DITHYRA. ANALOGIES. 



255 



ment, which passes through the apex or summit of one 

 of the valves : '' like the other bivalves/' as Cuvier ob- 

 serves, " they have a bilobed mantle, which," as in the 

 oysters, '^ h always open : instead of feet, they have two 

 fleshy arms, furnished with numerous filaments, which 

 are protruded or withdrawn at pleasure." Nearly all of 

 them possess (like their prototypes, the Cephalopoda and 

 the Tectibranchia) an internal bony support, which in 

 these consists of a sort of framework closely resembling 

 an internal skeleton, and rather complicated. Although 

 innumerable fossil shells of the Branchiopoda abound in 

 the older formations, very few occur in a recent state. 

 Our arrangement of the entire order will thus be in the 

 following tribes: — 1. The Macrotrachia, where there 

 are either one or two siphons ; 2. The Atrachia, hav- 

 ing none ; 3. The Tubulibranchia, or tubular shell- 

 fish, having an indistinct head, and an operculum to 

 their shell ; 4. The Cheliosomid^, with a cartilaginous 

 covering and two orifices ; and, 5. The Branchiopoda, 

 or anomian bivalves. The whole of these, it will now 

 appear, have their prototypes in the Gasteropoda. 



Analogies of the Orders Dithyra and the Gasteropoda. 



Tribes of the 

 Ditliyra. 



Macrotrachia. 



Atrachia. 

 Branchiopoda. 



Cheliosomid^. 

 Tubulibranchia. 



Analogies. 



Tribes of the 

 Gasteropoda. 



r Animal with the mantle formed 1 

 ■< into an elongated siphon, sim- > 

 C pie or double. J 



Mantle free, and without a siphon. 

 (■ Reciprocally representing the Ce- 1 

 i phalopoda. J 



TBody cheloniform, oval, covered^ 



< with testaceous or coriaceous > Cyclobranchia, 

 t plates. 3 



r Animal of the gastro|)od struc-T 



< ture, furnished with an obtuse?- 

 C head. 3 



Phytophaga, 

 Tectibranchia. 



Scutibranchia. 



This table is important, were it only to prove that 

 the usual divisions of the more typical bivalves accord- 

 ing to the number of their muscles, whether one or 

 two, is not a natural arrangement ; because it destroys 



