Well, there are two, the jardini and the leichardti. They both look very similar, except that leis seem slenderer than jars.
From a very young age jars will terrorize everything, and once they hit the 8" mark and get some muscle behind them it becomes even harder to keep them with anything else. Leis are usually more peaceful, but can be a little feisty, I've heard. But I wouldn't try to do a community arowana tank with a jar unless there were about 6 or more silvers, and they all had a really huge tank.