124 AMERICAN MARINE CONCHOLOGY. 



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

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

 usuall}" living buried vertically in the sand. 



Mactrid.e. 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; pallia! 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 deepl} 7 sinuated. 



Yenerid.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. 



b. 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.e. Shell orbicular, free, closed; hinge-teeth 1 or 2, 

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

 ligament inconspicuous or subinternal. 



Cardiid.e. 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. 



Chamidje. 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. 



Arcad.e. Shell free, regular, equivalve, with strong epidermis; 

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

 tinct; muscular impressions subequal. 



MYTlLiD.ffl. Shell free, equivalve, obliquely oval or elongated, 

 closed, umbones anterior, epidermis thick and dark, often fdamen- 

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



