MOLLUSCA. 197 
ejection of the excrements. The other large opening allows 
the water to enter to the mouth and gills. 
This tribe may be divided into two families. The first, 
Mytilde, will include the genera Mytilus, Modiolus, and 
Lithodomus, which are furnished with a byssus. The 
second, Unioride, will embrace Unio, Hyria, Anodonta, and 
Iridina. They want a byssus. 
M. Cuvier is disposed to place in this group the genera 
Cardita, Venericardia, and Crassatella. 
2d Tribe. 
Cloak closed posteriorly, and anteriorly forming three 
apertures. The first serves for the passage of the byssus, 
and is the largest. The second admits water to the bran- 
chize and mouth ; and the third is opposite the anus. The 
valves are closed by one adductor muscle. There are only 
two genera belonging to this tribe, Tridacna and Hippopus. 
In the two remaining tribes there are three openings in 
the cloak. Two of these are posterior, and near each other ; 
sometimes, indeed, they are tubular and united. ‘There is 
no byssus, but always a foot. 
3d Tribe. 
Anterior opening large, allowing the water free access to 
the mouth and gills, and the feet freedom of motion. The 
structure of the animals is yet too imperfectly examined to 
enable any one to establish families on permanent charac- 
ters. The attempt which Lamark has made may be con- 
sidered as a complete failure, independent of the wanton 
changes of nomenclature with which it is chargeable, whilst 
the efforts of Cuvier have not been attended with greater 
success. The following are the principal genera belonging 
to this tribe: Chama, Isocardia, Cardium, Donax, Cyclas, 
