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, 

 Mytildce, will include the genera Mytilus, Modiolus, and 

 Lithodomus, which are furnished with a byssus. The 

 second, Unioridce, 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- 

 chiae 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, 



