MITRASTEK CUMPACTUS. 07 



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

 5"6 mm. is to o"7 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 PI. IX, fig. o, the proportions of length to brcatlth 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 tlie 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. 



:5. MiTKASTKK coMPACTUS, Fovhes, sp. PL XVII, fig. 2; PI. XXVI, figs. 3, 3 a, 3 //, 3 c 



GoNiASTER COMPACTUS, Forbes, 1848. Mem. Geol. Surv. Gt. Brit., vol. ii, p. 468. 

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



tiary and Cretaceous Formations of Sussex, London, 

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



AsTROGONiUM cOMPACTUM, Dujavdiii e( Htipr. 1862. Hist. Nat. Zooph. fiehin. 



(Suites ii Buffon),p. 399. 



GoNiASTER coMPACTiTS, Forbes, 1878. In Dixon's Geology of Sussex (new edition, 



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



Specific Charnctcrx. — Outline pentagonal, slightly cvcloidal. Eight supero- 

 marginal plates on each side of the pentagon. Supei'o-marginal plates form a broad 

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

 paired supero-marginal plate tAvice as long as the othei', more proximal, supero- 

 marginal plates. Ten corresponding infero-marginalia. 



Matenal. — Only one specimen of this species is known. 'IMiis 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 ' Geology of Sussex.' As this figure is slightly 

 inaccui'ate I have had it redrawn and fui-ther details added on Plate XXVT. 



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

 subequal, closely-fitting plates. It is consideral)ly sunk in the specimen known. 



The supero-marginal ia bounding the disc form a uniform margin 5'15 mm. 

 broad. They are eiglit in niinil)er along each side, exclusive of the odd terminal 

 or ocular plates. The six middle plates are about P2 mm. long. Their breadth 

 is rather more than four times their length, a feature which distinguislios tlii-ni 



11 



