SHELL. 
being from tlieir rarity purchased at very 
high prices. 
The vast variety of forms which shells 
possess, not only renders arrangement more 
necessary, but at the same time occasions 
the formation of that arrangement the more 
difficult. Dissimilarity in size and colour, 
as well as in form, which occur in shells 
of the same species, at different ages, adds 
much to this difficulty. External characters, 
however, sufficiently marking specific dif- 
ferences, may almost always be discovered 
in shells, at every period of their existence ; 
and various attempts have hence been made, 
to dispose them in such a methodical ar- 
rangement, as should place each of them in 
its proper place ; and should thereby occa- 
sion them to be more readily recollected. 
Lister, Langius, and other early writers 
on testaceology, have proposed different 
methodical arrangements of shells, which, 
from their imperfections, have been in- 
tirely laid aside, and do not therefore re- 
quire to be here noticed. Indeedit appears 
to be unnecessary to go further back than 
to the arrangement of the celebrated Lin- 
naeus, which seems, in its turn, to be yield, 
ing to later systems, rendered more perfect 
by tlie constant accession of new and illus- 
trative specimens, and the consequent in- 
crease of knowledge. But previous to con- 
sidering the proper method of arrangement, 
and to examining into even the generic dif- 
ferences, it will be proper to ascertain the 
characters which belong to shells in gene- 
ral, and to determine the precise terms by 
which they can be best expressed. 
The division into univalves, bivalves, and 
mnltivalves is so clearly pointed out by na- 
ture, that it must be adopted in every well 
methodized arrangement, and may there- 
fore be safely admitted in these preliminary 
observations. Univalve shells are discoi- 
dal when the spiral is formed on a horizon- 
tal line, so tliat a section, made in that 
line, would divide the shell into two nearly 
equal parts ; fusiform when it bulges in the 
middle, and has the two extremities of 
nearly equal length ; turbinated, when the 
belly of the shell is very large and tumid 
compared with the spire, which is given 
off from its centre ; and turriculated when 
the turns of the spire, gradually increasing, 
form an elongated cone. 
The parts of univalve shells are, the back, 
which is the external bulging part of the 
turn opposite to the opening; the belly, 
the tumid part of the last turn, forming the 
right or outer lip ; that being termed the 
left which is formed on the columella, and 
which Linnasus distinguished as the colu- 
mellar ridge. The spire is formed on the su- 
perior part of the shell, the turns of which 
are formed round the columella, or little 
centi al column, reaching from the opening 
of the shell to the summit of the spire. 
These convolutions are numbered by reckon- 
ing that as one, which reaches from the ex- 
ternal to the inner lip, and in the same manner 
to the summit.; the line by which they are 
united being termed a suture. The spire 
itself is either pointed, obtuse, truncated, 
flattened, concave, convex, straight, ob- 
lique, or pyramidal. Its turnings are coro- 
nated, (beset with spines or tubercles), 
beaded, carinated, or canaliculated. The 
suture is cremilated, double, projecting, or 
effaced. The shell is said to be perforated, 
when a small umbilical cavity exists in the 
columella at the base of the shell, in the 
axis round which the spire is revolved ; and 
imperforate when there is here no cavity 
nor umbilicus. Besides being either umbi- 
licated or not, the columella may be flat- 
tened, truncated, caudated, canaliculated, 
spiral, and plaited, as in the Volutes. The 
umbilicus also may be slit, canaliculated, 
consolidated, crenulated, or dentated: it 
may also have a snperadded cullus. The 
opening varies much in different shells; 
and where it narrows it acquires the name 
of throat ; it terminates in a canal. The lip 
is eared, digitated, alated, grooved, slit, or 
wrinkled ; the form of the opening depend- 
ing much on that of the lip ; it hence becom- 
ing angular, round, semicircular, longitudi- 
nal, transverse, linear, gaping, compressed, 
or reversed. The openings of shells are some- 
times closed by a shelly or cartilaginous 
plano-convex body,termed operculum, which 
is disposed between the two lips, and fitted 
to the form of the opening. The flat surface 
of this body is marked by a spiral line. 
A bivalve shell has the two valves, which 
constitute it, connected at the basis, or in- 
ferior margin ; opposite to which is the su- 
perior margin ; the shell being supposed to 
be placed on the hinge, its margin is di- 
vided into the anterior and the posterior 
margin. The length of the shell is mea- 
sured from the inferior to the superior mar- 
gin, and the breadth from the anterior to 
the posterior margin. The belly is the 
most tumid part, and the beaks, the promi- 
nences over the hinge, are of a conoidal 
and spiral form. At the basis of the shell 
is the hinge, by which the valves are moved 
on each other ; and in the anterior chink a 
