124 AMERICAN MARINE CONCHOLOGT. 



ternal ; large, lineal, marginal, supported on a prominent pad or 

 fulcrum ; hinge-teeth usually 2-3, compressed, the posterior bifid ; 

 usually living buried vertically in the sand. 



MACTRIM. Shell equivalve, trigonal, close or slightly gaping; 

 cartilage contained in a deep triangular pit of the hinge-plate ; 

 hinge with two diverging cardinal teeth, and usually with anterior 

 and posterior laterals ; pallial sinus short, rounded. 



TELLINID^E. Shell free, regular; hinge with two cardinal teeth, 

 at most, in each valve, 'sometimes lateral teeth ; ligament external 

 or internal, on the shorter side of the shell; pallial impression 

 largely and deeply sinuated. 



VENERID^E. Shell regular, closed, suborbicular or oblong; 

 ligament external ; hinge with usually three diverging teeth in 

 each valve; muscular impressions oval, polished; pallial line 

 sinuated. 



6. Pallial line simple, without sinus. 



CYPRINID^E. Shell regular, equivalve, oval or elongated ; valves 

 close, solid ; epidermis thick and dark ; ligament external, con- 

 spicuous ; cardinal teeth 1-3 in each valve, and usually a posterior 

 lateral tooth ; pedal scars close to, or confluent with, the adductors; 

 pallial line simple. 



LUCINID^B. Shell orbicular, free, closed ; hinge-teeth 1 or 2, 

 laterals 1-1 or obsolete; muscular impressions 2, elongated, rugose; 

 ligament inconspicuous or subinternal. 



CARDIID^B. Shell regular, equivalve, free, cordate, ornamented 

 with radiating ribs ; posterior slope sculptured differently from the 

 front and sides ; cardinal teeth two, laterals 1-1 in each valve ; 

 ligament external, short and prominent; muscular impressions 

 subquadrate. 



CHAMID^B. Shell inequivalve, thick, attached ; beaks subspiral ; 

 ligament external; hinge-teeth 2 in one valve, 1 in the other; ad- 

 ductor impressions large, reticulated ; pallial line simple. 



B. Animal without /Siphons ; Pallial line simple. 



ARCADE. Shell free, regular, equivalve, with strong epidermis ; 

 hinge with a long row of similar, comb-like teeth ; pallial line dis- 

 tinct; muscular impressions subequal. 



MYTILID^E. Shell free, equivalve, obliquely oval or elongated, 

 closed, uinbones anterior, epidermis thick and dark, often filamen- 

 tose; ligament internal, submarginal, very long; hinge without 



