SHELL. 
thick recurved teeth, with two intermediate 
rounded cavities to receive the teetli of the 
other valve, and an elongated pit in which 
the ligament is placed ; in the superior 
valve are two exterior cavities for receiving 
the teeth of the other valve, and two teeth 
curving backwards, with a pit for the liga- 
ment. 
Charaa. This genus is appointed, by Bru- 
guiere and Lamarck, to those shells only 
which, in particular, possess unequal valves, 
and are adherent. The characters of the 
genus therefore are : a bivalve shell, with 
adhering unequal valves ; the hinge compo- 
sed of one oblique thick tooth, which may 
be crenulated or rough, and is articulated 
in a cavity in the opposite valve. 
It was necessary, therefore, to establish 
other genera for the reception of those 
shells which thus become excluded from 
the genus Chaina. Hence Bruguiere form- 
' ed too new genera; Cardita, having two 
teeth in the hinge ; and Tridacna, possessing 
the same number, but having the edges of 
the crescent crenulated and gaping. La- 
marck has proceeded still further; he has 
subdivided the genus Cardita from an iso- 
lated tooth, situated, in some species, under 
the corslet, as in Cardium isocardia, and 
makes this his genus Isocardia. He also 
makes another genus, Hippopus, the species 
of which are subtransverse, inequilateral 
shells ; the hinge with two compressed, in- 
verted teeth, the crescent filled up : of this 
genus is Chama gigax. But it is necessary 
still further to particularise these new 
genera. 
Cardita, a loose inequilateral shell, the 
hinge with two teeth ; the one at the base 
of the left valve, and the other longitudinal, 
parallel with the anterior face. 
Hippopus. An inequilateral, subtrans- 
verse shell ; the hinge with two compressed 
and inverted teeth ; the crescent filled up. 
Hippopus, it is to be remembered, differs 
from Tridacna by having its crenate filled, 
whilst in Tridacna it is gaping. 
I Tridacna. An inequilateral, subtrans- 
verse shell ; hinge with two teeth, compres- 
sed and inverted ; the crescent gaping. The 
Clrnnagigas of Linnaeus (the Tridacna gigas 
of Lamarck) is the only species under this 
genus. 
Cardinm. This genus remains unaltered 
by the French naturalists. The characters 
being : a sub-cordiform shell, with valves 
dentated on their edges; the hinge with 
four teeth; two cardinal, approximating 
obliquely .in each valve, and articulating 
across with their correspondent teeth ; the 
lateral teeth distant, and inverted in the 
opposite valve. 
Acardo. Is a genus very properly esta- 
blished by Commercon. It is founded on 
two species : the generic characters of 
which are : a shell composed of two, nearly 
equal, flat valves ; having neither hinge nor 
ligament ; a muscular impression appearing 
on the centre of each valve. 
Radiolites. This is a genus approximating 
to the preceding, but very properly sepa- 
rated from it ,by the form of the shell. The 
shell is of an irregular form, of unequal 
valves, striated on the outside ; the inferior 
valve turbinated; the superior convex, or 
conical ; no hinge nor ligament. 
Erodona. This genus was formed, by 
Daudin, from two shells in the cabinet of 
Favannes. It is intermediate between the 
Mya and Mactra. It is_ a subtransverse, 
bivalve shell, irregular and gaping ; one of 
the valves having a hollow tooth ; and 
the other a pit between two projections ; 
the ligament inserted on the tooth and in 
the pit. 
Mya, is a transverse shell, gaping at both 
ends ; the ligament on the inside ; the left 
valve having a cardinal tooth, compressed, 
rounded, perpendicular to the valve, and 
giving attachment to the ligament. This is 
the original genus of Linnaeus, from which 
Bruguiere and Lamarck have withdrawn 
many of the species to place them under 
the genera Vulsella, Glycemeris, and Unio ; ' 
leaving only those sea-shells which have a 
very projecting cardinal tooth in one of 
their valves. 
Glycemeris. A transverse shell, gap- 
ing at both extremities : a protuberant 
hinge without teeth ; the ligament on the 
outer part. Such is Mya glycemeris of 
Linnreus. 
Solen. A transverse shell, gaping at both 
extremities : the upper edge straight, or 
nearly so ; two or three teeth at the hinge' 
furnished by both valves. 
Tellina. Considerable confusion has 
arisen from the admission of species which ' 
by no means belong to this genus. Bru- 
gniere and Lamarck have divided the Tel- 
lens of Linnaeus into two new genera. 1. 
Tellina, a transverse or orbicular shell with 
equal valves, with a fold on the anterior 
side : one or two cardinal teeth, and two 
separated lateral teeth, as in Tellina vir- 
gata. 2. Caspa, a transverse shell, with 
