CONCHOLOGY. 
cipal tiibes, and which, after the Linnsean 
manner, may be denominated Multivalve, 
Bivalve, and Univalve. 
Any external part of a shell being of a 
testaceous substance, and either itself, 
forming a shield or covering for the animal, 
as in univalves, or in union with another, 
or others connected by a ligament, carti- 
lage, hinge, teeth, or other fastening, is 
denominated a valve. The shells, there- 
fore, consisting of a single piece, are called 
univalves, those of two parts bivalves, and 
those of many parts multivalves. Between 
bivalve and multivalve no distinction is 
drawn, shells consisting of more than two 
such parts being called multivalve, without 
any regard to the number. An amend- 
ment is proposed by some of the French 
writers, in a new order under the name of 
trivalve. 
Shells of the simplest form are arranged 
by some naturalists in the first class, from 
which they proceed progressively to those 
possessing the greatest number of valves, 
and being of the most intricate structure. 
This is an ancient and very simple mode of 
arrangement, and has its advocates in the 
present day. Linnaius reverses this order 
by beginning with the chiton, lepas, and 
pholas, which are shells of the multivalve 
and most complex structure, and ending 
with those of the simplest form. The for- 
mer seems most preferable. 
Univalve. In the examination of a shell 
of this order, the contour, or outline, is 
the first particular to be regarded. By this 
the conchologist is guided in his definition 
of simple, spiral, or turbinated shells, (or 
as the LinuEean school divides shells, uni- 
valves with a regular spire, and univalves 
without a regular spire) ; discoid, flattened, 
or turreted shells ; those with smooth or 
uneven anfractus ; the ventricose, alated, 
labiated, rostrated, and many other distinc- 
tions, all which strike the eye at the first 
view. It is- indeed, by attending to the 
contour, that the principal distinctions in 
shells of this kind are at once perceived, 
taking into consideration the back and front 
profile at the same time. Some few shells, 
as the nautilus ponipilius, and others of the 
same family, have flie spire revolving inter- 
nally, in which the outline offers less assist- 
ance in the primary definitions, but the 
number of such shells is very small. Next 
to the profile of the shell, the structure of 
the mouth, the pillar, and expansion of the 
inner lip, the gutter or canaliculation, and 
the umbilical opening, and operculum, if 
any, are to be considered, and, lastly, the 
work on the dnter surface, as well as the 
colours with which it is embellished. 
The base or bottom of the shell w'e consi- 
der that part upon which it rests when sup- 
ported in an erect position, with the summit 
or tip of the spire standing vertically. In 
such shells the tip is called the apex. The 
course of the spires or wreaths is from the 
left to right in most spiral shells, some few 
only being of the reversed or heterostro- 
phous kind, the whorls of which are in a 
contrary direction. When speaking of the 
right and left sides of a shell, it should be 
understood as having the aperture down- 
wards, and it will be then seen that in most 
shells the apertm-e or opening is on the left 
side, i. e. facing the right hand of the spec- 
tator. 
Base, the tip of the salient end of the 
shell, at the extremity opposite the apex of 
the spire ; in the rostrated kind of univalves 
it implies the tip of the beak. Some say 
the shell rests on its base when laid upon a 
flat surfiice with the mouth downwards : this 
is not correct, except in the patella tribe, 
and some other univalves which have no 
regular spire, as the dentalium, &c. Apex, 
the summit of the shell. Front, the face of 
the shell with the aperture placed directly 
in front of the observer. Back, that part 
of the shell which is immediately opposed 
to the preceding. Sides, those parts seen 
longitudinally in profile, to the right and 
left when the shell is viewed either in a 
front or back position. Body, of the shell, 
(corpus) the first whorl of the spire at the 
base. Belly, is to be distinguished from 
the body as it implies only the convex or 
swollen part of the first whorl, formed by 
the convexity of the aperture near the lip. 
Whorl, denotes one of the wreaths, turns, 
or evolutions of the shell. 
Spire, comprehends, in a general sense, 
all the whorls of the shell, the first or body 
wreath excepted. The form of the spire is 
of great consideration in the definition of 
shells, as it affords a prominent and distin- 
guished character ; it is in general flatfish, 
somewhat depressed or elevated-; some- 
times convex and slightly pointed ; or with 
the point obtuse; or much elevated and 
ending in a point ; plano-concave, pyrami- 
dal, subulate, or truncated. Mr. Adanson 
observes, that the disposition of the spires 
varies according to the plane they turn on, 
which is either horizontal, cylindrical, conic, 
or ovoid. These he conceives to be the 
four principal dispositions of the spites, but 
