57 
DONAX. 
(Plate VI. Fig. 4.) 
Shell having the margin often crenulate, and 
anteriorly very obtuse. Hinge with two primary 
teeth and one marginal, solitary, remote, rarely 
either double, triple, or deficient. (Fig. 3.) Liga- 
ment external. 
The shells of this genus are nearly all trian- 
gular, inequilateral, truncate, or concave on the 
anterior slope, and assume the form of a wedge. 
The single primary tooth in one of the valves is 
bifid. The shell usually gapes a little at each end. 
The derivation of the name from h6va^, an ar- 
row, may perhaps have been adopted on account 
of the rapidity with which the animal darts into 
the sand, on the approach of danger : his sagit- 
tate shell is well calculated for penetrating quickly 
the yielding substance of the shore. 
