peristome, do the pores 

 fall into double pairs ; and of these there are only two or three rows. 



The inter-ambulacral areas are more than three and a half times the width of the 

 ambulacral ; there are eight primary tubercles on each of the two rows that fill this 

 division of the test ; from the equator, where they are largest, to both poles, they gradually 

 diminish in size ; the tubercles are small, and not deeply perforated ; the bosses are large 

 and prominent (fig, 2 c), with coarsely crenulated summits ; they are surrounded by wide, 

 smooth areolas ; each tubercular plate (fig. 2 c) has on its zonal border, between the areola 

 and the pores, a series of six or seven minute tubercles, raised on small eminences ; and 

 on its centro-sutural border a series of six or seven like minute tubercles, which form 

 two lateral crescents around the base of the areola (fig. 2 d) ; the four uppermost plates 

 have a horizontal line of smaller granules extended across their apical border (fig. 2 a, b) 

 so that the primary tubercles of these upper plates have their areolas encircled by an 

 uninterrupted series of granules ; but in all the lower plates they are more or less confluent 

 above and below (fig. 2 c). 



The apical disc is large (fig. 2/) ; the antero-lateral genital plates are more than twice the 

 size of the others ; the right plate is the largest, and carries a prominent convex madrepori- 

 form body ; the postero-lateral and single plates are much reduced in size, in consequence 

 of the width of the anal opening (fig. 2 af) ; the oviductal holes are marginal, the ocular 



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