Page:Proceedings of the Royal Society of London Vol 4.djvu/462

Rh 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,

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