337 
mechanical principles essentially different from those embodied 
in previous arguments. He explains his 'planetesimal hypo- 
thesis," as he calls it, in the following terms: — "The pre- 
vailing form of tlie smaller nebulous areas of the heavens is 
a spiral, in most of which two arms are discernible. Such a 
form would be> developed from any nebulous body were ano- 
ther of sufficient mass to pass close to it, in the way that, 
say, comets sweep around the sun. It is further thought that 
the gaseous matter of the arms formed by such a disruptive 
approach would solidify into tiny planetesimals, which, in 
course of time, becoming concentrated by their mutual at- 
tractive forces, would produce relatively large masses of mat- 
ter (the planets), whirling in the same direction, around the 
parent body (the sun)." Mr. Mawson then shortly referred 
to some of the leading points of difference in the geological 
development of an earth, built up, after the 'planetesimal 
hypothesis," of aggregations of cold particles, and not origi- 
nally intensely heated, as required by the nebular hypothesis. 
Ordinary Meeting, August 1, 1905. 
The President (J. C. Verco, M.D., F.R.C.S.), in the 
chair. 
Exhibits.— A. H. C. Zietz, F.L.S., C.M.Z.S., a block 
of opal of a variety known as 'pineapple opal," from its 
shape. Unlike ordinary opal it is composed of large crystals. 
The specimen was found at the White Cliffs opal fields. The 
mineralogist of the Australian Museum, in Sydney, de- 
clared this variety to be a pseudomorph after Glauberite. 
He also exnibited another mineral of very similar structure, 
which has not yet been analysed. This was found at the 
Blinman Mine, embedded in clay. Mr. Zietz also exhibited two 
British slugs, found in an Adelaide garden, Li max arhorum, 
which in England lives in birch trees, and Limacf gagatus. 
The President, several molluscs, illustrating the change of 
form in the same species as it passes through the four stages 
of growth — embryonic, adolescent, mature, and the senile. 
Lafirus aurantiacus, gradually developing in weight, in the 
rudeness of the nodules, and in the size and cave-like appear- 
ance of the perforation. Voluta adcocki (Tate), in whicn, 
at a certain stage of growth, a change took place 
in the colour pattern. A Brachifes, from Port Lincoln, in 
which the minute valves of the embryo gradually open out and 
become connected by shelly matter, in the form of a large, 
tapering tube, resembling coral. Dr. Verco stated that a 
protoconch, found alone, had been described as a separate 
genus ( Siuusigera), but when older specimens were collected 
it was shown to be a Purpura; also that in some species the 
