GENERIC SYSTEM. 39 



into the opposite valve, as Mya, Solen, Tellina, 

 Donax ; or inserted, as Cardium, Mactra, Venus, 

 Spondylus, Chama, Area ; and 2dly, those without 

 teeth, as Ostrea, Anomia, Mytilus, Pinna. 



Univalves are distinguished into those, 1st, pos- 

 sessing a regular spire, which, as to their aperture, 

 are either effuse, as Conus, Cypraea, Bulla, Voluta ; 

 canaliculate, as Buccinum, Strombus, Murex; or 

 coarctate, as Argonauta, Nautilus, Trochus, Turbo, 

 Helix, Nerita, Haliotis : 2dly, having no spire, or 

 a very irregular one, as Patella, Dentalium, Ser- 

 pula. Teredo, Sabella. 



The Linnsean genera are thirty-six in number, 

 and are arranged according to the order of the 

 annexed Table, in which are inserted the total 

 amount of species hitherto described in each genus, 

 and the proportion of them which has been found 

 in and around the British isles, according to the 

 last edition of " Pennant's British Zoology." 



Many species, doubtless, are known which have 

 not yet made their way into the catalogues of con- 

 chological writers, and we have reason to suppose 

 that hundreds or thousands may be still unknown. 



No account is here taken of varieties. 



