MITRASTER RUGATUS. 6» 



2. MiTRASTER RUGATUS, Forbes, sp. PI. XVI, figs. 3 a — 5 d. 



GoNiASTEB (GoNiODiscus) RUGATUS, Forhes, 1848. Memoirs of the Geological 



Survey of Great Britain, vol. ii, 

 p. 471. 



— — — Forbes, 1850. In Dixon's Geology and 



Fossils of the Tertiary and Creta- 

 ceous Formatious of Sussex, London, 

 4to, p. 330, pi. xxi, figs. 2, 2*; 

 pi. zxiii, fig. 15. 



— — — Morris, 1854. Catalogue of British Fossils, 



2nd ed., p. 81. 

 AsTEOGONiUM EUOATUM, Dujardtn and Hupe, 1862. Hist. Nat. 



Zooph. Echin. (Suites a Buffon), 



p. 399. 

 GoNiASTEE (GoNiODiscus) EUOATUS, .FbrJcs, 1878. In Dixon's Geology of Sussex 



(new edition, Jones), p. 364, pi. sxiv, 



figs. 2, 2*; pi. xxvi, fig. 15. 



Body of small size. General form depressed. Abactinal surface flat. Actinal 

 surface also flat. Marginal contour pentagonal with almost straight sides. The 

 major radius is proportional to the minor radius as 100 : 75. The actual 

 measurements in an example from the Upper Chalk of Gravesend, preserved in 

 the British Museum, being, major radius, 17*5 mm.,; minor radius, 13'75 mm. 

 The rays are not produced beyond the contour of a true pentagon, and the radial 

 angles are not rounded. Margin moderately thick, the lateral wall being almost 

 vertical in consequence of the rapid bend from the abactinal and actinal surfaces ; 

 the bend to the actinal surface is, however, rather more gradual. 



The supero-marginal plates are three in number, counting from the median 

 interradial line to the extremity, or six from the tip of one ray to the tip of the 

 adjacent ray, exclusive of the odd terminal or " ocular " plate in each case. They 

 form a broad border to the abactinal area of the disk, of uniform breadth through- 

 out, which measures 6 mm. at the median interradial line in an example whose 

 diameter (R -|- r) measures about 31 mm. Excepting the ultimate plates, the 

 supero-marginal plates are approximately equal in size, the breadth being nearly 

 twice the length, the actual measurements in the above-mentioned specimen of 

 31 mm. in diameter being, length 3*5 to 3"6 mm., and breadth 6 mm. in the plate 

 adjacent to the median interradial line. The corresponding plate in another 

 example has a breadth of 5 mm. and length 2'75 ram., and in a third, breadth 

 5*5 mm. and length 2'75 to 2-8 mm. In the plate adjacent to the median inter- 



