MANUAL OF NATUKAL HISTOKY. 155 



with two primary teeth in one valve and 

 three in the other ; ligament internal. 



5. FAMILY. Astartes (Astartidse). Mantle almost 



entirely open ; siphons rudimentary, consist- 

 ing of two scarcely separated orifices ; foot 

 thick, hatchet-shaped, without a byssus ; shell 

 thick, closed ; hinge with primary teeth ; 

 ligament external ; pallial impression nearly 

 entire. 



6. FAMILY. Crassatellas (Crassatellidse). Animal 



? shell with the valves closed ; hinge 



with two cardinal teeth ; lateral teeth none ; 

 cartilage in an internal triangular pit. 



7. FAMILY. Lucinas (Lucinidse). Mantle-lobes 



half united, with two openings at the hind 

 part; foot cylindrical, folded on itself, hol- 

 low, the tube opening into the visceral cavity ; 

 shell more or less orbicular ; ligament ex- 

 ternal or sub-internal ; muscular scars large. 



8. FAMILY. False-Lucinas (Diplodontidse). Mantle- 



margins united except for a large inferior 

 opening for foot, and a small posterior anal 

 opening without any siphon; foot vermi- 

 form ; shell irregular, closed, with cardinal 

 teeth in each valve. 



9. FAMILY. Arks (Arcidse). Mantle freely open ; 



siphons none ; gills of separate filaments ; 

 foot bent, grooved, with slightly crimped 

 margins ; shell not pearly within, closed or 

 gaping at ventral margin; hinge multiden- 

 tate, teeth interlocking. 



