FOSSIL INDUSIiE. 119 
many of the marly beds of this formation is due 
to the presence of countless myriads of similar 
exuviee of the Cypris which give rise to divisions 
in the marl as thin as paper. Taking this fact 
in conjunction with the habit of these animals to 
moult and change their skin annually, together 
with their shell, he justly observes that a more 
convincing proof of the tranquillity of the waters, 
and of the slow and gradual process by which 
the lake was filled up with fine mud cannot be 
desired. 
Another proof of the length of time that must 
have elapsed during the deposition of these 
tertiary freshwater formations in Auvergne, is 
afforded near Cleremont by the occurrence of 
beds of limestone several feet in thickness, 
almost wholly made up of the Indusiae, or 
Caddis-like coverings, resembling the cases that 
enclose the larvae of our common May-fly. 
Mr. Lyell states that a single individual of 
these Indusiae is often surrounded by no less 
than a hundred minute shells of a small spiral 
univalve, (Paludina), fixed to the outside of this 
tubular case of a larva of the genus Phryganea. 
See Ly ell's Principles of Geology, 3rd edit. vol. 
iv. p. 100. It is difficult to conceive how strata 
like these, extended over large tracts of country, 
and laid one above another, with beds of marl 
and clay between them, should have contained 
the coverings of such multitudes of aquatic 
animals, by any other process than that of 
