Why is Linux the only OS in version identifier list that has a lowercase name?

[Zekereth via Digitalmars-d-learn]

So I was just testing some code and couldn't figure out why it 
wasn't working. My version block looked like this:

version(Linux)
{
...
}
Looking at the list(unless I'm missing something) Linux is the 
only OS that is lowercase. I'm guessing most people use Posix 
instead and never encounter this problem.

Regard,
Zekereth