Mollusea. termed muscular impressions. 
——" and lateral, subcentral, or simple, or composite. 
MOLLUSCA. 
The ¢eeth of the hinge of bivalves, since the days of Lan- Mollvses. 
gius, have been studied with care, and the characters which "\—= 
They are either separate 
This cha- 
racter was long employed by conchologists in their specific 
distinctions, and sometimes in the formation of the genera. 
Ithasbeen more recently employed by Lamark, as a character 
of the first importance in the division of the bivalves. He 
forms these shells into’ two sections, the first containing 
those shells which have the muscular impressions separate 
and lateral, and the second such as have only one subcen- 
tral, simple, or compound impression. However highly we 
respect the conchological labours of this naturalist, we can- 
not join with him in the present instance, and elevate a sub- 
ordinate character to a primary rank. If, by muscular im- 
pressions, he means those marks impressed on the valves of 
the shells by the muscles which serve to close it, then his 
character is unconnected with any of the primary functions 
of the inhabitant. For is it of much consequence whether 
the valves be brought into contact by the action of one 
muscle or by the assistanceof two? In so far, the charac- 
ter is evidently artificial, when the impressions of the ab- 
ductor muscles only are employed. But he evidently uses 
the term in a more extensive sense, to refer to those im- 
pressions left on the shell by some of the other muscles by 
which the animal is attached to it. To the mere concho- 
logist, these marks are of a very uncertain import, and 
can never enable him to construct natural families, and the 
student of the mollusca will employ more important distinc- 
tions. If we are to take all the muscular impressions into 
account, the arrangement of Lamark must undergo great 
alterations. Let us take the common mussel as an exam- 
ple. It is placed by the French Conchologist in the second 
section, as having only one muscular impression, although 
no less than four muscles adhere to each valve, destined for 
the performance of very different functions. The largest 
impression, which is situate near the obtuse end of the 
shell and towards the posterior margin, belongs to the ab- 
ductor muscle, employed in closing the valves. Connected 
with this impression there is a tongue-shaped mark, reach- 
ing nearly to the ligament. This mark is occasioned by 
one of the lateral muscles for supporting the byssus, and by 
one of the lateral muscles of the foot. The other muscle 
for supporting the byssus, is inserted under the teeth which 
occur at the beak. There is even another mark of adhe- 
sion on the margin of the shell, irregularly denticulated, 
occasioned by the fringed margin of the cloak, which is 
there united with the shell. This mark may be termed the 
marginal impression. Yo which of these marks then are 
we to attach the greatest importance? To the impression 
*of the one abductor muscle, which is common to all shells, 
—to the marks of the muscle of the byssus, or to the in- 
dented mark of the fringed margin of the cloak. Ifwe at- 
tend farther to Lamark’s arrangement, we shall find the 
Camacea separated from the Ostreacea, although the two 
families possess numerous external and internal points of 
resemblance. We regard the muscular impressions as fur- 
nishing a convenient character for the construction of genera, 
and the discrimination of species, but it is not worthy to 
occupy so high a rank as Lamark has assigned to it. 
As intimately connected with the muscular impressions, 
we may here notice the ligament. It is a horny elastic 
membrane, which serves to open the valves, when the ad- 
ductor muscle relaxes. It is placed on the exterior margin 
in some shells, and is concealed in others. When external, 
it is stretched when the shell is closed, and when it is inter- 
nal, it is compressed in similar circumstances. This cha- 
racter is very useful in the construction of genera, but it 
ought never to be employed in any of the higher divisions. 
It is not the index of any peculiar organization, neither does 
it serve to unite natural families. Lamark, without due con- 
sideration, regarded it as next in importance to the muscular 
impressions. ; 
they furnish have been employed, both in artificial and na- 
tural arrangements, in the construction of the primary divi- 
sions. It would have been of some advantage to the science, 
had conchologists ascertained the use of the teeth in the 
economy of the animal, before forming any divisions from 
their presence, absence, or position. They do not appear 
to be the index of any peculiar organization, neither can 
they be employed to bring together naturally allied families. 
The use of the adductor muscle is to close the shell; the 
ligament opens it; and the teeth of the hinge seem destined 
to modify and direct these movements. The characters 
furnished by these three parts of the shell appear to be near- 
ly of equal importance, and fit only to occupy a very subor- 
dinate place. Were the circumstances connected with the 
teeth of the hinge to become the foundation of the higher 
divisions, many natural families would be broken. Thus, 
the genus anodonta would be removed from the unio, al- 
though they are both fluviatile, possess one long subulated 
foot, one syphon in the form of a hole, the summit of the cloak 
furnished with cirri, the branchiz in part re-united, vivipa- 
rous, carrying the young in the branchiz. In short, it seems 
to be a character fit only for generic and specific distinc- 
tions. 
Bivalve shells have often been divided into equilateral 
and inequilateral. These differences do not appear to be 
the signs of any peculiar character of the animal, or any of 
its functions. They must influence, to a certain extent, the 
relation between the different parts, but this influence is not 
sufficiently obvious. The character thus furnished is of an 
uncertain kind. It is influenced by the age of the indivi- 
dual, and therefore can only be employed with caution in 
specific distincticns. 
The last character of the bivalves which we shall notice 
is the power which some of them possess of piercing stones 
and wood for the purpose of forming to themselves a retreat. 
These are termed borers. It was supposed by many that 
the animal secreted a liquor with which it dissolved the bo- 
dies into which it penetrated, but the sagacious Reaumur 
soon ascertained that the boring was performed by means of 
a rotatory movement of the larger valves. M. Fleurieu- 
Bellevue states, that the calcareous stone in which the ru- 
pellaria lithophaga is found, is often discoloured in the im- 
mediate neighbourhood of its recess. This may arise from 
other secretions of the animal, or even from the stagnant 
sea water in the hole, and not from the action of the phos- 
phoric acid, or any other solvent supposed to be employed 
by the animal. These would act equally on the shell as on 
the calcareous rock. But the borers are not confined to 
calcareous rocks, they also lodge in slate-clay, and other ar- 
gillaceous strata. This is very often the case with the Pho- 
lades. But this character can never be extensively employ- 
ed, as the same species which, at one time, may be found 
imbedded in stone, will be observed at another seated among 
the roots of sea-weed, or buried among gravel. 
From the preceding remarks it will appear obvious, that 
there are many characters furnished by the shell, which give 
us indications of corresponding peculiarities in the structure 
of the animal, and on that account ought to be employed in 
every natural arrangement. These characters have this cir- 
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 
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