ANIMALIA MOLLUSCA. 



107 



placed behind the hinge, and which throws open the valves 

 when the muscles relax. 



FAMILY I. OSTRACEA. 



The mantle open, without tubes or any particular aper- 

 tures ; foot very small or entirely wanting ; the greater part 

 are fixed either by their shells or by their threads to rocks or 

 other bodies under water. Two remarkable genera. 



Genus I. Ostrea. Oyster. 



No foot ; mantle provided with a double range of fringes ; 

 shell hinged, inequivalved and laminated ; hinge without 

 teeth; ligament small; a single muscular mass extending 

 from one valve to the other. 



Genus II. Avicula, Brug. 



Besides the single transverse muscular mass, another fasci- 

 culus extending from one valve to the other, and placed before 

 the mouth. Certain species are called Pintadinse, the most 

 celebrated of which yields the mother of pearl. 



FAMILY II. MYTILACEA. 



Mantle opening before, with another separate opening ; all 

 have a foot used in crawling, or at least in drawing out, 

 directing and placing the byssus. Three remarkable genera. 



Genus I. Mytilus, Lin. Sea Muscle. 



Shell swoln out into a triangle with equal valves ; one is 

 fixed by a byssus. [The rocks of the sea.] 



Genus II. Anodontea. 



No byssus ; hinge without a tooth ; in the interior of the 

 shell a silvery nacre, and sometimes the most brilliant co- 

 lours, purple and pink. [Fresh waters.] 



Genus III. Unio. 



Shell has a short cavity in one valve near the hinge, which 

 receives a short plate or tooth of the other, and behind it is a 

 long plate, which is inserted between two other plates on the 

 opposite side. [Fresh waters ; species of the United States 

 numerous.] 



