MANUAL OF NATURAL HISTORY. ] 49 



anal siphon ; mantle-lobes more or less disunited ; 

 siphons elongate, usually separate at their ends. 



1. FAMILY. Venuses (Veneridae). Siphons short, 



united for the greater part of their length ; 

 foot large, compressed ; shell regular, closed ; 

 hinge with three diverging cardinal teeth; 

 ligament external. Marine. 



2. FAMILY. Cyprinas (Cyprinidas). Siphons very 



short ; mantle-lobes free beneath ; shell 

 ovate, cordate, covered with an epidermis ; 

 hinge -teeth 3 3 ; anterior lateral teeth 

 none ; siphonal inflection none or rudimen- 

 tary. 



3. FAMILY. Glauconomes (Glauconomidse). Shell 



oblong, covered with a green epidermis ; 

 hinge- teeth 3-3, in right valve the hinder 

 elongate and bifid, in left valve the hinder 

 small and laminar ; lateral teeth none. 



4. FAMILY. Rock -Borers (Petricolidse). Siphons 



elongated, separate ; foot small, lanceolate, 

 with a byssal groove ; shell boring, gaping, 

 often irregular; ligament external; hinge- 

 teeth large and irregular. 



5. FAMILY. Gyrenoids (Cyrenoididse). Mantle- 



lobes free beneath, with two united siphons ; 

 gills two on each side ; shell oblong, ventri- 

 cose, covered with a thin epidermis ; cardinal 

 teeth 3-3 ; siphonal inflection none. 



6. FAMILY. Freshwater- Venuses (Corbiculidse). 



Siphons produced, more or less united ; foot 

 large, linguiform ; shell tumid, covered with 



