MITRASTER RUGATUS. 63 
2. Mitraster ruGaAtTUS, Forbes, sp. Pl. XVI, figs. 3a—5 d. 
GontasTeR (Gonropiscus) ruGaTUS, Forbes, 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 Formations of Sussex, London, 
4to, p. 330, pl. xxi, figs. 2, 2*; 
pl. xxiii, fig. 15. 
— — -- Morris, 1854. Catalogue of British Fossils, 
2nd ed., p. 81. 
ASTROGONIUM RUGATUM, Dujardin and Hupé, 1862. Hist. Nat. 
Zooph. Echin. (Suites 4 Buffon), 
p. 399. 
Goy1asteR (GoniIopiscus) ruGATUS, Forbes, 1878. In Dixon’s Geology of Sussex 
(new edition, Jones), p. 364, pl. xxiv, 
figs. 2, 2*; pl. 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-+7) 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 mm., and in a third, breadth 
5°5 mm. and length 2°75 to 2°8 mm. In the plate adjacent to the median inter- 
