124 AMERICAN MARINE CONCHOLOGY. 



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. 



MactriDuS;. 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. 



TELLiNiDiE. 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. 



Yeneridje. Shell i-egular, 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, icitJiout sinus. 



Cyprinid^. 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. 



LuciNiDiB. Shell orbiculai', free, closed ; hinge-teeth 1 or 2, 

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

 ligament inconspicuous or subinternal. 



Cardiid^. Shell regular, equivalve, free, cordate, ornamented 

 with radiating ribs ; posterior slope sculptured differentlj^ from the 

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

 ligament external, short and prominent ; muscular impressions 

 subquadrate. 



Chamid.e. 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. 



ARCADiE. 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, umbones anterior, epidermis thick and dark, often filaraen- 

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



