"HE'S NOT PETER, HE'S SCABBERS!" Ron yelled, trying to fore the rat back into his front pocket, but Scabbers was fighting to hard; Ron swayed and overbalanced, and Harry caught him am pushed him back down to the bed. Then, ignoring Black, Harry turned to Lupin.

 "There were witnesses who saw Pettigrew die," he said. "A whole street full of them..." 

"They didn't see what they thought they saw!" said Black savagely, still watching Scabbers struggling in Ron's hands. 

"Everyone thought Sirius killed Peter," said Lupin, nodding. "I believed it myself -- until I saw the map tonight. Because the Marauder's map never lies... Peter's alive. Ron's holding him, Harry."

 Harry looked down at Ron, and as their eyes met, they agreed, silently: Black and Lupin were both out of their minds. Their story made no sense whatsoever. How could Scabbers be Peter Pettigrew? Azkaban must have unhinged Black after all -- but why was Lupin playing along with him? 

Then Hermione spoke, in a trembling, would-be calm sort of voice, as though trying to will Professor Lupin to talk sensibly. 

"But Professor Lupin... Scabbers can't be Pettigrew... it just can't be true, you know it can't..." 

"Why can't it be true?" Lupin said calmly, as though they were in class, and Hermione had simply spotted a problem in an experiment with grindylows. 

"Because... because people would know if Peter Pettigrew had been an Animagus. We did Animagi in class with Professor McGonagall. And I looked them up when I did my homework -- the Ministry of Magic keeps tabs on witches and wizards who can become animals; there's a register showing what animal they become, and their markings and things... and I went and looked Professor McGonagall up on the register, and there have been only seven Animagi this century, and Pettigrew's name wasn't on the list."