CONCHOLOGY, 
of the teeth, or other process. In order to 
eoustitute a bivalve shell, it is only requisite 
that it be furnished with two connected 
valves, witliout regard to their resemblance 
in form or dimensions. Some of the bivalves 
have both valves formed alike ; in others 
they differ only in a slight degree, and 
again in others they are altogether dissimi- 
lar. The first of these is well exe.mplified 
by the solen genus ; in that of the Linnaean 
tellinae, we find examples both of tlie equi- 
valve shells, and those with the valves 
slightly different; of the last-mentioned 
kinds we have many, as the ostrea, spondy- 
lus, and anomia. Bivalve shells are often 
much compressed, some are gibbous, and 
when viewed at the side, or facing the liga- 
ment, have a cordated appearance, as in the 
Venus, and the Linnaean chama cor. Shells, 
having both valves alike, as before obsei-v- 
ed, are called equivalve. Equilateral valves 
imply tliose which have both sides of the 
same valve alike ; as for instance, when a 
longitudinal line is drawn from the beak to 
the opposite margin, the space on each 
side of the line is distinguished by the ap- 
pellation of the right and left side ; and 
when the form of both those spaces cor- 
respond, the shell is equilateral, as in the 
scallops (ostrea. Linn.) : the inequilateral 
valves are the reverse of this, a line drawn 
as above described, from the beak to the 
opposite margin, presenting two sides of a 
very different shape, as we see in most of the 
mactra, the donax, and tellina genera, and 
in the mya truncata especially. Snbequila- 
teral shells, or those having the valves near- 
ly equal at both sides, are sufficiently elu- 
cidated by shells of the cardiuin, or cockle 
genus, which are strictly “ bivalvis subas- 
quilatera.” 
All bivalve shells do not completely close 
their shells, though most of those before 
mentioned do so, such as the scallop, the 
donax, tellina, and cardium : in several 
other tribes of bivalves, when the shells are 
shut as closely as their form will allow, they 
still exhibit a kind of hiatus or gaping, ei- 
ther at the anterior or posterior end, or at 
both; and in some, when the valves aie 
shut, both the anterior and posterior parts 
are closed, but an opening appears on one 
side of the beak ; this last-mentioned cha- 
racter is very obvious in chama gigas. 
One of the first circumstances to be con- 
sidered, is, which part of a bivalve shell 
ought to be deemed the base, because when 
this^is determined, every other part will 
tall progressiyely in tlieir relative order 
under our observation. We name that part 
of the margin or limb which is situated in a 
direct line opposite the beak, the base of the 
shell. Linnaius, in order to establish the 
characters, and afford some apparent reason 
at least for the application of the terms he 
bestows on, the different parts of bivalves, 
reverses this position of the shell, and 
describes the beaks as the base of the shell. 
But the fact is, the natural position of the 
shell is in immediate contradiction to his 
axiom,, for the beaks are always uppermost, 
being either immediately vertical, or with 
a slight inclination obliquely, when tiie ani- 
mal moves along with its testaceous cover- 
ing on the back. A solitary example will 
perhaps occur occasionally, in which the 
beaks may be considerably inclined when 
the animal crawls, but none, we believe, are 
known which open the valves upwards, and 
proceed with the beaks under the body. 
The beaks, if only for tliis reason, are to be 
considered as the summit, and the margin 
opposite as the base. Many of tire bivalves 
are destitute of the locomotive power, or at 
least do not possess it in any material de- 
gree. 
Summit, a word applied in a general 
manner to the top or most elevated part of 
the two protuberances observable in the 
greater number of bivalves. Da Costo 
calls that part of the .shell, in which the 
teeth or hinge is placed, the summit or 
apex ; we regard it as the most elevated 
part of the beaks. Beak, the pointed ter- 
mination, apex, or tip of the protuberances 
last-mentioned, and which, in many shells, 
turn spirally downwards, or obliquely, so 
that the beak itself is seldom the most ele- 
vated part of the shell : though it is so some- 
times, as for instance in the mytilns edulis, 
or common muscle. Sides, the lateral 
parts of the valves distinguished by thq 
epithet of right and left side ; in com- 
mon language, the two valves of a shell 
are called the sides, but it is not under- 
stood as a term in conchology in this view. 
Margin, or limb, the whole circumference 
or outline of the shell, when laid flat down 
on one valve. Disk, the convex centre of 
each valve, or exterior surface. Anterior 
slope, that part of the shell in which the li- 
g-ament is situated ; in the front view of the 
anterior slope, the beaks fall back, or be- 
hind. Posterior slope, that immediately 
opposed to the former, and in which the 
beaks of the shell turn forward, Lunule, 
the lunulated depression below the beaks, 
either on the anterior or posterior slope. 
