ON THE COLOUR OF THE RIVER SPONGE. 259 



being only three wanting to equal the amount of existing 

 species), I think most surely, that every one will rather be 

 led to the opposite conclusion, that the number of species 

 was greater in that former period than in the present. With 

 respect to the number of genera, we have already proved 

 this position to a certainty, and we may, therefore, conclude 

 that this family presents the same condition as all those pre- 

 viously described, namely, that it was richer both in genera 

 and species, in former geological periods, than now. 



(To he continued.) 



Art. II, — On the Action of Light upon the Colour of the River 

 Sponge.' By John Hogg, M.A., F.R.S., L.S., C.P.S., &c.. Late 

 Fellow of St Peter's College, Cambridge. 



Having for several years past observed many variations in 

 the colour of different specimens of the river sponge [Spon- 

 gilla Jiuviatilis)^ (1*) although growing in, or inhabiting the 

 same rivulet, and during the same season of the year, I was 

 at first led to attribute the difference of age as the most pro- 

 bable cause of those variations. But a circumstance oc- 

 curred to me in the summer of 1837, which has indisputably 

 afforded the true cause of this variation in colour ; and, as 

 I consider it to be of great interest, I do not hesitate to pub- 

 lish a notice of that circumstance, and of the experiments 

 instituted by me, as w^ell towards its elucidation as towards 

 its full and direct confirmation. 



The circumstance alluded to is this : during August 1837, 

 whilst searching for some good specimens of the Spongilla 

 Jlaviatilis, which is not unfrequent in a rivulet of beautifully 

 clear water at Norton, in the county of Durham, for the pur- 

 pose of making observations on its supposed animal nature, 

 I dragged from the bottom of the stream a flat piece of tile, 

 having a patch of sponge growing upon its upper surface, of 

 a bright green colour, and also another patch of the same 

 species growing to its under surface, but which was of a pale 

 brown, or sand colour, and not in the least tinged with green. 



* Read before the Royal Society, June 21st. 1838, and communicated 

 to the ' Mag. Nat. Hist.' by the author. This paper is noticed at p. 457, 

 vol. 13, of the ' Philosophical Magazine,' also in the ' Bibliotheque Uni- 

 verselle' de Geneve, p. 207, tom. 19, where it is entitled " De Paction de 

 la lumiere sur la couleur de I'eponge de riviere." See also vol. 4, p. 72, 

 Proceedings of the Royal Society. 



2 The numerals refer to notes at the end of the paper. 

 Vol. IV.— No. 42. ns. 2 i 



