484 Royal Society. 
was also read, “ On a New Method of computing the Perturbations 
of the Planets whose eccentricities and inclinations are not small.” 
Communicated by G. B. Airy, Esq., F.R.S. 
The author announces that he has found a method by which the 
absolute perturbations of planets for any given time, with any given 
eccentricity and inclination of the orbit, may be calculated; and he 
exemplifies his method by applying it to the computation of the 
perturbations produced by Saturn on the comet of Encke, in every 
point of its orbit; a problem of which hitherto there existed no so- 
lution*. 
3. A paper was also in part read, entitled “On the minute struc- 
ture of the Skeletons or hard parts of the Invertebrata.” By W. B. 
Carpenter, M.D. Communicated by the President. 
Jan. 19.—The following papers were read :— 
1. “ Variation de la Déclinaison et Intensité Horizontale ob- 
servées & Milan pendant vingt-quatre heures consécutives le 25 et 
26 Novembre, et le 21 et 22 Décembre 1842.” Par Prof. Carlini, 
For. Mem. R.S. 
2. The reading of a paper, entitled “On the minute structure of 
the Skeletons or hard parts of Invertebrata,’ by W. B. Carpenter, 
M.D., was resumed and concluded. 
The present memoir is the first of a series which the author 
intends to communicate to the Society, and relates only to the Mol- 
lusca ; and he proposes, hereafter, to extend his inquiries to the 
skeletons of the Echinodermata, and the various classes of articu- 
lated animals. After adverting to the classifications of shells pro- 
posed by Mr. Hatchett and Mr. Gray +, from the propriety of which 
he finds reason to dissent, he proceeds to state the results of his 
microscopic examination of the texture of shells under the several 
following heads. First, shells having a prismatic cellular structure, 
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 calcified 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 
* See pres. vol. p. 303.—Epir. 
+ See Phil. Mag., S. 3, vol. iii, p. 452,—Eprr. 
