ORDER UNIMUSCULOSA. 205 



Tribe I. LAMELLIBRANCHIATA— lamella-gilled. 



The Cephalous mollusks dwelling, some in air, some in water, 

 and some in both elements, have different modes of respiring, and 

 their respiratory organ affords good distinguishing characters for 

 the subdivision of the Tribes into Orders. The Acephala are all 

 water- dwelling mollusks, and their respiratory organ is too little 

 varied to characterize an ordinal subdivision. It is represented 

 throughout the class by a pair of lamellse or leaflets on each side of 

 the viscera, quite distinct from the mantle, and they are usually 

 clothed with extremely delicate cilia, used by the animal for pro- 

 ducing an inflowing current from the surrounding fluid. These 

 beautifully ciliated leaflets are exquisitely shown in Mr. Goadby's 

 anatomical preparations. 



The Lamellibranchiate Acephala are divided into two Orders, 

 according to the mode by which the shell is affixed to the animal, 

 whether by one central adductor muscle or by two terminal muscles. 



1. Unimusculosa. Animal having its shell affixed by a central 



muscle in each valve, which is sometimes compound or fur- 

 nished with one or more small auxiliary muscles. 



2. Bimusculosa. Animal having its shell affixed by two dis- 



tinct terminal muscles, one at the anterior, the other at the 

 posterior end. 



Order I. UNIMUSCULOSA— one-muscled. 



Animal having its shell affixed by a central muscle in each valve, which is 

 sometimes compound, or furnished ivith small auxiliary muscles. 



It may appear unscientific to introduce a systematic division 

 founded on the character of a single pair of adductor muscles, when 

 the only British mollusk inhabiting fresh water that is referred to 

 it is possessed of more. But in viewing the entire series of Acephala, 

 both freshwater and marine, a very natural subdivision is sug- 

 gested by the separation into two Orders, of those in which the shell 

 is affixed to the animal by a central muscle in each valve, as in the 

 Oyster, and those in which the shell is affixed to the animal by two 

 lateral, or rather terminal, muscles, as in the Anodon. No characters 

 are permanent through any long series of natural objects, and it 



