250 CONCHIFERA. 



The British fluviatile. species belong to four fami- 

 lies, which may be thus described : — 



1. Cycladce. Shell oblong ; hinge with two di- 



verging teeth ; mantle lobes free beneath ; 

 siphons two ; foot oblong, wjthout any 

 beard. 



2. Dreissenadce. Shell triangular ; hinge tooth- 



less ; mantle lobes united ; siphons two, very 

 short ; foot very small, vermiform, with a 

 beard. 



3. PisidiadcB. 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 

 imited 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. 



