Miscellaneous . 425 



to consider tlie two "Woolwich forms as being variations of RoIj)hii ; the 

 })ecuhar form of the subcokimellar pUca, and other characters, not 

 admitting of the union of either with any other alhed species. The 

 specimen formerly in question must for the present be considered as 

 a large and unusual variety, or accidental deviation from the general 

 type of Cf. Rolphii. This deviation is particularly observable in the 

 form of the sj)ire, in the less-developed basal crest, and in the more 

 narrowly rimate and contracted periomphalus. There is also no trace 

 of the slight palatal callus, vanishing towards the base, which is 

 observable in the ordinary form found in other English localities and 

 on the contment. — W. H. Benson. 



On the Origin of Greensand, and its Formation in the Oceana of 

 the present Epoch. By Prof. J. W. Baii^ey. 



As an introduction to the subject of this paper, it is proper to 

 refer to various observations which have been made of facts inti- 

 mately related to those which I wish to present. That the calca- 

 reous shells of the Polythalamia are sometimes replaced by silica, 

 appears to have been first noticed by Ehrenberg, who, in a note 

 translated by Mr. Weaver, and published in the Philosophical 

 Magazine for 1841 (vol. xnii. p. 397), says: — 



" I may here remark that my continued researches on the Poly- 

 thalamia of the Chalk have convinced me that very frequently in the 

 earthy coating of flints, which is partly calcareous and partly sili- 

 ceous, the original calcareous-shelled animal forms have exchanged 

 their lime for silex without undergoing any alteration in figure, so 

 that while some are readily dissolved by an acid, others remain insolu- 

 ble; but in chalk itself, all similar forms are immediately dissolved." 



The first notice of casts of the cells and soft parts of the Poly- 

 thalamia was published by myself in the ' American Journal of 

 Science' for 1845, vol. xlviii., where I stated as follows: — 



"The specimens from Fort "Washington presented me with what 

 I believe have never been before noticed, viz. distinct casts of Poly- 

 thalamia. That these minute and perishable shells should, when 

 destroyed by chemical changes, ever leave behind them indestructible 

 memorials of their existence, was scarcely to be expected, yet these 

 casts of Polythalamia are abundant and easily to be recognized in 

 some of the Eocene marls from Fort "Washington." This notice 

 was accompanied bv figures of well-defined casts of Polvthalamia 

 (/. c. pi. 4. fig. 30, 31). 



Dr. Mantell also noticed the occurrence of casts of Polythalamia 

 and their soft prrts preserved in flint and chalk, and communicated 

 an account of them to the Royal Society of London, in May 184G. 

 In this paper he s{)eaks of the chambers of Polythalamia as being 

 frequently filled with chalk, flint, and silicate of iron (Phil. Trans. 

 184(j, p. 46()). To Ehrenberg, however, appears to be due the credit 

 of first distinctly announcing the connexion between the Polytha- 

 lamia and the formation of greensand, thus throwing the first light 

 upon the origin of a substance which has long been a \n\zzh to 



