MUSCULAIl IMPRESSIONS OE BIVALVES. 293 



are placed, is generally half-circular in form, and makes a 

 ledge under the dorsal margin. Upon this ledge are the 

 fulcrum of the ligament and the teeth. The fulcrum is a 

 thickening of part of this ledge on which the ligament is 

 placed. Tlie teeth, so called, are projections on the ledge 

 of one valve entering into corresponding depressions of the 

 other. Those which are placed centrally, or under the 

 umboes, generally diverging from each other, are called the 

 cardinal teeth. Those which are more or less remote from 

 these, towards the ends of the shell, are called lateral teeth. 

 Some bivalves have no teeth on their hinges, and others have 

 rows of teeth. PectunculMS, for instance, has the hinge 

 lamina semicircular, with a curved row of small, angular 

 teeth on each side : these are serial teeth. Sometimes the 

 teeth are hificl, or having double terminations. 



MUSCULAR IMPRESSIONS OP BIVALVES. 



On the inner, concave surface of the valve will be observed 

 one or two slightly depressed areas or impressions, slightly 

 differing in texture from the rest of the surface. These are 

 made by the attachment of the large adductor muscles, used 

 in drawincs; the valves of the shell tog-ether. In the ovster 

 a familiar example is seen. Here there is one central 



