CONCHIFERA OR BIVALVES. 109 



67. Hippopus. — Shell equivalve, regular, 

 inequilateral; valves closed, transverse, two 



compressed teeth in each valve ; ligament ex- 

 ternal ; shell imbricated with numerous tuber- 

 cles. Animal similar to that of the Tridacna. 

 — 1 species recent. 



The Hippopus has no opening in the valves, 

 and is therefore presumed to be destitute of a 

 byssus. It is not nearly so large as the Tri- 

 dacna^ and is a common shell in collections, 

 although there is only one species, the Hippopus 

 maculatusy from the Indian Ocean. 



Family 2.—Mytilida>.* 



The Mytilidce, or Mussel family, possesses 



* Les MytilaceeSy Lamarck. 



