12 BRITISH MARINE TESTACEOUS MOLLUSCA : 
together by the best mode that agrees with their compo- 
sition. 
It has long been the fashion, without any particular good 
reason, to commence the Acephala with Pholas, Teredo,&c., and 
to terminate them with the Pectines, Ostree, and Anomie, &c. 
I admit, as regards the essential points of natural order, that 
it is not very material whether Pholas and Teredo stand first 
or last in the scale. But in the classification I have adopted, 
which is founded on the progressive advancement of the repro- 
ductive organs, and having removed the Brachiopoda, which 
custom has placed at the head of the bivalves, to a position of 
less pretension, it has become necessary to invert nearly the 
usual order of arrangement, that animals of similar relations 
may be associated. This change entirely higes on, and is 
the result of, the transference of the Brachiopoda from the 
position they have so long occupied; otherwise the ancient 
distribution would have been nearly as satisfactory. But the 
false position of the Brachiopoda, according to our views, has 
admitted of no alternative. 
In conformity with these observations, the Anomie, Ostree, 
and Pectines naturally follow the Brachiopoda with which they 
have relations, and are succeeded by the Mytilide, &c., and 
brought, according to the intervening genera of the synopsis, 
to the Gastrochenide ; the remaining families of the Myade, 
Solenide, and Pholadide, are. thus placed at the head of the 
list, and form a very natural group; and I think that their 
decidedly higher organization —I particularly allude to the 
Pholades — and superior functions, as those of excavation, 
together with the compound structure of their shells, as is 
evidenced by the complication of the accessorial appendages 
as well as the consideration of the mcreased importance of the 
siphonal tubes and the enveloping mantle, bring them by 
these advances In composition into closer connection with the 
Gasteropoda than with the Ascidiz, in the vicinity of which 
they have been placed from their muscular siphonal sheaths 
and closed mantle, which have been considered to bear a 
resemblance to the coriaceous envelopes of those animals. We 
have no difficulty in admitting Venerirupis into the family of 
