382 MOLLUSCA BIVALVIA. 



TI. Valves closed by two adductor muscles. 



A. Hinge with teeth. 



a. Teeth numerous in both valves. 



b. Teeth of the hinge simple. Arcade. 



c. Hinge teeth on a straight line ; a transverse area be- 

 tween the beaks. 

 Area. 

 Cticullea. 



cc. Hinge-teeth on a bent line. 

 Pectunculus. 

 Nucula. 

 bb. Teeth of the hinge transversely striated. Trigoniadce. 

 Trigonia. 

 (Castalia.) 

 aa. Hinge with a single tooth. 

 Avicula. 



AA. Hinge without teeth. 

 (Meleagrina.) 

 Pinna. 



