The Citroen U55 Currus Cityrama was a futuristic, double-decker sightseeing bus commissioned in 1956 by Groupe Cityrama in Paris.  Built by Carrosserie Currus using a Citroen Type 55 truck, its avant-garde design looked like a road-going luxury sea vessel.  Designer Albert Lemaitre drew inspiration from the sleek, curved hulls of the Bateaux Mouches excursion boats that traveled the River Seine.  Built as a double-decker with curved Plexiglas wrap-around panels enclosing both decks to ensure passengers had completely unobstructed views of Paris' landmark sights.  The upper deck roof was constructed of glass panels that could be retracted or completely removed for open-air touring during summer months - as the sheer volume of glass created a "greenhouse effect," making the cabin intensely hot on sunny days despite rudimentary ventilation systems.
More pics, information and video here.