156 MANUAL OF NATURAL HISTORY. 



10. Family. — Solen-Arks (Solenellidse). Mantle 



open the entire length, margin double, with 

 a single anal siphon ; labial palps elongate ; 

 foot compressed, geniculate, with a disk with 

 crenate margins ; shell thin, gaping slightly 

 posteriorly, not pearly within ; hinge-margin 

 with comb-like teeth ; ligament external, 

 prominent. 



11. Family. — Pearly -Arks (Nuculidse). Mantle 



completely open, with or without siphonal 

 tubes ; one of the labial palps curled, with 

 fimbriated margins ; gills foliaceous ; foot 

 deeply grooved, forming an ovate disk with 

 serrated margins ; shell pearly within ; teeth 

 comb-like ; ligament internal. 



12. Family. — Cockle- Arks (Trigoniidse). Mantle- 



lobes free, prolonged into short siphons ; gills 

 foliaceous ; foot large, geniculate, with a disk ; 

 shell equivalve, with radiating ribs, pearly 

 within ; hinge with a few broad, lamelliform, 

 grooved teeth interlocking with each other. 



13 Family. — Pond-Mussels (Unionidse). Mantle 

 freely open, branchial orifice fringed, anal 

 plain or tube-like ; gills foliaceous ; foot 

 broad, compressed, without a byssal groove ; 

 shell pearly within, covered with an epider- 

 mis ; hinge variable ; ligament external. Flu- 

 viatile. 



14. Family. — Nile -Mussels (Mutelidse). Mantle 

 freely open, united behind and prolonged 

 into two short siphons ; gills foliaceous ; foot 



