49 



In some populations the shells show a remarkable uniform- 

 ity indicating presumably that they have reached a state of 

 genetic equilibrium. Such populations usually occupy isolated 

 sites where the probability of receiving additional migrants is 

 remote, or the migrants received may be so similar as to cre- 

 ate no disturbance. On the other hand populations occupying 

 areas subject to invasion by divergent alien elements are apt 

 to be more or less heterogeneous. Artificial structures, such 

 as docks and breakwaters, offer an interesting study from this 

 standpoint. Many areas inhabited by populations of Thais 

 are subject to destruction or profound alteration through the 

 initiation of engineering projects. 



In some instances the amount of variation within a single 

 population is so extensive it cannot be given adequate express- 

 ion within the limits of a single plate. 



Many problems relating to this highly polymorphic species 

 must remain on a tentative basis pending the initiation of 

 adequate genetic and physiologic experimentation. 



