which he had made several observations. He described the mode of using 
the combined instruments, as well as other ways of applying the two 
together, and promised to show some of the effects produced by it at the 
next meeting of the section. 
GEOLOGICAL SECTION. 
February 22.— Mr. W. Sanders, F.R.S., President of the section, 
in the chair. 
Mr. W. W. Stoddart, F.G.S., made a communication upon Ammon- 
ites planorbis and its varieties. This was one of the most important, 
though rare, Lias fossils of the district, and three distinct species of it had 
been described by Sowerby and D'Orbigny, under the names of Ammon- 
ites Johnstonii, Sow., A. torus, and A. tortilis, D'Orb. Dr. Wright, of 
Cheltenham, who had been much occupied in studying the Ammonites of 
the Lias, was convinced that these three were simply varieties of one and 
the same species, A. planorbis. Mr. Stoddart exhibited typical specimens 
of each, and with the aid of drawings, explained what was meant by the 
keel, and the suture, stating that the different varieties of the same 
species were generally the same in the configuration of the sutures. The 
Lias beds were divided into zones, each characterised by its own Ammonite 
(which was only found with those limits), as the Planorbis zone, the 
Bucklandi zone, &c, and A. Planorbis belonging to the group Arietis, 
which was subdivided into those with no keel, as A. sauzeanus, A. planor- 
bis, &c, and those with a keel, as A. Bucklandi, A. Turneri, &c. 
In the conversation which followed, Mr. Jordan, the President, and 
others, urged the importance of searching for and studying the interme- 
diate links between two strongly marked forms, in order to ascertain what 
were really species, and what only varieties, British Mollusca generally, 
Foramenifera, and two fossils, Navicula longicostata and Terebratula orni- 
thocephala, were mentioned as examples of this. 
Mr. W. L. Carpenter exhibited a piece of silica, deposited from a 
boiling solution of silicate of soda, which was being decomposed by a 
mineral acid. Under a pressure of 2 feet of water it had acquired, in 3 
hours' time, in great part, the hardness, semi-transparency, conchoidal 
fracture, and other characteristics of flint, with which it was chemically 
identical, except in respect of the quantity of water combined with it. 
The President exhibited some tables illustrating various classifica- 
tions of fish, which he had prepared with the intention of uringing the 
subject of the fossil fish of the neighbourhood before the section at some 
future time. Various characters had been taken as bases for classification, 
some being adapted best for fossil fishes, others for recent. Mr. Sanders 
had endeavoured to combine both, imparting into Prof. Owen's classifica- 
tion of fossil fish, some of the divisions usually adopted in works of Natural 
History for recent fish, in order to obtain one adapted to his purpose. 
