436 



as the Pinna, and which are composed of a multitude of flattened 

 hexagonal calcareous prisms, originally deposited in continuous 

 layers of hexagonal cells, and thus constituting a calcined epithe- 

 lium, analogous with the enamel of the teeth. Secondly, those con- 

 sisting of membranous shell-substance, the basis of which, after the 

 removal of its calcareous portion, presents nothing but a membra- 

 nous film, of greater or less consistence, composed of several layers, 

 but without the appearance of any cellular tissue : this membrane 

 the author regards as being derived from the mantle, of which it 

 was originally a constituent part, by the development of nucleolated 

 cells ; and the various corrugations and foldings of which it is sus- 

 ceptible in different species, introducing many diversities into the 

 structure of the shells of this class. Thirdly, shells having a nacreous 

 structure, and exhibiting the phenomena of iridescence ; a property 

 which the author ascribes to the plicated form of the membrane of 

 the shell, combined with a secondary series of transverse corruga- 

 tions. Fourthly, shells exhibiting a tubular structure, formed by 

 cylindrical perforations occurring among the several layers, and 

 varying in diameter from about the 20,000th to the 3500th part of 

 an inch ; but measuring on an average about the 6000th part of an 

 inch, and presenting a striking analogy with the dentine or ivory 

 of the teeth. The last sections of the paper relate to the epidermis 

 and the colouring matter of shells. 



References are made, in many parts of the paper, to illustrative 

 drawings ; which, however, the author has not yet supplied, 



January 26, 1843. 



Sir JOHN WILLIAM LUBBOCK, Bart., V.R and Treasurer, 

 in the Chair. 



The following papers were read, viz. : — 



1. "Observations on certain cases of Elliptic Polarization of 

 Light by Reflection," by the Rev. Baden Powell, M.A., F.R.S., 

 Savilian Professor of Geometry in the University of Oxford. 



The author, by way of introduction, passes in review the la- 

 bours of various inquirers on the subject of the elliptic polarization 

 of light, and notices more particularly those of Sir David Brewster, 

 who first discovered this curious property, as recorded in the Phi- 

 losophical Transactions for 1830; of Mr. Airy, in the Cambridge 

 Transactions for 1831 and 1832; and of Professor Lloyd, in the 

 Philosophical Transactions for 1840, and in the Reports of the 

 British Association for 1841. He then proceeds to give an account 

 of his own experimental examination of the phenomena of elliptic 

 polarization in the reflection of light from various surfaces, by ob- 

 serving the modifications of the polarized rings under different con- 

 ditions, both of surface and of incidence, and by endeavouring to 

 ascertain both the existence and amount of ellipticity, as shown by 



