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, Mytiius, Pinna. 
Univalves are distinguished into those, 1st, pos- 
sessing a regular spire, which, as to their aperture, 
are either effuse, as Conus, Cyprsea, 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 Linnaean 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. 
