43 
observed that the members present had been taken almost a complete 
round through the sciences. He then proceeded to make some remarks 
upon the mathematical laws according to which many of these shells were 
formed. The revolution of a certain curve round its axis, sliding at the 
same time in the direction of its axis, describing a logarithmic or equi- 
angular spiral, would produce a form closely resembling these shells, the 
whorls of which were separated from each other by a geometric series of 
distances. This was a very uniform and simple way of working. In the 
case of the Nautilus, the form was produced by the revolution of an ellipse 
round its minor axis, the length of the major axis being simultaneously in- 
creased. The spiral on the operculum was probably thus formed : when 
the operculum was too small to close the mouth of the shell completely, 
it was twisted round to fit exactly against one side, while the other side 
was filled up with new material ; it did not grow equally on all sides at 
once. 
The time for adjournment having long passed, the meeting separated. 
MEETINGS OF SECTIONS. 
GEOLOGICAL SECTION. 
Thursday, March 28th, 186/.— Mr. Thos. Pease, F.G.S., Vice- 
President of the Society, in the chair. 
Major Austin read a short communication on the occurrence of Otop- 
teris in the Lower Lias. 
According to Lindley and Hutton, who established the genus, Otopteris was 
probably a simple pinnated plant, with a thickish petiole, to which leaflets were 
attached by the lower half of the base, the upper half being free and auricled j 
the leaflets were devoid of midrib, and furnished with veins which originated in the 
base, and curved right and left to the margins, only forking as they proceeded 
towards the extremity. All these characters are plainly observable in the specimen 
on the table, and are also represented in the drawing. 
So rare are these fossil plants, that I am only aware of two other specimens 
of Otopteris ever having been found in the lias, namely, one at Membury, near 
Axminster, and the other in the same formation at Polden Hill, near Bridgewater, 
in Somersetshire, Both these specimens were the same species, Otopteris obtusa. 
