84 CLASSES, ORDERS, AND SUB-ORDERS. 



Of these five orders, two have been further divided. The sub- 

 orders of the Filibranchiata are : 



1 . Anomiacea one aorta ; anterior adductor small ; shell 



with plug perforating right valve. 



2. Arcacas two aortas ; both adductors large ; shell without plug. 



3. Mytilacea one aorta; anterior adductor small; shell with- 



out plug. 



The sub-orders of the Eulamellibranchiata include the great 

 majority of the British bivalves, but they are not easily distinguish- 

 able, from description owing to the many exceptions that have to be 

 provided for. We have, therefore, not used them in this system of 

 identification. They are, with the families represented in our fauna. 



1. Submytilacea. 



Carditidas, Astartidae, Cyprinidse, Unionidas, Dreissensiidas, 

 Lucinidae, Erycinidse, Galeommidae, Cyraenidae. 



2. Tellinacea. 



Telliiiidas, Scrobiculariidae, Donacidae, Mactridas. 



3. Veneracea. 



Veneridae, Petricolidae. 



4. Cardiacea. 



Cardiidse. 



5. Myacea. 



Psammobiidag, Myidae, Solenidas, Glycimeridae, Gastro- 

 chcenidae. 



6. Pholadacea. 



Pholadidae, Teredinidae. 



7. Anatinacea. 



Pandoridae, Lyonsiidae, Anatinidae. 



