12 BEITISH 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, Ostrete, and Anomia, &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 hinges 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 Anomia, Ostrece, 

 and Pectines naturally follow the Brachiopoda with which they 

 have relations, and are succeeded by the Mytilida, &c., and 

 brought, according to the intervening genera of the synopsis, 

 to the Gastroch&nidce ; the remaining families of the Myadae, 

 Solenida, and Pholadidce, 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 fuiictions, 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 increased 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 Ascidise, 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 



