KEPORTS OF MEETINGS. 
167 
with the consideration of sponges, the skeletons of some of 
which are directly related to the subject he was about to discuss. 
In certain varieties of sponge we find the framework arranged 
in the form of lattice-work, filled up with cubical meshes. Of 
these we meet with several varieties, such as the Lithistid, the 
skeleton of which is made up of spicules composed of four rays, 
terminating in botryoidal expansions, which interlace with one 
another. In addition to them, there are Hexactinillid and 
Eenierid sponges, the latter of which have skeletons composed 
of sheaths containing numerous fibres resembling fine needles. 
All these consist mainly of silica. Among fossil sponges are 
some, which we cannot find among those at present existing, and 
vice versa. In the former we frequently find silica replaced by 
carbonate of lime, an instance of which was produced by 
Professor Sollas as occurring in a specimen sent to him from the 
green sand at Folkestone. The formation of flint is intimately 
connected with the existence of sponge, arising as it does from 
the deposition of sponge spicules, which we find in all flint. In 
order to account for such transformation, we must admit that, in 
some stage of the process, the silica becomes more or less 
soluble ; and to explain this solubility, some writers have framed 
the hypothesis that it is due to humic acid, which certainly 
might produce the effect ; hut this acid has not as yet been 
proved to exist at the bottom of the sea. That ordinary flint is 
to a certain extent soluble, is shown in the case of flint walls, 
where we find the freshly-exposed black surface of the flint 
gradually disappearing, and leaving behind a white, chalky 
deposit. Professor Sollas was inclined to attribute the solubility 
to the great pressure existing at considerable depths, aided by 
the peculiar action of spiculin, an active principle contained in 
the spicules. But the subject is still involved in doubt. 
The last meeting of the year took place on the evening of 
December 2nf, when Mr. J. W. White gave “ Remarks on the 
