FOSSIL INDUSLE. 
119 
many of the marly beds of this formation is due 
to the presence of countless myriads of similar 
exuviae of the Cypris which give rise to divisions 
the marl as thin as paper. Taking this fact 
m conjunctiou with the habit of these animals to 
nioult and change their skin annually, together 
"iti theii shell, he justly observes that a more 
onvincing pi oof 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 
Another proof of the length of time that must 
have elapsed during the deposition of these 
tCTtiary 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 Indusise, or 
^addis-bke coverings, resembling the cases that 
endose the larvae of our common May-fly. 
Mr. Lyell states that a single individual of 
lese ndusiae is often surrounded by no less 
than a hundred minute shells of a small spiral 
univalve, (Paludina), fixed to the outside of this 
See Pliryganea. 
t fn * P""eiples of Geology, 3id edit. vol. 
lik ^ difficult to conceive how strata 
anrl <^’^er large tracts of country, 
mi 1 another, with beds of marl 
th > ^ tmtween them, should have contained 
anhiv,r*'7“^* multitudes of aquatic 
by any other process than that of 
