I include this post on the Citroën SM (1970-75) under the "What Were They Thinking" category. Not because the car had wretched or inexplicable styling, but because I think the concept of the SM was flawed from a marketing standpoint.
As its Wikipedia entry indicates, the SM was a result of Citroën's purchase of a controlling interest in Maserati, the famed Italian sports and racing car firm. The concept was to create a high-performance Maserati-powered Citroën DS derivative.
According to the first link above, the SM was successful as a performance car. However, Citroën did not have a reputation as a builder of high-performance cars in the Maserati sense. So it was necessary to create a strong public image for the SM. This failed for at least two reasons. First, Citroën went bankrupt in 1974 and was rescued by Peugeot which spun off Maserati and dropped both the SM and the DS upon which it was based. Therefore, whatever efforts were being made to cast Citroën as a high-performance brand were doomed by the firm's financial troubles before they might have taken root. Secondly, the SM's styling did not exemplify high-performance, as I shall point out in the Gallery section below.
This, and most of the other photos are auction firm images, the one at the top of the post is from Bonhams, for instance. The SM's styling as been credited to Citroën head stylist Robert Opron, who must have worked with engineering staff and used wind tunnel testing to yield a reported (by Wikipedia) aerodynamic drag coefficient of 0.26.
Low drag was achieved in part due to the front being smoothed via clear panels over the headlamps and by a fender skirt over the rear wheel openings. A ridge at the rear of the hatch/trunk opening makes an upwards detour over the license plate area, this creating a small aerodynamic spoiler.
The SM was partly derived from the DS model introduced in the fall of 1955 at the Paris auto show, a radical car in its day. Comparing this photo to the one above it, a similarity of styling spirit and detailing is apparent. For example, rear wheels are covered. The roofline is thin. Windshields are similar and both models have slender A and B pillars. And then there is the typical French body shoulder (by the bottom edge of the windows) drop-off starting at the A-pillar, a common style feature on French car models built by Citroën, Renault, Peugeot and some others in the 1930s and beyond. The SM, therefore, is clearly a Citroën and not, say, a Maserati.