MOLLUSCA. 73 
cumstance to recommend them, that they are obvious and. 
permanent. The objects which furnish them can be pre- 
served in our cabinets, and serve to perpetuate our recollec- 
tion of the appearances which the more perishable parts 
have exhibited. 
There is yet another class of characters to be considered, 
very variously rated by different authors. These characters 
are taken from the situation in which shells are found, whe- 
ther on the land, in fresh water, or in the sea. 
This mode of dividing testaceous bodies has not been 
sufficiently attended to by conchologists, who have, in ge- 
neral, condemned the plan, as founded upon an improper 
principle, viz., the classification of animals from the places 
which they frequent, instead of the forms which they exhi- 
bit. Such a mode of arranging the higher divisions of the 
different classes we would readily censure; but when em- 
ployed in the inferior subdivisions of the testacea, we regard 
it as an important and a natural character. We ask the true 
naturalist to say, which is the most important character, the 
hinge having teeth or wanting projections, and the animal 
residing in fresh water or in the sea? We anticipate with con- 
fidence the preference which would be given to the latter, 
although the decision might provoke a sneer in a mere col- 
lector. Nature has evidently drawn a line of separation 
between the three tribes, which it is not difficult to perceive. 
The terrestrial testacea are destined to live on vegetable 
matter. Their organs of respiration are suited to the me- 
dium in which they reside. Their organs of feeling are, in 
general, more numerous than those of the fluviatile or marine 
shells. The tentacula of the latter seldom exceed two, 
while in the land shells the tentacula are, in general, four in 
number, The eyes are likewise differently placed; in the 
E 
