PENTACEROS OCELLATUS. 



5. Pentaceros ocellatus, Forbes, sp. PI. XXV, figs. 4,4 a. 



Oreaster ocellatus, Forbes, 1848. Mem. Geol. Surv. Great Brit., vol. ii, 

 p. 468. 



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



Tertiary and Cretaceous Formations of Sussex, 

 p. 329, pi. xxi, fig. 13. 

 Dujardin et Hup,', 1862. Hist, Nat. Zoopli. Echin. 

 (Suites a Buff on), p. 389. 



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



Jones), pp. 364, 370, pi. xxi, fig. 3. 

 Pentaceros — McPherson, IF., 1902. Rep. Brighton Nat. Hist. Soc. 



Specific Characters. — Ventrolateral plates (as probably also the dorsal plates) 

 depressed and finely striated on their truncated surface so as to simulate the 

 surface of a madreporite, with sides rugged and ocellato-punctate. Between these 

 plates smaller ossicles of a similar character are interspersed. 



Material. — But one specimen of this species was known to Forbes. This is 

 preserved in the British Museum of Natural History (Dixon Coll., E. 2571). It is 

 a mass of ossicles which look as if they were derived from the dorsal surface of the 

 disc. They are more spheroidal and somewhat larger than the ossicles of the 

 ventral surface of the more nearly perfect example discovered by Mr. William 

 McPherson in the Senonian Marsupites band at Brighton. This he presented 

 to the British Museum (Natural History) in 1901 (E. 5012). 



Description. — The disc and arms are unknown. The specimen no. E. 5012 shows 

 a well-preserved portion of the ventral surface. The mouth-angles were occupied 

 by single initial rhomboidal ossicles. To these succeed the ventro-lateral ossicles 

 which border the ambulacral groove. These are pentagonal ossicles of very 

 uniform size. The length of the exposed sides of the ossicles bordering the 

 groove is 4 - -l mm. and the greatest breadth of an ossicle 4 - 2 mm. The remaining 

 ventro-lateral plates are hexagonal, but of almost the same dimensions, although 

 the plates appear to become a little larger distally. 



The plates overlap one another considerably, rendering precise measurement 

 difficult. Between the larger plates are interspersed large numbers of smaller 

 and more irregular ossicles which fill up the angles between their sides. The 

 whole test would be thus very strongly built. 



Both larger and smaller plates are curiously similar in appearance. The 



