MITRASTER COMPACTUS. 67 



of length to breadth are in the case of the interradial supero-marginal plates as 

 5" 6 mm. is to 37 mm. Further, the tuberculation in this specimen may or may 

 not extend over the whole of the abactinal area, the variations being on adjacent 

 plates, and the abactinal gibbosity is not strongly developed. In the example 

 figured on PL IX, fig. 3, the proportions of length to breadth in the case of the 

 interradial supero-marginal plates are as 4'5 mm. is to 3*6 mm. ; the granulated 

 areas more generally stop short of the distal edge of the supero-marginal plate and 

 the abactinal gibbosity is well pronounced. In view of these considerable variations 

 it is difficult to refer these forms to more than one species. 



3. Miteastee compactus, Forbes, sp. PI. XVII, fig. 2; PI. XXVI, figs. 3, 3 a, '3 b, 3 c. 



Goniaster compactus, Forbes, 1848. Mem. Geol. Surv. Gt. Brit., vol. ii, p. 468. 

 — Forbes, 1850. In Dixon's Geology and Fossils of the Ter- 



tiary and Cretaceous Formations of Sussex, London, 

 p. 333, pi. xxii, fig. 3. 



Astrogonium compactum, Dujardin et Hupe, 1862. Hist. Nat. Zooph. Echin. 

 (Suites a. Buff on), p. 399. 



Goniaster compactus, Forbes, 1878. In Dixon's Geology of Sussex (new edition, 

 Jones), p. 366, pi. xxii, fig. 3. 



Specific Character*. — Outline pentagonal, slightly cycloidal. Eight supero- 

 marginal plates on each side of the pentagon. Supero-marginal plates form a broad 

 margin, and the breadth of each is about four times its length. Base of ultimate 

 paired supero-marginal plate twice as long as the other, more proximal, supero- 

 marginal plates. Ten corresponding infero-marginalia. 



Material. — Only one specimen of this species is known. This formed a portion 

 of Mr. Willett's collection and is now preserved in the Brighton Museum. It 

 apparently escaped the observation of the late Dr. Wright, for the figure on Plate 

 XVII is copied from that in Dixon's { Greology of Sussex.' As this figure is slightly 

 inaccurate I have had it redrawn and further details added on Plate XXVI. 



Description. — The dorsal surface of the disc is covered with a number of small, 

 subequal, closely-fitting plates. It is considerably sunk in the specimen known. 



The supero-marginalia bounding the disc form a uniform margin 5*15 mm. 

 broad. They are eight in number along each side, exclusive of the odd terminal 

 or ocular plates. The six middle plates are about 1'2 mm. long. Their breadth 

 is rather more than four times their length, a feature which distinguishes them 



11 



