What are the pilot rating requirements for amphibians? I know that both land and sea planes require pilot ratings in order to act as pilot-in-command. 

What is the requirement to act as pilot-in-command of an amphibian? 

Is there a third rating or just seaplane and land ratings? 

What about flights taking off and landing exclusively on either water or land but not both?
According to the Seaplane Pilots Association you can operate an amphibious airplane on hard-surface runways with a land class rating but need a seaplane class rating to take off or land on water, and vice versa.

As far as the regs, it seems that the airplane can simply be considered either land or seaplane class depending on how you are operating it.