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CONCHIFERA. 
The .British fluviatile species belong to four fami- 
lies, which may be thus described 
1. CycladcB. Shell oblong; hinge with two di- 
verging teeth; mantle lobes free beneath; 
siphons two ; foot oblong, without any 
beard. 
2. DreissenodcB. Shell triangular; hinge tooth- 
less ; mantle lobes united ; siphons two, very 
short ; foot very small, vermiform, with a 
beard. 
3. Pisidiadce, Shell ovate ; hinge with two di- 
verging teeth ; mantle lobes free beneath ; 
siphons one ; foot oblong, beardless. 
4. Unionidce, Shell oblong; hinge toothless, or 
with large lateral teeth ; mantle lobes free all 
round, bearded behind; foot large, oblong, 
without any beard. 
Order I. VENERACEA. 
Fam. I. CYCLAD^. 
Animal with the mantle lobes united at each end ; 
pedal opening moderate ; foot compressed, sub- 
quadrate, or becoming strap-shaped ; siphons 
united at the base, diverging at the top. 
Shell subcordate, porcellanous, thin, covered with 
a hard, olive, horny periostracum ; hinge teeth 
two or three, diverging ; lateral teeth distinct, 
laminar. 
