What The Hell Is That?

I received one of these mystery fruit from a friend of mine who, in turn, received them in a bag of assorted organic fruits and vegetables.  I am as clueless as her as to what it might be.  It has a hard rind - firmer than a lemon - and roughly the same size, maybe a little smaller.  I would guess some kind of citrus.

I cut it open and it smells like some kind of melon, or at least I think it's melon scented, I can't quite put my finger on it.  Also it has seeds that look like small watermelon seeds.  

The moral of the story is, what the hell is that?  Some unholy expirement which nature never intended?  I can't identify it.  The good news is that it has led to the next innovative idea from Craw Fu Enterprises:  WhatTheHellIsThat.com.  This is a web site where you upload a photo of something you can't identify and the What The Hell Is That experts will identify it for you.  After I get it up and running I'll sell it to Google for a billion dollars then buy a Hawaiian island and retire.