Iterating over jQuery $(this).attr('class').split(" ") gives odd results

I've got a page where I'm trying to fetch arrays of classes for lots of divs which share a common class. For example:

<div class="common lorem ipsum"></div>
<div class="common dolor sit"></div>
<div class="common hello world"></div>

I want to fetch each common class div and get an Array of it's classes. At the moment, I'm doing it by using this bit of jQuery:

$('.common').each(function(index) {
  var classes = $(this).attr('class').split(" ");
  for(var i in classes) {
    alert(classes[i]);
  }
});

Looking at the first resulting classes variable gives this:

classes: Array (3)
0: "common"
1: "lorem"
2: "ipsum"
length: 3
__proto__: Array

The problem is that the for(var i in classes) seems to be iterating over the __proto__ Array and delving down into that as well - has anybody ever come across this before? I'm using the latest version of Chrome (6.0.453.1).