two New Freshwater Shells. 15 
summits. Now it is obvious, that this peculiar form, is of all 
others the best adapted to strengthen and protect the elastic bris~ 
tles which they enclose: both appendages, therefore, are in unison 
with each other, and leave not a doubt in my mind, (setting aside 
the personal testimony of its discoverer) that the whole shell is in 
a perfectly natural state. 
It is difficult to conjecture in what way the formation of the 
shell accords with the economy of its inhabitant. We know that 
testaceous mollusca, are the food of several kinds of fish, both ma- 
rine and fresh-water; may not these bristles be intended by nature 
to defend the animal from such enemies? they would certainly be 
very repulsive to the lips of any fish; and in all probability would 
penetrate, as deep as possible into the skin. The weapons of pro- 
tection or of defence with which nature has furnished different 
tribes of animals, are as various as they are wonderful. In the 
testaceous mollusca, they are confined alone to the shelly covering 
of the animal, who, as long as his castle is armed and entire, with- 
draws into its walls, secures the entrance, and remains passively 
secure. 
0b. 2. Since the above was written, Mr. Broderip informs me, 
another specimen of this shell has come into his possession : “care- 
fully cleared, and every vestige of bristle removed, the hollow co- 
ronations remain.” 
Unio GiGas. 
Specific character. 
U. testa ovato-oblonga, depress4, anticé alata et sulcis obliquis, 
divaricatis subradiata; posticé brevissima; dente laterali (utri- 
usque valve) solitario; umbonibus brevibus, retusis. 
Shell ovate-oblong, depressed anterior side winged and marked 
by oblique grooves in different directions ; pdsterior side very short; 
lateral teeth, one in each valve; umbones small, retuse. 
Description. 
This is truly a gigantic shell; far exceeding in size any other 
yet discovered as inhabiting the fresh-water, and presenting cha- 
racters which leave no doubt that it has hitherto remained unknown 
to all conchological writers. Its extreme length is rather more 
