4 



BRITISH FOSSILS. 



Allied British Species. 



In the collection of Mr. Tennant there is a very beautiful, though imperfect, specimen 

 of a Discoidea, nearly allied to, but quite distinct from cylindrica ; it was found in the 

 upper green sand of Somerset. It measures a little more than an inch in diameter, by 

 nearly eight-twelfths of an inch in height ; is hemispheric, with a tendency to become 

 subcylindrical, and has plates horizontally proportioned and ornamented as in the cylin- 

 drical but differing materially in their relative number ; there being six (instead of four) 

 ambulacrals, and, consequently, as many pairs of pores, to each interambulacral. The 

 anus, moreover, is much nearer the margin, and larger than in the allied species. 



The Discoidea favrina of Desor (Monog. Galerites, p. (52, pi. 7, f. 12-14, called rotula 

 on the plate), from green sand near Rouen, seems to be its near ally, if not identical ; 

 though it also resembles considerably the same author's D. conica (t. 7, f. 17-22). Unfor- 

 tunately neither figures nor descriptions indicate sufficiently the distinctive character of 

 the relative numbers of pairs of pores and interambulacral plates. The original Niicleolites 

 or Galerites rotula of Alexander Brongniart {see Cuvier, Ossemens fossiles, vol. ii. p. 614, 

 plate IX., fig. 13), appears, judging from the figure, to be only a rather large specimen of 

 Galerites (^Discoidea) suhuculus, and is quoted as such by M. Desor, in his synonymy 

 of that species. As such it is quite distinct from the Discoidea rotula of Agassiz, in the 

 *' Echiuodermes fossiles de la Suisse" (1st part, p. 90, t. 6, f. 10-12) ; but, nevertheless, 

 the same figure of Brongniart is again cited by M. Desor, as a representation of Discoidea 

 rotula, at page 61 of his " Monograph of Galerites." Dr. E. Sismonda, in his " Memoir on 

 the Fossil Echini of Nice" (1843), enumerates Discoidea rotula, from the green sand near 

 Nice, and cites Alexander Brongniart's figure, as well as those given by Agassiz in his 

 *' Descriptions of the Fossil Echini of Switzerland." Unfortunately no figure is given of 

 the Italian species, nor are the relations of the ambulacral, as compared with the inter- 

 ambulacral plates mentioned in his description ; but for the comparison he makes of it to 

 the Discoidea cylindrica, it is not improbable that our fossil may prove identical specifically 

 with that from Nice. In Mr. Morris's " Catalogue of British Fossils," Discoidea rotula 

 is enumerated as a British fossil from the lower chalk of Maidstone and Dover, with a 

 reference to the figures of Brongniart and Agassiz, cited above. This mention of D. 

 rotula has escaped Mr. M'^Coy, who again enumerates a fossil under that name, referring 

 to Agassiz alone as an authority, in his list of Mesozoic Radiata not included in Morris's 

 catalogue, and preserved in the geological collection of the University of Cambridge 

 (Annals of Nat. Hist., 2nd ser., vol. ii., p. 420) : the specimen he cites is from the upper 

 chalk of Norwich. 



I think it not improbable that in the end we shall have to adopt the specific appellation 

 Favrina for the green sand species ; and that the chalk specimens alluded to will prove 

 varieties of cylindrica ; but a comparison of the types themselves only can settle the 

 matter. 



E. Forbes. 



April, 1849. 



