506 PHILOSOPHY OF ZOOLOGY. 



This tribe may be divided into families. The first, My- 

 tilusida, will include the genera Mytilus, Modiolus and Li- 

 thodomus, which are furnished with a byssus. The second, 

 Unioda, will embrace Unio, Ilyiria, Anodonta, and Iridina. 

 These are destitute of 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 Hip- 

 popus. 



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 acess to 

 the mouth and gills, and the feet freedom of motion. 



The structure of the animals is yet too imperfectly ex- 

 amined, to enable any one to establish families on permanent 

 characters. The attempt which Lamarck has made may be 

 considered as a complete failure, independent of the wanton 

 changes of nomenclature with which it is chargeable. The 

 following are the principal genera belonging to this tribe : 

 Cliama, Iscordia, Cardium, Donax, Cyclas, Corbis, Tellina, 

 Loripes, Lucina, Venus, Capsa, Petricola, Corbula, and 

 Mactra. 



In the genus Chama, the two posterior apertvu'es are in 

 the form of short tubes, the anterior one is small, and indi- 



