250 CONCHIFERA. 



The British fluviatile species belong to four fami- 

 lies, which may be thus described : 



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



verging teeth; mantle lobes free beneath; 

 siphons two; foot oblong, without any 

 beard. 



2. Dreissenada. Shell triangular; hinge tooth- 



less ; mantle lobes united ; siphons two, very 

 short; foot very small, vermiform, with a 

 beard. 



3. Pisidtadce. Shell ovate ; hinge with two di- 



verging teeth ; mantle lobes free beneath ; 

 siphons one ; foot oblong, beardless. 



4. Unionida. Shell oblong; hinge toothless, or 



with large lateral teeth ; mantle lobes free all 

 round, bearded behind; foot large, oblong, 

 without any beard. 



ORDER I. VENERA CEA. 



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. 



