370 Royal Society. 



PROCEEDINGS OF LEARNED SOCIETIES. 

 royal society. June 21, 1838. 



" On the action of light upon the colour of the River Sponge." 

 By John Hogg, M.A., F.L.S., C.P.S., &c, Fellow of St. Peter's Col- 

 lege, Cambridge. Communicated by Thomas Bell, Esq., F.R.S. 



The author found that the green colour of the Spongilla fluviatilis, 

 or river sponge, is acquired solely through the agency of light, and 

 is lost when the sponge is removed from its influence. As this does 

 not appear to be the case with Actinice, the Hydra viridis, or any 

 other Polype, the author is disposed to consider this production as 

 being nearer allied to the Algae or Fungi, than to any tribe belonging 

 to the animal kingdom*. % 



" On the Geometrical Forms of Turbinated and Discoid Shells." 

 By the Rev. H. Moseley, Professor of Natural Philosophy and As- 

 tronomy in King's College, London. Communicated by Thomas Bell, 

 Esq., F.R.S. 



This paper is occupied by an investigation of certain mathemati- 

 cal principles which the author considers as governing the formation 

 of turbinated and discoid shells. According to these views, all such 

 shells may be conceived to be generated by the revolution about a 

 fixed axis of the perimeter of a geometrical figure, which, remaining 

 always similar to itself, increases continually its dimensions. The 

 spiral lines which are observable on the opercula of certain classes 

 of shells, taken in connexion with the well-known properties of the 

 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 in- 

 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 



* [Mr. Gray arrived at the same conclusion, but on physiological grounds, 

 some years since. See Zoological Journal, vol. i. p. 50.— Edit.] 



