372 Linneean Society. 



animal's growth ; and therefore, the whole number of whorls and 

 parts of whorls will, at any period, be proportional to its age. 



The form of the molluscous animal remaining always similar to 

 itself, the surface of the organ by which it deposits its shell will 

 vary as the square of the linear dimensions ; but as the deposition 

 of its shell must vary as the cube of the same dimensions, there must 

 be an increased functional activity of the organ, varying as the sim- 

 ple linear dimensions. 



Since to each species of shell there must correspond a particular 

 number expressing the ratio of the geometrical progression of the 

 similar successive linear dimensions of the whorls ; and since the 

 constant angle of the particular logarithmic spiral, which is aflfected 

 by that species of shell, is deducible from this number, the author 

 considers that, connected as the form of the shell is with the cir- 

 cumstances of the animal's growth and the manner of its existence, 

 this number, or the angle of the particular spiral, determinable as 

 it is in each case by actual measurement, may be available for the 

 purposes of classification, and may suggest relations by which, 

 eventually, they may become linked with characteristic forms, and 

 modes of molluscous existence. 



The concluding portion of the paper contains a mathematical dis- 

 cussion of certain geometrical and mechanical elements of a con- 

 choidal surface. These are, the extent of the surface itself; the vo- 

 lume contained by it ; the centre of gravity of the surface, and also 

 of the volume, in each case, when the generating figure revolves 

 about a fixed axis without any other motion, and also when it has, 

 besides this, a motion of translation in the direction of that axis ; 

 and, lastly, the angle of the spiral. The author states that his ob- 

 ject in this inquiry is the application of these elements to a discussion 

 of the hydraulic theory of shells. The constant angle of the spiral, 

 which each particular species aflfects, being connected by a necessary 

 relation with the cEConomy of the material of the habitation of each, 

 with its stability, and the condition of its buoyancy, it is therefore 

 necessary to determine the value of this angle. 



LINN^AN SOCIETY. 



Nov. 6, 1838.— Mr. Forster, V.P., in the Chair. 



Read a letter from Mr. Jonathan Couch, F.L.S., giving an account 



of a single specimen of Wilson's Petrel (Procellaria Wilsoni) having 



been found dead in a field near Polperro in Cornwall, about the 



middle of August last, at a time when the stormy petrel (P. pe- 



