Just as he was about to grab another book, he heard the older girl speak. Actually surprised that she did indeed know him, he masked it with a polite impassive look. "I am Bellatrix. And you?"
With this, he proceeded to take the book off the shelf while still keeping eye contact with her. Another common Slytherin rule: Never let a fellow snake see your back, for one never knows if a knife is wielded in one hand.