INTRODUCTION. 35 



Hinge lamina, Teeth and Fulcrum of the Ligament. 



In a great variety of cases, there is a thickening of the sub- 

 stance of the shell within, under the dorsal margin; this is 

 named the hinge lamina. It is sometimes merely callous ; 

 but in many cases it has raised teeth in both valves, those in 

 one valve entering into corresponding cavities in the other. 

 Those which are placed immediately below the umbones, and 

 seem to take their rise from beneath them, are called cardinal 

 teeth ; those at a distance from the umbones, which are seen 

 to lie along the upper margin of the shell are named lateral 

 teeth . 



When the cardinal teeth terminate in a double point, which 

 is not unfrequently the case, they are said to be bifid. The 

 lateral teeth, in various species, are distinguished as termi- 

 nating near to, or at a distance from the umbones. In the 

 Nuculae and Arcse there is a row of teeth placed across the 

 hinge lamina. In which case, the lateral cannot be distin- 

 guished from the cardinal teeth. 



Muscular Impression. 



Fig. 74. a, anterior ; p, posterior ; m i, muscular impressions. 



Lamarck divides the Bivalve shells into two general orders ; 



d2 



