CONCHIFERA OR BIVALVES. 107 



Division I. — Ligament long and marginal. 

 3 families, Tridacnidce, Mytilidce, Malleidce. 



Family 1. — Tridacnidce.* 

 The Tridacnidce, or Clams, are the giants of 

 the acephalous Mollusca, nearly all the species 

 attaining a large size ; they are transverse and 

 equivalve, and attach themselves by a fibrous 

 muscle to rocks and corals. They are generally 

 found in exposed situations. — Only two genera. 

 66. Tridacna. — Shell regular, equivalve, 



inequilateral, transverse; two compressed blunt 

 teeth in each valve ; a wide opening on one 

 side of the bosses for the passage of the byssus ; 

 ligament external. Animal, foot cylindrical 

 and thick. — 7 species recent. 



These shells are brought from the Indian 

 Ocean, where they attain so large a size that a 

 single person can with difficulty carry one 



* Les Tridacnees, Lamarck. 



