Royal Society. 381 



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. 



Feb. 9. — " On the Structure and Mode of Action of the Iris :" by 

 C. It. Hall, Esq. Communicated by P. M. Roget, M.D., Sec. R.S. 

 After reciting the various discordant opinions entertained at dif- 

 ferent periods by anatomists and physiologists, relative to the struc- 

 ture and actions of the iris, the author proceeds to give an account 

 of his microscopical examination of the texture of this part of the 

 eye, in different animals. He considers the radiated plicae, which 

 are seen on the uvea in Mammalia, as not being muscular ; but he 

 agrees with Dr. Jacob in regarding them as being analogous in 

 structure to the ciliary processes. The white lines and elevations 

 apparent on the anterior surface of the human iris, he supposes to 

 be formed by the ciliary nerves which interlace with one another in 

 the form of a plexus. The iris, he states, is composed of two portions ; 

 the first, consisting of a highly vascular tissue, connected by vessels 

 with the choroid, ciliary processes, sclerotica and cornea, and abun- 

 dantly supplied with nerves, which, in the human iris, appear, in a 

 front view, as thread-like striae ; and which are invested, on both 

 surfaces, by the membrane of the aqueous humour. They are more 

 or less thickly covered with pigment, which, by its varying colour, 

 imparts to the iris on the anterior surface its characteristic hue ; and, 

 by its darkness on the posterior surface, renders an otherwise semi- 

 transparent structure perfectly opake. The second component por- 

 tion of the iris consists of a layer of concentric muscular fibres, which 

 fibres, in Man and Mammalia generally, are situated on the posterior 

 surface of the pupillary portion of the iris ; but which in Birds ex- 

 tend much nearer to the ciliary margin, and consequently form a 

 much broader layer. In Fishes and in some Reptiles they do not 

 exist at all. 



The author then proceeds to inquire into the bearings which 



