48 PAL.EONTO LOGY 



connected with a fundamental difference in the method 

 of opening the shell in the two groups. The lamellibranch 

 has no divaricator muscles : when its adductors contract 

 so as to close the shell tightly, the elastic ligament is 

 subjected to tension ; when they relax, the elasticity of 

 the ligament draws the two cardinal areas towards one 

 another and the valves open. Throughout life, however, 

 the adductors are never completely relaxed, and the 

 ligament is always more or less stretched, so that it is 



FIG. ir. PECTUNCULUS GLYCIMERIS (LINNE), PLIOCENE. 

 Exterior of right valve. (Natural size, not full grown. ) (Original.) 



not until after death that the ligament has full play and 

 the valves then yawn apart more than ever during life. 



Fossil lamellibranch shells are most commonly found 

 in one of three conditions, according to the rapidity of 

 their burial after death, (i) When quickly buried (or 

 when the animal lived and died in a burrow), the 

 valves remain tightly closed, the weight of the sediment 

 counteracting the tendency of the ligament to open them : 

 in this case the interior is filled with calcite or other 



