Page:Proceedings of the Royal Society of London Vol 4.djvu/102

Rh logarithmic or equiangular spiral, appear to have suggested the idea, that not only the boundary of the operculum, which measures the sectional expansion of a shell, but also the spiral lines, which in general are well marked both externally and internally in the shell itself, are curves of this nature.

From an examination of the spirals marked on opercula, it appears that the increase of their substance takes place on one margin only; the other margin still retaining the spiral form, and acquiring an iu- crease of length by successive additions in the direction of the curve. As in the logarithmic spiral the distances of successive spires, mea- sured on the same radius vector produced from the pole, from each other, are respectively in geometrical progression, if similar distances between the successive whorls on the opercula of shells be found to observe the same law, it will follow that these whorls must have a similar form; and that such is the case, the author shows by a variety of numerical results obtained by careful measurements on three different opercula of shells of the order Turbo. That such is the law of nature in the formation of this class of shells is rendered probable by the instances adduced by the author, in which a con- formity to this law is found to exist.

From the known properties of the logarithmic spiral the author concludes that the law of the geometrical description of turbinated shells is, that they are generated by the revolution about a fixed axis, (namely, the axis of the shell,) of a curve, which continually varies its dimensions according to the law, that each linear incre- ment shall vary as the existing dimensions of the line of which it is the increment. If such be the law of nature, the whorls of the shell, as well as the spires on the operculum, must have the form of the logarithmic spiral ; and that this is likewise the case is sho"UTi by the almost perfect accordance of numerical results, deduced from the property of that curve, with those deduced from a great variety of careful measurements made of the distances between successive whorls on radii vectores drawn on shells of the Turbo diiplicatiis. Turbo 'phasianus, Buccinum suhulatum, and in a fine section of a Nautilus pompilius. The author further states that, besides the results given in the paper,' a great number of measurements were similarly made upon other shells of the genera Trochus, Stromhus, and MureXy all confirmatory of the law in question.

One of the interesting deductions which the author has derived ^ from the prevalence of this law in the generation of the shells of a large class of mollusca, is that a distinction maybe expected to arise with regard to the growth of land and of aquatic shells, the latter serving both as a habitation and as a float to the animal w^hich forms it; and that, although the facility of varying its position at every period of its growth may remain the same, it is necessary that the enlargement of the capacity of the float should bear a constant ratio to the corresponding increment' of its body ; a ratio which always assigns a greater amount to the increment of the capacity of the shell than to the corresponding increment of the bullv of the animal.

Another conclusion deducible from the law of formation here con-