He not only knows that Mademoiselle Stangerson defended herself with the revolver, but he knows what the weapon was that was used to attack her.

Monsieur Darzac tells me it was a mutton-bone.

Why is Monsieur de Marquet surrounding this mutton-bone with so much mystery?
No doubt for the purpose of facilitating the inquiries of the agents of the Surete?
He imagines, perhaps, that the owner of this instrument of crime, the most terrible invented, is going to be found amongst those who are well-known in the slums of Paris who use it.

But who can ever say what passes through the brain of an examining magistrate ?" Rouletabille added with contemptuous irony.
"Has a mutton-bone been found in The Yellow Room ?" I asked him.
"Yes, Monsieur," said Robert Darzac, "at the foot of the bed; but I beg of you not to say anything about it." (I made a gesture of assent.) "It was an enormous mutton-bone, the top of which, or rather the joint, was still red with the blood of the frightful wound.

It was an old bone, which may, according to appearances, have served in other crimes.