198 



THE SCIENCE OF LIFE. 



of a bivalve are generally equal, except in Brachiopods 

 and in the Oyster. The umbones, or beaks, are a little 

 in front of the center, and turn toward the mouth of the 

 animal. The valves are joined by a ligament near the 



FlG. 96. a. b. Length of the shell, c. d. Height, e. Lunula, above which is the sum- 

 mit, d. The ventral or inferior edge. 



A B 



FIG. 97. A. The line across marks the thickness of bivalves. B. a. Anterior extrem- 

 ity, b. Posterior, c. d. Muscular impressions, e. f. Palleal impression, g. Lower 

 edge of the left valve. 



umbones, and often also by a " hinge " formed by the 

 " teeth " of one valve locking into cavities of the other. 

 The aperture of a univalve is sometimes closed by a 

 horny or calcareous plate, called an " operculum" 



