Mollusca. 
S—>\-—> ordination. 
Unival ves. 
Canalicula- 
ted shells. 
MOLLUSCA., 
relative value of the characters used, and of their true sub- 
This is a subject of vast moment, and too little 
attended to by naturalists. We do not here consider that 
character as of the highest kind, which is the most general, 
but that which has the greatest influence over the faculties 
and instincts of the animal. Had this subject been studied 
with greater care, the science of conchology would, at this 
period, have been in a more flourishing condition. To as- 
certain, in some degree, this subordination of character, is 
the object of the following observations. 
The division of the testaceous mollusca into three orders, 
as adopted by Linneus, is confessedly artificial. The Mul- 
tivalvia of that author possess no characters in common, nei- 
ther can they boast of a general resemblance. The first 
genus, Chiton, consists of animals which belong to the ce- 
phalous and gasteropodous mollusca. The animals of the 
genus Lepas approach more nearly to the crustacea than 
the mollusca ; while the animals, inhabiting the genus Pho- 
las, belong to the acephalous mollusca, and are closely con- 
nected with the Myz and Solenes.. The shells of the first 
genus are merely calcareous scales, arranged transversely on 
the back of the animal. The shells of the second genus 
are variously articulated, fixed, and either sessile or pedun- 
culated. The shells of the.third genus are bivalves, with a 
few accessory calcareous plates. It is to be hoped that mo- 
dern conchologists will avoid so incongruous a combination. 
The two remaining divisions of Linnzus, the bivalves 
and univalves, are not only obvious, but natural. They in- 
dicate the existence of certain forms peculiar to the animals 
whese shells are thus separated in the system. The uni- 
valve shells are inhabited by animals which possess a head, 
and whose organs of motion are either tentacula situated on 
the head, or a foot spread over the belly, as in the slug. 
The animals of the bivalve shells, on the other hand, are 
destitute of a head; some of them have no locomotive 
power; and, in others, the organ of motion is a fleshy foot, 
which the animal can protrude at pleasure. These cireum- 
stances point out the connection which subsists between 
the organs of the animal, and the external forms of the shell; 
a connection which, in every system, ought to be carefully 
attended to. 
It is somewhat difficult to point out, among the univalves, 
the true subordination, or relative importance of the cha- 
racters employed by conchologists in describing them. We 
have much to learn of their anatomy, and hence we cannot 
with certainty point out the relation of the parts of the shells 
to the organs which those parts protect. The form andstruc- 
ture of the mouth of the shell, however, may be expected to fur- 
nish characters of the first-rate importance, and have always 
attracted the notice of the student of testaceous bodies. The 
very shape of the animal, together with its ordinary habits, 
must necessarily depend, in 2 great measure, on the form of 
the mouth. i 
In many genera, the mouth of the shell towards the base 
is produced, and terminates in a groove or beak. These 
univalves are termed canaliculated, and are readilv distin- 
guished from those whose mouth is entire. The differences 
in the form of the shell in these two divisions is an mdex of 
equally remarkable differences in the form of the animals. 
The canaliculated shells contain animals possessed of an 
elongated tube for the purposes of respiration, and this canal 
is destined for its reception and protection when expanded. 
The animals whose shells are destitute of this canal, are 
likewise destitute of this lengthened respiratory tube. Cir- 
cumstances of this kind induce us to believe, that shells, 
agreeing in external form, in general, contain animals of a 
similar organization. We consider this division of the uni- 
valves into canaliculated and entire, as obvious and natural. 
The next character, in point of importance, appears to 
depend on the direction of the revolutions of the spire. In 
general, when a spiral univalve is placed upon its base, oz 
VOL. Kv. 
337 
mouth, with its summit towards the observer; the mouth Mollusea. 
will open on the right side of its axis or pillar, and the =——~ 
whorls will be observed to revolve from right to left, be- 
ginning at the base, and ending at the summit. ‘These 
shells are termed dextral. In a few shells, however, this 
order is reversed. The mouth occurs on the left side of the 
pillar in the above-mentioned position, and the whorls from 
the mouth to the summit revolve from left to right. Shells 
of this sort are termed sznistral, sometimes also heterostro- 
phe or heteroclite, and are generally called by dealers unique. 
In the dewtral shells, the animals have the external open- Dextral 
ings of the rectum, penis, and uterus, on the right side of shells. 
the body, and the heart on the left. In the sindstral shells, 
these organs are placed on the opposite sides. Thus the 
openings of the rectum and organs of generation are on the 
left. side, while the heart is situated on the right. Here 
again we have an external character impressed on the shell, 
which indicates certain arrangements in the organs of the 
animals. We are aware that some conchologists consider 
the sinistral shells as accidental varieties, and on that ac- 
count regard the character which is indicated as of inferior 
importance. Bosc, indeed, says, “ La cause de cette varia- 
tion dans la direction des. spires, vient des circonstances 
dans lequelles s’est trouvé Vanimal au moment de sa’ nais- 
sance, et dun obstacle quwil a.trouvé lorsqu’il a voulu tour- 
ner sa téte du c6té que la nature lui a indiqué.” This ex- 
planation might have been received, had such changes in 
the direction of the whorls been confined to one individual 
or two, of particular species. But when we observe all the 
individuals of particular species, nay even of genera with 
their whorls thus invariably reversed, we are disposed to re- 
gard the occurrence as connected with the primary struc- 
ture of the animal, and not as the result of accident. Be- 
sides, the viscera of the animal of a reversed shell are not 
placed in the same position in relation to its back or belly, 
as the animals of the dextral species. A simple change of 
direction in the spire, therefore, will not convert a dextral 
mto a sinistral species, and the character must be consider- 
ed as of a higher order than those employed for the separa- 
tion of the species merely. We consider sinistral shells as 
belonging to distinct genera from those which are dextral, 
it being inexpedient to make use of the character for higher 
divisions. 
Among many of the univalves, the animal is furnished Opereulat- 
with a lid, by means of which it can close up the entrance ed shells: 
of the shell after it has withdrawn itself into the cavity. It 
is in general corneous, sometimes also calcareous. It is 
usually flat, and attached to the superior and posterior part 
of the foot of the animal. The shells which possess this lid 
are generally termed operculated shells. They must not 
be confounded with those land shells of which the animals 
form a temporary covering to the mouth, previous to winter, 
for the purpose of protecting them from the vicissitudes of 
the weather. This lid, in the former case, is permanent, in 
the latter deciduous ; in the former it adheres to the ani- 
wal, being in connection with it; in the latter only to the 
margins of the mouth of the shell. 
This character was first employed by Adanson in the con- 
struction of the second section of his class univalves, and 
has been more or less attended to by succeeding concholo- 
gists. It is certainly a very general character, and at first 
sight might be supposed worthy of forming some of the 
higher divisions. It appears but rarely in the land shells, 
more frequently in fresh water shells, and generally in the 
marine species. It does not, as yet, appear to be connect- 
ed with any peculiar organization, although it must influ- 
ence to a certain extent the economy of the animal.. Were 
we however, to employ it in higher divisions than generic 
ones, some confusion would certainly arise. It would cause 
the separation of many genera which are nearly allied, and 
even divide several. natural families. Thus, for example, 
2U 
