EXPLANATION OF PLATE IX. 



Fig. 19. — Posterior views of live mussels in aquarium showing "siphons" 

 open in act of feeding and discharging wastes. Reading from left to right 

 are: — One P. laevissima d^ , L. fragilis 9 (next three) and L. suborbiculata 9 

 (two at extreme right). Young of L. Jragilis (lateral view) and Q. quadrula 

 (umbonal view) lie in front. 



Fig. 20 — Continued aquarium view showing narrowly open "siphons" 

 of A. rariplicata on the left and the extremely extended "siphons" of P. 

 laevissima o^ on the right (lateral view). 



