264 PLASTIC CLAY FORMATION. 



I believe it is this bivalve that Mr. Webster has designated by the 

 name of Cytherea Scutellaria. It very nearly resembles the " cytheree 

 bombee" of Lamarck, "des marnes superieures du gypse*." 



9. A species of cyrene 9 



This shell was also found in the blue clay (ISTo. 9), and is remarkable' 

 from its being the only decided instance of a fresh-water shell that has 

 been discovered in that deposit f. 



10. Ostrea. 



The bed, No. 10, consists principally of a broad, flat species of oyster, 

 presenting nothing peculiar in its form or appearance. Another species 

 is sometimes found, in which the upper valve is nearly flat, and the lower 

 one very convex; this shell is precisely similar to a specimen in Mr. 

 Parkinson's cabinet, marked ostrea vesicularisp 



11. Tooth of the Squalus niustelus. 



This specimen is from the blue clay (No. 8), and perfectly resembles 

 the teeth of the recent fish. 



Of the strata above enumerated, Nos. 8, 9, 10, are considered by 

 Professor Buckland, as analogous to the plastic clay beds of Woolwich, 

 (Nos. 7, 8. PI. xiii. Geolog. Trans, vol. iv.) which contain also the same 

 species of cerithium and cyclas. 



No. 4 of Castle Hill, is the ash coloured sand of Woolwich, in dimi- 

 nished thickness. The breccia, No. 3, corresponds with the Reading oyster 

 bed, which " though inconsiderable in thickness, seems constantly to occur 



* GeograpMe Miner alogi que, p. 276, PI. ii. fig. 7. 



f The fossil in question was submitted to the examination of Mr. G. B. Sowerby, who 

 favoured me with the following observations. 



" The specimen you have sent me has evei-y character of a cyrene, and of being a fresh- 

 •water bivalve. I would, however, suggest the necessity of extreme caution in noting the 

 stratum from which it was obtained. If from the plastic clay, it is the first that I have seen 

 decidedly from that formation, of a Jresh water origin ; all the fossils from the London and 

 plastic clay being marine. This shell, too, bears considerable resemblance to some from 

 Woolwich ; as well as to a species that occurs abundantly in the stratum of melange between 

 the two fresh water beds at Headen hill, in the Isle ef Wight." 



J In a recent communication from M. Brongniart, that distinguished geologist questions 

 the propriety of this appropriation. He remarks, that in the environs of Paris, the ostrea 

 vesicularis of Lamarck is confined to the upper, or flinty chalk. It is therefore probable, that 

 the English fossil belongs to a diiFerent species. 



