MANUAL OF NATURAL HISTORY. 149 



anal siphon ; mantle-lobes more or less disunited ; 

 siphons elongate, usually separate at their ends. 



1. Family. — Venuses (Veneridse). 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 (Cyprinidse). 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. — Cyrenoids (Cyrenoididee). 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 



