The "Nadar" signature seems genuine enough, it is also visible on the photograph of the Paris studio of Nadar in Naomi Rosenblum's "A world history of photography". That book also says Nadar operated a studio in Marseille during the '80s and '90s, but it doesn't say if this replaced his Paris studio (35 Boulevard des Capucines), and in fact the same Paris studio photo is dated as "after 1880"... so he may have run two at the time together with his son Paul.
Funny, there is a bit of conflicting info here as to the whereabouts of Nadar or his son, as this MoMA page says:
"After staying in Marseille from 1895 to 1904, where he opened a studio, Nadar returned to the region of Paris."
Also, that page lists 25 Boulevard des Capucine as the address of the studio, instead of 35...
Although this particular image of the outside of his studio is repeated a multitude on the internet, there doesn't seem to be a depiction of the interior of his studio including the cameras he used...
My "The history of photography as seen through the Spira collection book" shows an "Express Détective Nadar" magazine camera, second model, that Paul Nadar seems to have been using ca. 1889. It used 9x12 cm dry plates.
I also had to laugh when I saw this animated rotating self portrait by Nadar :
Another interesting image is this one with Nadar doing aerial photography (Lithograph):
A more serious version of his own cartoon: