McGonnagal the cat

[rachelhardisty at yahoo.com]

No: HPFGUIDX 18643

Hi, I have been lurking for a while now and although this may have 
come up before, I can't find the answer.

My question is this - in the first chapter of PS/SS, McGonnagal is 
sitting (as a cat) on the Dursley's fence all day.  When Dumbledore 
arrives he sits next to her and says 'hello Minerva' or something, 
she changes back, and says huffily 'how did you know it was me'?  
Well, surely if they know each other well (assuming they were both 
teaching at Hogwarts back then), Dumbledore would have seen Minerva 
in cat form many times?  She hardly keeps it a secret since she 
changes into a cat infront of Harry's class (in COS?).  How come she 
was surprised that he recognised her?

Also, if Dumbledore taught transfiguration before Minerva, isn't it a 
bit strange that HE'S not an animagus?  I know that if he turned out 
to be another unregistered animagus it would be a boring plotline 
since we already have found 4, but even so.