15. 



back vmder the shell, its margins are reflected, and it is then 

 withdra-wn. If these movements are executed on mud, the lower, 

 reflected margin of the foot buries itself, and gives the necess- 

 ary purchase for the animal to right itself. If instead, they 

 are executed on a smooth, hard surface, where no purchase can be 

 obtai'16^* these movements are likely to be replaced by leaping 

 movements. 



The leaping movements consist of bending the foot back 

 vmder the shell, turning the shell nearly on its dorsal margin, 

 planting the sTirface of the expa;T.ded "sole" on the bottoi^i* and 

 giving a quick downward movement. 



If the foot does not slip, the shell is generally turned 

 end for end and thrown some incZaes. If the foot slips, as more 

 frequently happens in smooth -bottomed aquaria, it ahoots^ out aa - 

 t-05H:^9Pi^ with wonderful rapidity and causes a posterior movement. 

 Other movements are common but will not be described. It seems 

 very hard to imagine that the foot could possibly be used as a 

 creeping organ, and its present function, for which its structiire 

 so admirabl;r fits it, is tiH burro^^ift^ in soft mud. 



BYSSAL GLAtro. 

 Figures 4 and 5 bg. 

 In none of the specimens that came under rny obser^j-a- " 



