21 



The sholl is the af;eut , assisted by the foot, as I liave 

 actually observed in young specimens under the microscope. 

 The whole structure of the shell and the arrancenent of 

 its adductor muscles indicate this mode. The teeth on the 

 anterior edges of the valves point outwards and backwards. 

 On both dorsal and ventral portions of each valve (figs. 

 IS and 19) there are stout calcareous knobs, which form a 

 double pivot by which the valves swing upon each other, by 

 the contraction of the adductor muscles. The mode of bur- 

 rowing is as follows: Wliile the foot performs a cupping 

 action, so as to draw the shell close against the surface 

 of the wood, the posterior adductor muscle contracts, so 

 that the teeth on the shell rasp away the v/ood. The valves 

 are brought to the original position by the small adductor. 

 The comparatively very large posterior adductor is there- 

 fore the active a^ent that does the work, aided by the 

 foot; the shell is the tool by which it is done. 



Ingestion of Wood and Food.- As in other Lamellibranchs , 

 a constant stream of water is passing through the siphons 

 when they are extended into the water, which serves for 

 respiration and also contains the small organisms which 



