152 MANUAL OF NATURAL HISTORY. 



5. Family. — Lantern-Shells (Laternulidse). Siphons 



slender, separated, with fringed orifices ; foot 

 small ; mantle almost entirely closed ; shell 

 inequivalve, gaping at the hinder extremity ; 

 ligament internal; usually a free ossicle at 

 the hinge ; a siphonal inflection. 



6. Family. — Arctic-Gapers (Glycimeridee). Siphons 



elongated, united ; mantle-lobes united ; shell 

 more or less gaping at the sides, transversely 

 elongated ; ligament large, prominent, exter- 

 nal, on a fulcrum ; hinge simple or with a 

 few primary teeth. 



7. Family. — Gapers (Myidae). Siphons greatly 



elongated, united to their ends ; foot small ; 

 mantle almost entirely closed ; shell oblong, 

 gaping at the extremities ; cartilage in a 

 spoon-shaped cavity in one of the valves ; 

 hinge edentulate ; no free ossicle. 



8. Family. — Pearly - Gapers (Pholadomyidse). 



Mantle- lobes united ; siphons none ; an open- 

 ing under siphonal orifice ; foot bifurcate ; 

 shell transverse, pearly within, gaping pos- 

 teriorly ; hinge without teeth ; ligament ex- 

 ternal. 



9. Family. — Pod- Gapers (Solenomyidse). Mantle al- 



most entirely open, with a single cirrhated 

 orifice behind ; foot cylindrical, truncate, 

 ending in a fimbriated disk ; shell transverse- 

 ly oblong, with a thick epidermis extending 

 beyond the margins ; hinge with a cardinal 

 tooth in each valve. 



