From the Styx link to http://www.pantheon.org (Encyclopedia Mythica) :
It is thought that Charon, the old ferry man who ferries the dead onto the underworld, crosses the river
Styx where the dragon tailed dog Cerberus guards, allowing all souls to enter but none to leave. This is a
misconception, Charon crosses the river Acheron where also Cerebus stands his eternal guard.