Where is I/O reference

New to arduino. In getting started I ran the blink program. It uses digitalpin13 however when I look at my mega2650 pin out it shows that hooked up to PB07. Where is the reference that assigns PB07 to digitalpin13? Thanks

pins_arduino.h seems a likely place to look.

https://www.google.co.uk/search?q=arduino+mega2560+pinout+images&client=firefox-a&hs=77l&rls=org.mozilla:en-US:official&channel=nts&biw=1024&bih=610&source=lnms&tbm=isch&sa=X&ei=NxFNVODWFqyV7AaG5IG4AQ&ved=0CAYQ_AUoAQ