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Whitney that in some cases such so-called “volcanic ash” 
never came from the volcano at all but was the result of the 
drying up of lakes, in which the Diatomacee had grown and 
multiplied, by the injection of lava or the draining off of the 
water by the disturbance of the earth. In other cases where 
the Diatomaceous remains did come from the volcano, that 
they got there in the following manner: when the volcano 
became inactive the crater had gradually become filled with 
water until a lake was formed, in which appeared, throve and 
rapidly multiplied the Diatomacez and similar organisms. 
Now if the mountain should break forth again, of course such 
a lake would be destroyed and its contents be ejected and 
spread over the surrounding country. In fact, as in every 
case, with one uncertain exception, that of Mount Erebus, 
the ejected Diatomacez are invariably such forms as live in 
fresh water only and of a character which have not, as yet, 
been found in strata below the surface of the earth, so that it 
was extremely doubtful if we ever have subterranean Diato- 
maceze ejected from volcanos. 
May 29th, 1871. 
The President in the chair. Sixteen persons present. 
The following paper was read by title, to be published in 
the Annals of the Lyceum. 
On the Jaws and Lingual Dentition of Helix Turbiniformis and 
other Terrestrial Molluscs. 
By W. G. Binney & T. BLAND. 
In this paper the authors describe and figure the jaw of 
Helix turbiniformis, of Jamaica, showing a form hitherto un- 
observed in the genus. It approaches very closely that of 
Cylindrella rosea, of which a photograph was given in the 
