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 d6va^, 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. 



