196 MOLLUSCA. 



1st Subdivision. 



Cloak open. 



There are no syphons, the anterior margin of the cloak 

 being as open as the mouth of the shell. When the valves 

 open, the water comes immediately in contact with the 

 branchiae and mouth. The margin of the mantle has a 

 double fringe of filaments. 



1st Tribe. 



Valves closed by one adductor muscle. 



A. Pectenidce. Animals free or fixed only by a byssus. 

 Furnished with a foot. 



Into this family, contemplated by Lamark, the following 

 ill assorted genera may be placed : Pecten, Lima, Pedum, 

 Plagiostoma, Vulsella, Placuna, Gryphsea, Perna, Gervil- 

 lea, Inoceramus, Malleus, and Crenatula. 



B. OstreadcB. Shell cemented to foreign bodies. Body 

 destitute of a foot. 



To this family the following genera are related : Ostrea, 

 Anomia, Spondylus, and Plicatula. 



2d Tribe. 



Shell closed by two adductor muscles. 



The two genera, Avicula and Meleagrina (of Lamark), 

 form one family of this tribe ; the genus Pinna another ; 

 and the Arcadae a third, including Area, Pectunculus, Nu- 

 cula, Cucullaea, Trigonia, and Castalia. 



2d Subdivision. 



Cloak more or less closed, forming syphons. 



The further division of this group depends on the modi- 

 fications of the syphons, or aperture of the cloak. 



1st Tribe. 



The union of the cloak forming only one syphon. k This 

 is situate posteriorly opposite the anus, and serves for the 



