62 
both fresh and salt water. All these shells still leave 
us, therefore, in doubt. But what does the Serpula prove ? 
No Serpula is known to live in fresh water, and the one 
we have along with these Mussels is too tender to have 
been removed far ; so if the other shells belong to fresh 
water, they must have been brought down by a river 
into the sea ; but they are extremely well preserved, and 
many as tender as the Serpula, which makes us rather 
incline to the opinion that they are all Marine, at least 
those which occur in the same stratum. 
That river shells should be washed down to the coast 
and mix with marine ones is probable : even large masses 
or islands, consisting of decayed vegetable matter with 
the shelly remains of animals that lived in lakes, may by 
floods be carried into rivers, and by them down to the 
sea, and be deposited upon the ordinary sediment in the 
bed of the ocean. The analogy of the various shells in the 
formations we allude to, rather favours this hypothesis. 
We leave it to geologists to compare a number of facts, 
respecting situation and many other circumstances, to 
determine the question. 
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