THE LAMELLIBRANCHIA 49 



material deposited from solution (except in those genera 

 Whose valve-margins do not meet for their whole length).. 

 (2) If the muscles decay away before burial, the valves 

 yawn open, and sediment drifting in prevents their 

 closing, but they remain united. This case is well 

 illustrated by the specimens of Pectunculus brevirostris that 



FIG. 12. PECTUNCULUS GLYCIMERIS (LINNE), PLIOCENE. 



Interior of left valve. (Natural size.) From above down are seen in order 

 umbo, cardinal area, curved row of teeth, edge of hinge-plate, 

 adductor impressions, pallial line, crenulate ventral margin. (Original.) 



abound in the Bognor rock, a band of hard sandstone in 

 the London Clay of the Sussex coast. (3) If burial is 

 long delayed, the valves are drifted about until not only 

 the muscles but the ligament also is decayed ; then the 

 valves become completely separated. This is the case 

 with the abundant valves of P. glycimevis in the Red 

 Crag of Suffolk. Each of these three conditions may be 



4 



