OF NORTHUMBERLAND AND DURHAM. 101 
Survey of Great Britain,’ have demonstrated the use of local 
Faunas like the present, in furnishing materials for working out 
some of the most interesting problems that Natural History af- 
fords ; but for the true solution of problems of this kind, it is not 
only necessary that local Catalogues should be as complete as 
possible in the productions of the district, but that care should 
also be taken not to introduce species by mistake, or on imper- 
fect evidence, which might give a false idea of their geographi- 
cal range, and vitiate the generalizations founded upon them, 
Impressed with this conviction, we have been particularly care- 
ful in scrutinizing the earlier published Catalogues, and com- 
paring them with the geographical limits of species as far as already 
known ; in doing this, we have been led to reject some species 
only found in a dead state, which deference to previous authori- 
ties might otherwise have induced us to retain. 
One source of error, which, without great care, is likely to vi- 
tiate our Fauna, is the quantity of shells brought in ballast by 
the coal ships. This ballast, dredged in the rivers and harbours 
of the south of England, is, in consequence of the dues payable 
for its deposit on shore, frequently cast overboard at sea, and 
shells from it are occasionally washed up on the adjoining sands. 
We have endeavoured to ascertain as far as possible what shells 
are attributable to this source, but we cannot be sure that in every 
case we have succeeded. 
The exposed nature of the coast is not favourable to the use of 
the dredge, and consequently very few explorations have been 
made with that useful aid to the naturalist. Our own exploits 
in that way, we must confess to have been nearly complete failures, 
owing principally to our inability to stand the exposure and mo- 
tion of an open sea. Mr. King and Mr. Howse have made at- 
tempts at deep water dredging, on the cod-fishing grounds at a 
considerable distance from the shore, the results of which have 
appeared in the “Annals of Natural History.” Though these can 
scarcely be called successful, there cannot be a doubt that under 
more favourable circumstances excellent results might be ob- 
tained from deep-water dredging off our coast, but the expense 
and discomfort of the undertaking, as prosecuted in the fishing 
