146 BIVALVES. 



Bivalves is principally derived from the formation 

 of the hinge, and the general appearance of the 

 shell. 



PARTS OF A BIVALVE SHELL. 



The valves. 



The cardo or hinge, the part where the valves 

 are united. 



The beaks or apices, the points of the valves 

 near the hinge. 



The base, the part of the shell opposite the 

 beaks. 



The umbones or bosses, the swelled parts near 

 the beaks. 



The ligament. 



The margin of valves. 



The area or anterior slope, the margin of the 

 valves near the ligament. 



The areola or posterior slope, the margin of 

 the valves, the other side of the ligament. 



The cavity. 



The disk, the convex part of the valves be- 

 tween the umbones and margin. 



The length, the direction of the shell from the 

 beak to the base. 



The breadth, the direction at right angles with 

 the length. 



The auriuclcz* or ears, small appendages 

 laced at the side of the hinge. 



* Auricula, a small ear ; the diminutive of the Latin auris, an 

 ear. 



