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. 
Macrrip&. 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. 
TELLINIDH. 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. 
VeNERIDs&. Shell regular, closed, suborbicular or oblong; 
ligament external; hinge with usually three diverging teeth in 
each valve; muscular impressions oval, polished; pallial line 
sinuated.. 
db. Pallial line simple, without sinus. 
Cyprinip&. 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. 
Lucinip#. Shell orbicular, free, closed; hinge-teeth 1 or 2, 
laterals 1-1 or obsolete; muscular impressions 2, elongated, rugose; 
ligament inconspicuous or subinternal. 
Carpip#. Shell regular, equivalve, free, cordate, ommeieaiets 
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. 
Cuamip#. 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. 
Arcap#. Shell free, regular, equivalve, with strong epidermis; 
hinge with a long row of similar, comb-like teeth ; pallial line dis- 
tinct; muscular impressions subequal. 
Myritip%. Shell free, equivalve, obliquely oval or elongated, 
closed, umbones anterior, epidermis thick and dark, often filamen- 
tose; ligament internal, submarginal, very long; hinge without 
