The literary device in use in this sentence is metaphor.
In the sentence, the author explains how Bartleby is looking at an area that is practically empty now, but used to be busy with people ("sole spectator of a solitude which he has seen all populous"). He then directly compares Bartleby with Marius, who had looked upon a once-busy, but now-destroyed, Carthage.
If the sentence had read
"And here Bartleby... all populous - like a sort of innocent and transformed..."
then it would have been a simile, not a metaphor.
However, because the author simply states that Bartleby IS "a sort of innocent and transformed Marius", without a word like "like" before it, it is a metaphor.
