DONAX. 



(Plate VI. fig. 4.) 



Shell having the margin often crenulate, and an- 

 teriorly 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 triangu- 

 lar, inequilateral^ truncate or concave on the an- 

 terior 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 So'vaf, 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 quick- 

 ly the yielding substance of the shore. 



