Yes, he looks pretty grim here, but he had scoliosis and had recently suffered a catastrophic accident while riding across Clapham Common.
Gideon Mantell frequently led walks over Wandsworth and Clapham Commons, to introduce people to the remarkable geology and paleontology.
He says words to the effect, just as Columbus had discovered a new hemisphere, so science was disclosing new worlds near at hand, under out feet.
But you don't have to travel to the other side of the planet to see these new worlds, or to make great discoveries. You don't have to be special.
Just come to Wandsworth Common... Enlightenment is available to absolutely anybody who simply sets foot on it.
...examine the "new worlds beneath our feet, which all are at liberty to visit and explore... It is reckoned one of the noblest and best privileges of our nature to be able to examine and to value the perfections of the Creator... enabling us to investigate fresh scenes of admire the perfections of the Creator in spheres unknown, unsuspected before" [CHECK QUOTE]
NB: Mantell is writing before Darwin...
[Blake: "And did those feet in ancient time walk upon Englands mountains green..."]
Within a few years, Mantell had identified 4 of the 5 genera of dinosaurs then known. Some fabulous speculative reconstructions of his dinosaurs can still be seen in the grounds of Crystal Palace.
[MID-C19 DINOSAURS AT CRYSTAL PALCE.]
[Curiously, this makes another connection.] Quite literally. The railway line that runs through Wandsworth Common station was originally built to carry passengers from London to the Crystal Palace at Sydenham, when it moved there from Hyde Park in 1856.